mm/vmscan.c: prevent useless kswapd loops

[linux/fpc-iii.git] / fs / userfaultfd.c

/*
 *  fs/userfaultfd.c
 *
 *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
 *  Copyright (C) 2008-2009 Red Hat, Inc.
 *  Copyright (C) 2015  Red Hat, Inc.
 *
 *  This work is licensed under the terms of the GNU GPL, version 2. See
 *  the COPYING file in the top-level directory.
 *
 *  Some part derived from fs/eventfd.c (anon inode setup) and
 *  mm/ksm.c (mm hashing).
 */

#include <linux/list.h>
#include <linux/hashtable.h>
#include <linux/sched/signal.h>
#include <linux/sched/mm.h>
#include <linux/mm.h>
#include <linux/poll.h>
#include <linux/slab.h>
#include <linux/seq_file.h>
#include <linux/file.h>
#include <linux/bug.h>
#include <linux/anon_inodes.h>
#include <linux/syscalls.h>
#include <linux/userfaultfd_k.h>
#include <linux/mempolicy.h>
#include <linux/ioctl.h>
#include <linux/security.h>
#include <linux/hugetlb.h>

static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;

enum userfaultfd_state {
        UFFD_STATE_WAIT_API,
        UFFD_STATE_RUNNING,
};

/*
 * Start with fault_pending_wqh and fault_wqh so they're more likely
 * to be in the same cacheline.
 *
 * Locking order:
 *      fd_wqh.lock
 *              fault_pending_wqh.lock
 *                      fault_wqh.lock
 *              event_wqh.lock
 *
 * To avoid deadlocks, IRQs must be disabled when taking any of the above locks,
 * since fd_wqh.lock is taken by aio_poll() while it's holding a lock that's
 * also taken in IRQ context.
 */
struct userfaultfd_ctx {
        /* waitqueue head for the pending (i.e. not read) userfaults */
        wait_queue_head_t fault_pending_wqh;
        /* waitqueue head for the userfaults */
        wait_queue_head_t fault_wqh;
        /* waitqueue head for the pseudo fd to wakeup poll/read */
        wait_queue_head_t fd_wqh;
        /* waitqueue head for events */
        wait_queue_head_t event_wqh;
        /* a refile sequence protected by fault_pending_wqh lock */
        struct seqcount refile_seq;
        /* pseudo fd refcounting */
        atomic_t refcount;
        /* userfaultfd syscall flags */
        unsigned int flags;
        /* features requested from the userspace */
        unsigned int features;
        /* state machine */
        enum userfaultfd_state state;
        /* released */
        bool released;
        /* memory mappings are changing because of non-cooperative event */
        bool mmap_changing;
        /* mm with one ore more vmas attached to this userfaultfd_ctx */
        struct mm_struct *mm;
};

struct userfaultfd_fork_ctx {
        struct userfaultfd_ctx *orig;
        struct userfaultfd_ctx *new;
        struct list_head list;
};

struct userfaultfd_unmap_ctx {
        struct userfaultfd_ctx *ctx;
        unsigned long start;
        unsigned long end;
        struct list_head list;
};

struct userfaultfd_wait_queue {
        struct uffd_msg msg;
        wait_queue_entry_t wq;
        struct userfaultfd_ctx *ctx;
        bool waken;
};

struct userfaultfd_wake_range {
        unsigned long start;
        unsigned long len;
};

static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
                                     int wake_flags, void *key)
{
        struct userfaultfd_wake_range *range = key;
        int ret;
        struct userfaultfd_wait_queue *uwq;
        unsigned long start, len;

        uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
        ret = 0;
        /* len == 0 means wake all */
        start = range->start;
        len = range->len;
        if (len && (start > uwq->msg.arg.pagefault.address ||
                    start + len <= uwq->msg.arg.pagefault.address))
                goto out;
        WRITE_ONCE(uwq->waken, true);
        /*
         * The Program-Order guarantees provided by the scheduler
         * ensure uwq->waken is visible before the task is woken.
         */
        ret = wake_up_state(wq->private, mode);
        if (ret) {
                /*
                 * Wake only once, autoremove behavior.
                 *
                 * After the effect of list_del_init is visible to the other
                 * CPUs, the waitqueue may disappear from under us, see the
                 * !list_empty_careful() in handle_userfault().
                 *
                 * try_to_wake_up() has an implicit smp_mb(), and the
                 * wq->private is read before calling the extern function
                 * "wake_up_state" (which in turns calls try_to_wake_up).
                 */
                list_del_init(&wq->entry);
        }
out:
        return ret;
}

/**
 * userfaultfd_ctx_get - Acquires a reference to the internal userfaultfd
 * context.
 * @ctx: [in] Pointer to the userfaultfd context.
 */
static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
{
        if (!atomic_inc_not_zero(&ctx->refcount))
                BUG();
}

/**
 * userfaultfd_ctx_put - Releases a reference to the internal userfaultfd
 * context.
 * @ctx: [in] Pointer to userfaultfd context.
 *
 * The userfaultfd context reference must have been previously acquired either
 * with userfaultfd_ctx_get() or userfaultfd_ctx_fdget().
 */
static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
{
        if (atomic_dec_and_test(&ctx->refcount)) {
                VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
                VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
                VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
                VM_BUG_ON(waitqueue_active(&ctx->fault_wqh));
                VM_BUG_ON(spin_is_locked(&ctx->event_wqh.lock));
                VM_BUG_ON(waitqueue_active(&ctx->event_wqh));
                VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
                VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
                mmdrop(ctx->mm);
                kmem_cache_free(userfaultfd_ctx_cachep, ctx);
        }
}

static inline void msg_init(struct uffd_msg *msg)
{
        BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);
        /*
         * Must use memset to zero out the paddings or kernel data is
         * leaked to userland.
         */
        memset(msg, 0, sizeof(struct uffd_msg));
}

static inline struct uffd_msg userfault_msg(unsigned long address,
                                            unsigned int flags,
                                            unsigned long reason,
                                            unsigned int features)
{
        struct uffd_msg msg;
        msg_init(&msg);
        msg.event = UFFD_EVENT_PAGEFAULT;
        msg.arg.pagefault.address = address;
        if (flags & FAULT_FLAG_WRITE)
                /*
                 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
                 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
                 * was not set in a UFFD_EVENT_PAGEFAULT, it means it
                 * was a read fault, otherwise if set it means it's
                 * a write fault.
                 */
                msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
        if (reason & VM_UFFD_WP)
                /*
                 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
                 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
                 * not set in a UFFD_EVENT_PAGEFAULT, it means it was
                 * a missing fault, otherwise if set it means it's a
                 * write protect fault.
                 */
                msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
        if (features & UFFD_FEATURE_THREAD_ID)
                msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
        return msg;
}

#ifdef CONFIG_HUGETLB_PAGE
/*
 * Same functionality as userfaultfd_must_wait below with modifications for
 * hugepmd ranges.
 */
static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
                                         struct vm_area_struct *vma,
                                         unsigned long address,
                                         unsigned long flags,
                                         unsigned long reason)
{
        struct mm_struct *mm = ctx->mm;
        pte_t *ptep, pte;
        bool ret = true;

        VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));

        ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));

        if (!ptep)
                goto out;

        ret = false;
        pte = huge_ptep_get(ptep);

        /*
         * Lockless access: we're in a wait_event so it's ok if it
         * changes under us.
         */
        if (huge_pte_none(pte))
                ret = true;
        if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
                ret = true;
out:
        return ret;
}
#else
static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
                                         struct vm_area_struct *vma,
                                         unsigned long address,
                                         unsigned long flags,
                                         unsigned long reason)
{
        return false;   /* should never get here */
}
#endif /* CONFIG_HUGETLB_PAGE */

/*
 * Verify the pagetables are still not ok after having reigstered into
 * the fault_pending_wqh to avoid userland having to UFFDIO_WAKE any
 * userfault that has already been resolved, if userfaultfd_read and
 * UFFDIO_COPY|ZEROPAGE are being run simultaneously on two different
 * threads.
 */
static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
                                         unsigned long address,
                                         unsigned long flags,
                                         unsigned long reason)
{
        struct mm_struct *mm = ctx->mm;
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd, _pmd;
        pte_t *pte;
        bool ret = true;

        VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));

        pgd = pgd_offset(mm, address);
        if (!pgd_present(*pgd))
                goto out;
        p4d = p4d_offset(pgd, address);
        if (!p4d_present(*p4d))
                goto out;
        pud = pud_offset(p4d, address);
        if (!pud_present(*pud))
                goto out;
        pmd = pmd_offset(pud, address);
        /*
         * READ_ONCE must function as a barrier with narrower scope
         * and it must be equivalent to:
         *      _pmd = *pmd; barrier();
         *
         * This is to deal with the instability (as in
         * pmd_trans_unstable) of the pmd.
         */
        _pmd = READ_ONCE(*pmd);
        if (pmd_none(_pmd))
                goto out;

        ret = false;
        if (!pmd_present(_pmd))
                goto out;

        if (pmd_trans_huge(_pmd))
                goto out;

        /*
         * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
         * and use the standard pte_offset_map() instead of parsing _pmd.
         */
        pte = pte_offset_map(pmd, address);
        /*
         * Lockless access: we're in a wait_event so it's ok if it
         * changes under us.
         */
        if (pte_none(*pte))
                ret = true;
        pte_unmap(pte);

out:
        return ret;
}

/*
 * The locking rules involved in returning VM_FAULT_RETRY depending on
 * FAULT_FLAG_ALLOW_RETRY, FAULT_FLAG_RETRY_NOWAIT and
 * FAULT_FLAG_KILLABLE are not straightforward. The "Caution"
 * recommendation in __lock_page_or_retry is not an understatement.
 *
 * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_sem must be released
 * before returning VM_FAULT_RETRY only if FAULT_FLAG_RETRY_NOWAIT is
 * not set.
 *
 * If FAULT_FLAG_ALLOW_RETRY is set but FAULT_FLAG_KILLABLE is not
 * set, VM_FAULT_RETRY can still be returned if and only if there are
 * fatal_signal_pending()s, and the mmap_sem must be released before
 * returning it.
 */
vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
{
        struct mm_struct *mm = vmf->vma->vm_mm;
        struct userfaultfd_ctx *ctx;
        struct userfaultfd_wait_queue uwq;
        vm_fault_t ret = VM_FAULT_SIGBUS;
        bool must_wait, return_to_userland;
        long blocking_state;

        /*
         * We don't do userfault handling for the final child pid update.
         *
         * We also don't do userfault handling during
         * coredumping. hugetlbfs has the special
         * follow_hugetlb_page() to skip missing pages in the
         * FOLL_DUMP case, anon memory also checks for FOLL_DUMP with
         * the no_page_table() helper in follow_page_mask(), but the
         * shmem_vm_ops->fault method is invoked even during
         * coredumping without mmap_sem and it ends up here.
         */
        if (current->flags & (PF_EXITING|PF_DUMPCORE))
                goto out;

        /*
         * Coredumping runs without mmap_sem so we can only check that
         * the mmap_sem is held, if PF_DUMPCORE was not set.
         */
        WARN_ON_ONCE(!rwsem_is_locked(&mm->mmap_sem));

        ctx = vmf->vma->vm_userfaultfd_ctx.ctx;
        if (!ctx)
                goto out;

        BUG_ON(ctx->mm != mm);

        VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
        VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));

        if (ctx->features & UFFD_FEATURE_SIGBUS)
                goto out;

        /*
         * If it's already released don't get it. This avoids to loop
         * in __get_user_pages if userfaultfd_release waits on the
         * caller of handle_userfault to release the mmap_sem.
         */
        if (unlikely(READ_ONCE(ctx->released))) {
                /*
                 * Don't return VM_FAULT_SIGBUS in this case, so a non
                 * cooperative manager can close the uffd after the
                 * last UFFDIO_COPY, without risking to trigger an
                 * involuntary SIGBUS if the process was starting the
                 * userfaultfd while the userfaultfd was still armed
                 * (but after the last UFFDIO_COPY). If the uffd
                 * wasn't already closed when the userfault reached
                 * this point, that would normally be solved by
                 * userfaultfd_must_wait returning 'false'.
                 *
                 * If we were to return VM_FAULT_SIGBUS here, the non
                 * cooperative manager would be instead forced to
                 * always call UFFDIO_UNREGISTER before it can safely
                 * close the uffd.
                 */
                ret = VM_FAULT_NOPAGE;
                goto out;
        }

        /*
         * Check that we can return VM_FAULT_RETRY.
         *
         * NOTE: it should become possible to return VM_FAULT_RETRY
         * even if FAULT_FLAG_TRIED is set without leading to gup()
         * -EBUSY failures, if the userfaultfd is to be extended for
         * VM_UFFD_WP tracking and we intend to arm the userfault
         * without first stopping userland access to the memory. For
         * VM_UFFD_MISSING userfaults this is enough for now.
         */
        if (unlikely(!(vmf->flags & FAULT_FLAG_ALLOW_RETRY))) {
                /*
                 * Validate the invariant that nowait must allow retry
                 * to be sure not to return SIGBUS erroneously on
                 * nowait invocations.
                 */
                BUG_ON(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
#ifdef CONFIG_DEBUG_VM
                if (printk_ratelimit()) {
                        printk(KERN_WARNING
                               "FAULT_FLAG_ALLOW_RETRY missing %x\n",
                               vmf->flags);
                        dump_stack();
                }
#endif
                goto out;
        }

        /*
         * Handle nowait, not much to do other than tell it to retry
         * and wait.
         */
        ret = VM_FAULT_RETRY;
        if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
                goto out;

        /* take the reference before dropping the mmap_sem */
        userfaultfd_ctx_get(ctx);

        init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
        uwq.wq.private = current;
        uwq.msg = userfault_msg(vmf->address, vmf->flags, reason,
                        ctx->features);
        uwq.ctx = ctx;
        uwq.waken = false;

        return_to_userland =
                (vmf->flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
                (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
        blocking_state = return_to_userland ? TASK_INTERRUPTIBLE :
                         TASK_KILLABLE;

        spin_lock_irq(&ctx->fault_pending_wqh.lock);
        /*
         * After the __add_wait_queue the uwq is visible to userland
         * through poll/read().
         */
        __add_wait_queue(&ctx->fault_pending_wqh, &uwq.wq);
        /*
         * The smp_mb() after __set_current_state prevents the reads
         * following the spin_unlock to happen before the list_add in
         * __add_wait_queue.
         */
        set_current_state(blocking_state);
        spin_unlock_irq(&ctx->fault_pending_wqh.lock);

        if (!is_vm_hugetlb_page(vmf->vma))
                must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
                                                  reason);
        else
                must_wait = userfaultfd_huge_must_wait(ctx, vmf->vma,
                                                       vmf->address,
                                                       vmf->flags, reason);
        up_read(&mm->mmap_sem);

        if (likely(must_wait && !READ_ONCE(ctx->released) &&
                   (return_to_userland ? !signal_pending(current) :
                    !fatal_signal_pending(current)))) {
                wake_up_poll(&ctx->fd_wqh, EPOLLIN);
                schedule();
                ret |= VM_FAULT_MAJOR;

                /*
                 * False wakeups can orginate even from rwsem before
                 * up_read() however userfaults will wait either for a
                 * targeted wakeup on the specific uwq waitqueue from
                 * wake_userfault() or for signals or for uffd
                 * release.
                 */
                while (!READ_ONCE(uwq.waken)) {
                        /*
                         * This needs the full smp_store_mb()
                         * guarantee as the state write must be
                         * visible to other CPUs before reading
                         * uwq.waken from other CPUs.
                         */
                        set_current_state(blocking_state);
                        if (READ_ONCE(uwq.waken) ||
                            READ_ONCE(ctx->released) ||
                            (return_to_userland ? signal_pending(current) :
                             fatal_signal_pending(current)))
                                break;
                        schedule();
                }
        }

        __set_current_state(TASK_RUNNING);

        if (return_to_userland) {
                if (signal_pending(current) &&
                    !fatal_signal_pending(current)) {
                        /*
                         * If we got a SIGSTOP or SIGCONT and this is
                         * a normal userland page fault, just let
                         * userland return so the signal will be
                         * handled and gdb debugging works.  The page
                         * fault code immediately after we return from
                         * this function is going to release the
                         * mmap_sem and it's not depending on it
                         * (unlike gup would if we were not to return
                         * VM_FAULT_RETRY).
                         *
                         * If a fatal signal is pending we still take
                         * the streamlined VM_FAULT_RETRY failure path
                         * and there's no need to retake the mmap_sem
                         * in such case.
                         */
                        down_read(&mm->mmap_sem);
                        ret = VM_FAULT_NOPAGE;
                }
        }

        /*
         * Here we race with the list_del; list_add in
         * userfaultfd_ctx_read(), however because we don't ever run
         * list_del_init() to refile across the two lists, the prev
         * and next pointers will never point to self. list_add also
         * would never let any of the two pointers to point to
         * self. So list_empty_careful won't risk to see both pointers
         * pointing to self at any time during the list refile. The
         * only case where list_del_init() is called is the full
         * removal in the wake function and there we don't re-list_add
         * and it's fine not to block on the spinlock. The uwq on this
         * kernel stack can be released after the list_del_init.
         */
        if (!list_empty_careful(&uwq.wq.entry)) {
                spin_lock_irq(&ctx->fault_pending_wqh.lock);
                /*
                 * No need of list_del_init(), the uwq on the stack
                 * will be freed shortly anyway.
                 */
                list_del(&uwq.wq.entry);
                spin_unlock_irq(&ctx->fault_pending_wqh.lock);
        }

        /*
         * ctx may go away after this if the userfault pseudo fd is
         * already released.
         */
        userfaultfd_ctx_put(ctx);

out:
        return ret;
}

static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
                                              struct userfaultfd_wait_queue *ewq)
{
        struct userfaultfd_ctx *release_new_ctx;

        if (WARN_ON_ONCE(current->flags & PF_EXITING))
                goto out;

        ewq->ctx = ctx;
        init_waitqueue_entry(&ewq->wq, current);
        release_new_ctx = NULL;

        spin_lock_irq(&ctx->event_wqh.lock);
        /*
         * After the __add_wait_queue the uwq is visible to userland
         * through poll/read().
         */
        __add_wait_queue(&ctx->event_wqh, &ewq->wq);
        for (;;) {
                set_current_state(TASK_KILLABLE);
                if (ewq->msg.event == 0)
                        break;
                if (READ_ONCE(ctx->released) ||
                    fatal_signal_pending(current)) {
                        /*
                         * &ewq->wq may be queued in fork_event, but
                         * __remove_wait_queue ignores the head
                         * parameter. It would be a problem if it
                         * didn't.
                         */
                        __remove_wait_queue(&ctx->event_wqh, &ewq->wq);
                        if (ewq->msg.event == UFFD_EVENT_FORK) {
                                struct userfaultfd_ctx *new;

                                new = (struct userfaultfd_ctx *)
                                        (unsigned long)
                                        ewq->msg.arg.reserved.reserved1;
                                release_new_ctx = new;
                        }
                        break;
                }

                spin_unlock_irq(&ctx->event_wqh.lock);

                wake_up_poll(&ctx->fd_wqh, EPOLLIN);
                schedule();

                spin_lock_irq(&ctx->event_wqh.lock);
        }
        __set_current_state(TASK_RUNNING);
        spin_unlock_irq(&ctx->event_wqh.lock);

        if (release_new_ctx) {
                struct vm_area_struct *vma;
                struct mm_struct *mm = release_new_ctx->mm;

                /* the various vma->vm_userfaultfd_ctx still points to it */
                down_write(&mm->mmap_sem);
                /* no task can run (and in turn coredump) yet */
                VM_WARN_ON(!mmget_still_valid(mm));
                for (vma = mm->mmap; vma; vma = vma->vm_next)
                        if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
                                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
                                vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
                        }
                up_write(&mm->mmap_sem);

                userfaultfd_ctx_put(release_new_ctx);
        }

        /*
         * ctx may go away after this if the userfault pseudo fd is
         * already released.
         */
out:
        WRITE_ONCE(ctx->mmap_changing, false);
        userfaultfd_ctx_put(ctx);
}

static void userfaultfd_event_complete(struct userfaultfd_ctx *ctx,
                                       struct userfaultfd_wait_queue *ewq)
{
        ewq->msg.event = 0;
        wake_up_locked(&ctx->event_wqh);
        __remove_wait_queue(&ctx->event_wqh, &ewq->wq);
}

int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
{
        struct userfaultfd_ctx *ctx = NULL, *octx;
        struct userfaultfd_fork_ctx *fctx;

        octx = vma->vm_userfaultfd_ctx.ctx;
        if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
                vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
                return 0;
        }

        list_for_each_entry(fctx, fcs, list)
                if (fctx->orig == octx) {
                        ctx = fctx->new;
                        break;
                }

        if (!ctx) {
                fctx = kmalloc(sizeof(*fctx), GFP_KERNEL);
                if (!fctx)
                        return -ENOMEM;

                ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
                if (!ctx) {
                        kfree(fctx);
                        return -ENOMEM;
                }

                atomic_set(&ctx->refcount, 1);
                ctx->flags = octx->flags;
                ctx->state = UFFD_STATE_RUNNING;
                ctx->features = octx->features;
                ctx->released = false;
                ctx->mmap_changing = false;
                ctx->mm = vma->vm_mm;
                mmgrab(ctx->mm);

                userfaultfd_ctx_get(octx);
                WRITE_ONCE(octx->mmap_changing, true);
                fctx->orig = octx;
                fctx->new = ctx;
                list_add_tail(&fctx->list, fcs);
        }

        vma->vm_userfaultfd_ctx.ctx = ctx;
        return 0;
}

static void dup_fctx(struct userfaultfd_fork_ctx *fctx)
{
        struct userfaultfd_ctx *ctx = fctx->orig;
        struct userfaultfd_wait_queue ewq;

        msg_init(&ewq.msg);

        ewq.msg.event = UFFD_EVENT_FORK;
        ewq.msg.arg.reserved.reserved1 = (unsigned long)fctx->new;

        userfaultfd_event_wait_completion(ctx, &ewq);
}

void dup_userfaultfd_complete(struct list_head *fcs)
{
        struct userfaultfd_fork_ctx *fctx, *n;

        list_for_each_entry_safe(fctx, n, fcs, list) {
                dup_fctx(fctx);
                list_del(&fctx->list);
                kfree(fctx);
        }
}

void mremap_userfaultfd_prep(struct vm_area_struct *vma,
                             struct vm_userfaultfd_ctx *vm_ctx)
{
        struct userfaultfd_ctx *ctx;

        ctx = vma->vm_userfaultfd_ctx.ctx;

        if (!ctx)
                return;

        if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
                vm_ctx->ctx = ctx;
                userfaultfd_ctx_get(ctx);
                WRITE_ONCE(ctx->mmap_changing, true);
        } else {
                /* Drop uffd context if remap feature not enabled */
                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
                vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
        }
}

void mremap_userfaultfd_complete(struct vm_userfaultfd_ctx *vm_ctx,
                                 unsigned long from, unsigned long to,
                                 unsigned long len)
{
        struct userfaultfd_ctx *ctx = vm_ctx->ctx;
        struct userfaultfd_wait_queue ewq;

        if (!ctx)
                return;

        if (to & ~PAGE_MASK) {
                userfaultfd_ctx_put(ctx);
                return;
        }

        msg_init(&ewq.msg);

        ewq.msg.event = UFFD_EVENT_REMAP;
        ewq.msg.arg.remap.from = from;
        ewq.msg.arg.remap.to = to;
        ewq.msg.arg.remap.len = len;

        userfaultfd_event_wait_completion(ctx, &ewq);
}

bool userfaultfd_remove(struct vm_area_struct *vma,
                        unsigned long start, unsigned long end)
{
        struct mm_struct *mm = vma->vm_mm;
        struct userfaultfd_ctx *ctx;
        struct userfaultfd_wait_queue ewq;

        ctx = vma->vm_userfaultfd_ctx.ctx;
        if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_REMOVE))
                return true;

        userfaultfd_ctx_get(ctx);
        WRITE_ONCE(ctx->mmap_changing, true);
        up_read(&mm->mmap_sem);

        msg_init(&ewq.msg);

        ewq.msg.event = UFFD_EVENT_REMOVE;
        ewq.msg.arg.remove.start = start;
        ewq.msg.arg.remove.end = end;

        userfaultfd_event_wait_completion(ctx, &ewq);

        return false;
}

static bool has_unmap_ctx(struct userfaultfd_ctx *ctx, struct list_head *unmaps,
                          unsigned long start, unsigned long end)
{
        struct userfaultfd_unmap_ctx *unmap_ctx;

        list_for_each_entry(unmap_ctx, unmaps, list)
                if (unmap_ctx->ctx == ctx && unmap_ctx->start == start &&
                    unmap_ctx->end == end)
                        return true;

        return false;
}

int userfaultfd_unmap_prep(struct vm_area_struct *vma,
                           unsigned long start, unsigned long end,
                           struct list_head *unmaps)
{
        for ( ; vma && vma->vm_start < end; vma = vma->vm_next) {
                struct userfaultfd_unmap_ctx *unmap_ctx;
                struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;

                if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) ||
                    has_unmap_ctx(ctx, unmaps, start, end))
                        continue;

                unmap_ctx = kzalloc(sizeof(*unmap_ctx), GFP_KERNEL);
                if (!unmap_ctx)
                        return -ENOMEM;

                userfaultfd_ctx_get(ctx);
                WRITE_ONCE(ctx->mmap_changing, true);
                unmap_ctx->ctx = ctx;
                unmap_ctx->start = start;
                unmap_ctx->end = end;
                list_add_tail(&unmap_ctx->list, unmaps);
        }

        return 0;
}

void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf)
{
        struct userfaultfd_unmap_ctx *ctx, *n;
        struct userfaultfd_wait_queue ewq;

        list_for_each_entry_safe(ctx, n, uf, list) {
                msg_init(&ewq.msg);

                ewq.msg.event = UFFD_EVENT_UNMAP;
                ewq.msg.arg.remove.start = ctx->start;
                ewq.msg.arg.remove.end = ctx->end;

                userfaultfd_event_wait_completion(ctx->ctx, &ewq);

                list_del(&ctx->list);
                kfree(ctx);
        }
}

static int userfaultfd_release(struct inode *inode, struct file *file)
{
        struct userfaultfd_ctx *ctx = file->private_data;
        struct mm_struct *mm = ctx->mm;
        struct vm_area_struct *vma, *prev;
        /* len == 0 means wake all */
        struct userfaultfd_wake_range range = { .len = 0, };
        unsigned long new_flags;

        WRITE_ONCE(ctx->released, true);

        if (!mmget_not_zero(mm))
                goto wakeup;

        /*
         * Flush page faults out of all CPUs. NOTE: all page faults
         * must be retried without returning VM_FAULT_SIGBUS if
         * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
         * changes while handle_userfault released the mmap_sem. So
         * it's critical that released is set to true (above), before
         * taking the mmap_sem for writing.
         */
        down_write(&mm->mmap_sem);
        if (!mmget_still_valid(mm))
                goto skip_mm;
        prev = NULL;
        for (vma = mm->mmap; vma; vma = vma->vm_next) {
                cond_resched();
                BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
                       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
                if (vma->vm_userfaultfd_ctx.ctx != ctx) {
                        prev = vma;
                        continue;
                }
                new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
                prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
                                 new_flags, vma->anon_vma,
                                 vma->vm_file, vma->vm_pgoff,
                                 vma_policy(vma),
                                 NULL_VM_UFFD_CTX);
                if (prev)
                        vma = prev;
                else
                        prev = vma;
                vma->vm_flags = new_flags;
                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
        }
skip_mm:
        up_write(&mm->mmap_sem);
        mmput(mm);
wakeup:
        /*
         * After no new page faults can wait on this fault_*wqh, flush
         * the last page faults that may have been already waiting on
         * the fault_*wqh.
         */
        spin_lock_irq(&ctx->fault_pending_wqh.lock);
        __wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
        __wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
        spin_unlock_irq(&ctx->fault_pending_wqh.lock);

        /* Flush pending events that may still wait on event_wqh */
        wake_up_all(&ctx->event_wqh);

        wake_up_poll(&ctx->fd_wqh, EPOLLHUP);
        userfaultfd_ctx_put(ctx);
        return 0;
}

/* fault_pending_wqh.lock must be hold by the caller */
static inline struct userfaultfd_wait_queue *find_userfault_in(
                wait_queue_head_t *wqh)
{
        wait_queue_entry_t *wq;
        struct userfaultfd_wait_queue *uwq;

        VM_BUG_ON(!spin_is_locked(&wqh->lock));

        uwq = NULL;
        if (!waitqueue_active(wqh))
                goto out;
        /* walk in reverse to provide FIFO behavior to read userfaults */
        wq = list_last_entry(&wqh->head, typeof(*wq), entry);
        uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
out:
        return uwq;
}

static inline struct userfaultfd_wait_queue *find_userfault(
                struct userfaultfd_ctx *ctx)
{
        return find_userfault_in(&ctx->fault_pending_wqh);
}

static inline struct userfaultfd_wait_queue *find_userfault_evt(
                struct userfaultfd_ctx *ctx)
{
        return find_userfault_in(&ctx->event_wqh);
}

static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
{
        struct userfaultfd_ctx *ctx = file->private_data;
        __poll_t ret;

        poll_wait(file, &ctx->fd_wqh, wait);

        switch (ctx->state) {
        case UFFD_STATE_WAIT_API:
                return EPOLLERR;
        case UFFD_STATE_RUNNING:
                /*
                 * poll() never guarantees that read won't block.
                 * userfaults can be waken before they're read().
                 */
                if (unlikely(!(file->f_flags & O_NONBLOCK)))
                        return EPOLLERR;
                /*
                 * lockless access to see if there are pending faults
                 * __pollwait last action is the add_wait_queue but
                 * the spin_unlock would allow the waitqueue_active to
                 * pass above the actual list_add inside
                 * add_wait_queue critical section. So use a full
                 * memory barrier to serialize the list_add write of
                 * add_wait_queue() with the waitqueue_active read
                 * below.
                 */
                ret = 0;
                smp_mb();
                if (waitqueue_active(&ctx->fault_pending_wqh))
                        ret = EPOLLIN;
                else if (waitqueue_active(&ctx->event_wqh))
                        ret = EPOLLIN;

                return ret;
        default:
                WARN_ON_ONCE(1);
                return EPOLLERR;
        }
}

static const struct file_operations userfaultfd_fops;

static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
                                  struct userfaultfd_ctx *new,
                                  struct uffd_msg *msg)
{
        int fd;

        fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
                              O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
        if (fd < 0)
                return fd;

        msg->arg.reserved.reserved1 = 0;
        msg->arg.fork.ufd = fd;
        return 0;
}

static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
                                    struct uffd_msg *msg)
{
        ssize_t ret;
        DECLARE_WAITQUEUE(wait, current);
        struct userfaultfd_wait_queue *uwq;
        /*
         * Handling fork event requires sleeping operations, so
         * we drop the event_wqh lock, then do these ops, then
         * lock it back and wake up the waiter. While the lock is
         * dropped the ewq may go away so we keep track of it
         * carefully.
         */
        LIST_HEAD(fork_event);
        struct userfaultfd_ctx *fork_nctx = NULL;

        /* always take the fd_wqh lock before the fault_pending_wqh lock */
        spin_lock_irq(&ctx->fd_wqh.lock);
        __add_wait_queue(&ctx->fd_wqh, &wait);
        for (;;) {
                set_current_state(TASK_INTERRUPTIBLE);
                spin_lock(&ctx->fault_pending_wqh.lock);
                uwq = find_userfault(ctx);
                if (uwq) {
                        /*
                         * Use a seqcount to repeat the lockless check
                         * in wake_userfault() to avoid missing
                         * wakeups because during the refile both
                         * waitqueue could become empty if this is the
                         * only userfault.
                         */
                        write_seqcount_begin(&ctx->refile_seq);

                        /*
                         * The fault_pending_wqh.lock prevents the uwq
                         * to disappear from under us.
                         *
                         * Refile this userfault from
                         * fault_pending_wqh to fault_wqh, it's not
                         * pending anymore after we read it.
                         *
                         * Use list_del() by hand (as
                         * userfaultfd_wake_function also uses
                         * list_del_init() by hand) to be sure nobody
                         * changes __remove_wait_queue() to use
                         * list_del_init() in turn breaking the
                         * !list_empty_careful() check in
                         * handle_userfault(). The uwq->wq.head list
                         * must never be empty at any time during the
                         * refile, or the waitqueue could disappear
                         * from under us. The "wait_queue_head_t"
                         * parameter of __remove_wait_queue() is unused
                         * anyway.
                         */
                        list_del(&uwq->wq.entry);
                        add_wait_queue(&ctx->fault_wqh, &uwq->wq);

                        write_seqcount_end(&ctx->refile_seq);

                        /* careful to always initialize msg if ret == 0 */
                        *msg = uwq->msg;
                        spin_unlock(&ctx->fault_pending_wqh.lock);
                        ret = 0;
                        break;
                }
                spin_unlock(&ctx->fault_pending_wqh.lock);

                spin_lock(&ctx->event_wqh.lock);
                uwq = find_userfault_evt(ctx);
                if (uwq) {
                        *msg = uwq->msg;

                        if (uwq->msg.event == UFFD_EVENT_FORK) {
                                fork_nctx = (struct userfaultfd_ctx *)
                                        (unsigned long)
                                        uwq->msg.arg.reserved.reserved1;
                                list_move(&uwq->wq.entry, &fork_event);
                                /*
                                 * fork_nctx can be freed as soon as
                                 * we drop the lock, unless we take a
                                 * reference on it.
                                 */
                                userfaultfd_ctx_get(fork_nctx);
                                spin_unlock(&ctx->event_wqh.lock);
                                ret = 0;
                                break;
                        }

                        userfaultfd_event_complete(ctx, uwq);
                        spin_unlock(&ctx->event_wqh.lock);
                        ret = 0;
                        break;
                }
                spin_unlock(&ctx->event_wqh.lock);

                if (signal_pending(current)) {
                        ret = -ERESTARTSYS;
                        break;
                }
                if (no_wait) {
                        ret = -EAGAIN;
                        break;
                }
                spin_unlock_irq(&ctx->fd_wqh.lock);
                schedule();
                spin_lock_irq(&ctx->fd_wqh.lock);
        }
        __remove_wait_queue(&ctx->fd_wqh, &wait);
        __set_current_state(TASK_RUNNING);
        spin_unlock_irq(&ctx->fd_wqh.lock);

        if (!ret && msg->event == UFFD_EVENT_FORK) {
                ret = resolve_userfault_fork(ctx, fork_nctx, msg);
                spin_lock_irq(&ctx->event_wqh.lock);
                if (!list_empty(&fork_event)) {
                        /*
                         * The fork thread didn't abort, so we can
                         * drop the temporary refcount.
                         */
                        userfaultfd_ctx_put(fork_nctx);

                        uwq = list_first_entry(&fork_event,
                                               typeof(*uwq),
                                               wq.entry);
                        /*
                         * If fork_event list wasn't empty and in turn
                         * the event wasn't already released by fork
                         * (the event is allocated on fork kernel
                         * stack), put the event back to its place in
                         * the event_wq. fork_event head will be freed
                         * as soon as we return so the event cannot
                         * stay queued there no matter the current
                         * "ret" value.
                         */
                        list_del(&uwq->wq.entry);
                        __add_wait_queue(&ctx->event_wqh, &uwq->wq);

                        /*
                         * Leave the event in the waitqueue and report
                         * error to userland if we failed to resolve
                         * the userfault fork.
                         */
                        if (likely(!ret))
                                userfaultfd_event_complete(ctx, uwq);
                } else {
                        /*
                         * Here the fork thread aborted and the
                         * refcount from the fork thread on fork_nctx
                         * has already been released. We still hold
                         * the reference we took before releasing the
                         * lock above. If resolve_userfault_fork
                         * failed we've to drop it because the
                         * fork_nctx has to be freed in such case. If
                         * it succeeded we'll hold it because the new
                         * uffd references it.
                         */
                        if (ret)
                                userfaultfd_ctx_put(fork_nctx);
                }
                spin_unlock_irq(&ctx->event_wqh.lock);
        }

        return ret;
}

static ssize_t userfaultfd_read(struct file *file, char __user *buf,
                                size_t count, loff_t *ppos)
{
        struct userfaultfd_ctx *ctx = file->private_data;
        ssize_t _ret, ret = 0;
        struct uffd_msg msg;
        int no_wait = file->f_flags & O_NONBLOCK;

        if (ctx->state == UFFD_STATE_WAIT_API)
                return -EINVAL;

        for (;;) {
                if (count < sizeof(msg))
                        return ret ? ret : -EINVAL;
                _ret = userfaultfd_ctx_read(ctx, no_wait, &msg);
                if (_ret < 0)
                        return ret ? ret : _ret;
                if (copy_to_user((__u64 __user *) buf, &msg, sizeof(msg)))
                        return ret ? ret : -EFAULT;
                ret += sizeof(msg);
                buf += sizeof(msg);
                count -= sizeof(msg);
                /*
                 * Allow to read more than one fault at time but only
                 * block if waiting for the very first one.
                 */
                no_wait = O_NONBLOCK;
        }
}

static void __wake_userfault(struct userfaultfd_ctx *ctx,
                             struct userfaultfd_wake_range *range)
{
        spin_lock_irq(&ctx->fault_pending_wqh.lock);
        /* wake all in the range and autoremove */
        if (waitqueue_active(&ctx->fault_pending_wqh))
                __wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
                                     range);
        if (waitqueue_active(&ctx->fault_wqh))
                __wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, range);
        spin_unlock_irq(&ctx->fault_pending_wqh.lock);
}

static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
                                           struct userfaultfd_wake_range *range)
{
        unsigned seq;
        bool need_wakeup;

        /*
         * To be sure waitqueue_active() is not reordered by the CPU
         * before the pagetable update, use an explicit SMP memory
         * barrier here. PT lock release or up_read(mmap_sem) still
         * have release semantics that can allow the
         * waitqueue_active() to be reordered before the pte update.
         */
        smp_mb();

        /*
         * Use waitqueue_active because it's very frequent to
         * change the address space atomically even if there are no
         * userfaults yet. So we take the spinlock only when we're
         * sure we've userfaults to wake.
         */
        do {
                seq = read_seqcount_begin(&ctx->refile_seq);
                need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) ||
                        waitqueue_active(&ctx->fault_wqh);
                cond_resched();
        } while (read_seqcount_retry(&ctx->refile_seq, seq));
        if (need_wakeup)
                __wake_userfault(ctx, range);
}

static __always_inline int validate_range(struct mm_struct *mm,
                                          __u64 start, __u64 len)
{
        __u64 task_size = mm->task_size;

        if (start & ~PAGE_MASK)
                return -EINVAL;
        if (len & ~PAGE_MASK)
                return -EINVAL;
        if (!len)
                return -EINVAL;
        if (start < mmap_min_addr)
                return -EINVAL;
        if (start >= task_size)
                return -EINVAL;
        if (len > task_size - start)
                return -EINVAL;
        return 0;
}

static inline bool vma_can_userfault(struct vm_area_struct *vma)
{
        return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
                vma_is_shmem(vma);
}

static int userfaultfd_register(struct userfaultfd_ctx *ctx,
                                unsigned long arg)
{
        struct mm_struct *mm = ctx->mm;
        struct vm_area_struct *vma, *prev, *cur;
        int ret;
        struct uffdio_register uffdio_register;
        struct uffdio_register __user *user_uffdio_register;
        unsigned long vm_flags, new_flags;
        bool found;
        bool basic_ioctls;
        unsigned long start, end, vma_end;

        user_uffdio_register = (struct uffdio_register __user *) arg;

        ret = -EFAULT;
        if (copy_from_user(&uffdio_register, user_uffdio_register,
                           sizeof(uffdio_register)-sizeof(__u64)))
                goto out;

        ret = -EINVAL;
        if (!uffdio_register.mode)
                goto out;
        if (uffdio_register.mode & ~(UFFDIO_REGISTER_MODE_MISSING|
                                     UFFDIO_REGISTER_MODE_WP))
                goto out;
        vm_flags = 0;
        if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
                vm_flags |= VM_UFFD_MISSING;
        if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
                vm_flags |= VM_UFFD_WP;
                /*
                 * FIXME: remove the below error constraint by
                 * implementing the wprotect tracking mode.
                 */
                ret = -EINVAL;
                goto out;
        }

        ret = validate_range(mm, uffdio_register.range.start,
                             uffdio_register.range.len);
        if (ret)
                goto out;

        start = uffdio_register.range.start;
        end = start + uffdio_register.range.len;

        ret = -ENOMEM;
        if (!mmget_not_zero(mm))
                goto out;

        down_write(&mm->mmap_sem);
        if (!mmget_still_valid(mm))
                goto out_unlock;
        vma = find_vma_prev(mm, start, &prev);
        if (!vma)
                goto out_unlock;

        /* check that there's at least one vma in the range */
        ret = -EINVAL;
        if (vma->vm_start >= end)
                goto out_unlock;

        /*
         * If the first vma contains huge pages, make sure start address
         * is aligned to huge page size.
         */
        if (is_vm_hugetlb_page(vma)) {
                unsigned long vma_hpagesize = vma_kernel_pagesize(vma);

                if (start & (vma_hpagesize - 1))
                        goto out_unlock;
        }

        /*
         * Search for not compatible vmas.
         */
        found = false;
        basic_ioctls = false;
        for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
                cond_resched();

                BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
                       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));

                /* check not compatible vmas */
                ret = -EINVAL;
                if (!vma_can_userfault(cur))
                        goto out_unlock;

                /*
                 * UFFDIO_COPY will fill file holes even without
                 * PROT_WRITE. This check enforces that if this is a
                 * MAP_SHARED, the process has write permission to the backing
                 * file. If VM_MAYWRITE is set it also enforces that on a
                 * MAP_SHARED vma: there is no F_WRITE_SEAL and no further
                 * F_WRITE_SEAL can be taken until the vma is destroyed.
                 */
                ret = -EPERM;
                if (unlikely(!(cur->vm_flags & VM_MAYWRITE)))
                        goto out_unlock;

                /*
                 * If this vma contains ending address, and huge pages
                 * check alignment.
                 */
                if (is_vm_hugetlb_page(cur) && end <= cur->vm_end &&
                    end > cur->vm_start) {
                        unsigned long vma_hpagesize = vma_kernel_pagesize(cur);

                        ret = -EINVAL;

                        if (end & (vma_hpagesize - 1))
                                goto out_unlock;
                }

                /*
                 * Check that this vma isn't already owned by a
                 * different userfaultfd. We can't allow more than one
                 * userfaultfd to own a single vma simultaneously or we
                 * wouldn't know which one to deliver the userfaults to.
                 */
                ret = -EBUSY;
                if (cur->vm_userfaultfd_ctx.ctx &&
                    cur->vm_userfaultfd_ctx.ctx != ctx)
                        goto out_unlock;

                /*
                 * Note vmas containing huge pages
                 */
                if (is_vm_hugetlb_page(cur))
                        basic_ioctls = true;

                found = true;
        }
        BUG_ON(!found);

        if (vma->vm_start < start)
                prev = vma;

        ret = 0;
        do {
                cond_resched();

                BUG_ON(!vma_can_userfault(vma));
                BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
                       vma->vm_userfaultfd_ctx.ctx != ctx);
                WARN_ON(!(vma->vm_flags & VM_MAYWRITE));

                /*
                 * Nothing to do: this vma is already registered into this
                 * userfaultfd and with the right tracking mode too.
                 */
                if (vma->vm_userfaultfd_ctx.ctx == ctx &&
                    (vma->vm_flags & vm_flags) == vm_flags)
                        goto skip;

                if (vma->vm_start > start)
                        start = vma->vm_start;
                vma_end = min(end, vma->vm_end);

                new_flags = (vma->vm_flags & ~vm_flags) | vm_flags;
                prev = vma_merge(mm, prev, start, vma_end, new_flags,
                                 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
                                 vma_policy(vma),
                                 ((struct vm_userfaultfd_ctx){ ctx }));
                if (prev) {
                        vma = prev;
                        goto next;
                }
                if (vma->vm_start < start) {
                        ret = split_vma(mm, vma, start, 1);
                        if (ret)
                                break;
                }
                if (vma->vm_end > end) {
                        ret = split_vma(mm, vma, end, 0);
                        if (ret)
                                break;
                }
        next:
                /*
                 * In the vma_merge() successful mprotect-like case 8:
                 * the next vma was merged into the current one and
                 * the current one has not been updated yet.
                 */
                vma->vm_flags = new_flags;
                vma->vm_userfaultfd_ctx.ctx = ctx;

        skip:
                prev = vma;
                start = vma->vm_end;
                vma = vma->vm_next;
        } while (vma && vma->vm_start < end);
out_unlock:
        up_write(&mm->mmap_sem);
        mmput(mm);
        if (!ret) {
                /*
                 * Now that we scanned all vmas we can already tell
                 * userland which ioctls methods are guaranteed to
                 * succeed on this range.
                 */
                if (put_user(basic_ioctls ? UFFD_API_RANGE_IOCTLS_BASIC :
                             UFFD_API_RANGE_IOCTLS,
                             &user_uffdio_register->ioctls))
                        ret = -EFAULT;
        }
out:
        return ret;
}

static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
                                  unsigned long arg)
{
        struct mm_struct *mm = ctx->mm;
        struct vm_area_struct *vma, *prev, *cur;
        int ret;
        struct uffdio_range uffdio_unregister;
        unsigned long new_flags;
        bool found;
        unsigned long start, end, vma_end;
        const void __user *buf = (void __user *)arg;

        ret = -EFAULT;
        if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
                goto out;

        ret = validate_range(mm, uffdio_unregister.start,
                             uffdio_unregister.len);
        if (ret)
                goto out;

        start = uffdio_unregister.start;
        end = start + uffdio_unregister.len;

        ret = -ENOMEM;
        if (!mmget_not_zero(mm))
                goto out;

        down_write(&mm->mmap_sem);
        if (!mmget_still_valid(mm))
                goto out_unlock;
        vma = find_vma_prev(mm, start, &prev);
        if (!vma)
                goto out_unlock;

        /* check that there's at least one vma in the range */
        ret = -EINVAL;
        if (vma->vm_start >= end)
                goto out_unlock;

        /*
         * If the first vma contains huge pages, make sure start address
         * is aligned to huge page size.
         */
        if (is_vm_hugetlb_page(vma)) {
                unsigned long vma_hpagesize = vma_kernel_pagesize(vma);

                if (start & (vma_hpagesize - 1))
                        goto out_unlock;
        }

        /*
         * Search for not compatible vmas.
         */
        found = false;
        ret = -EINVAL;
        for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
                cond_resched();

                BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
                       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));

                /*
                 * Check not compatible vmas, not strictly required
                 * here as not compatible vmas cannot have an
                 * userfaultfd_ctx registered on them, but this
                 * provides for more strict behavior to notice
                 * unregistration errors.
                 */
                if (!vma_can_userfault(cur))
                        goto out_unlock;

                found = true;
        }
        BUG_ON(!found);

        if (vma->vm_start < start)
                prev = vma;

        ret = 0;
        do {
                cond_resched();

                BUG_ON(!vma_can_userfault(vma));

                /*
                 * Nothing to do: this vma is already registered into this
                 * userfaultfd and with the right tracking mode too.
                 */
                if (!vma->vm_userfaultfd_ctx.ctx)
                        goto skip;

                WARN_ON(!(vma->vm_flags & VM_MAYWRITE));

                if (vma->vm_start > start)
                        start = vma->vm_start;
                vma_end = min(end, vma->vm_end);

                if (userfaultfd_missing(vma)) {
                        /*
                         * Wake any concurrent pending userfault while
                         * we unregister, so they will not hang
                         * permanently and it avoids userland to call
                         * UFFDIO_WAKE explicitly.
                         */
                        struct userfaultfd_wake_range range;
                        range.start = start;
                        range.len = vma_end - start;
                        wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
                }

                new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
                prev = vma_merge(mm, prev, start, vma_end, new_flags,
                                 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
                                 vma_policy(vma),
                                 NULL_VM_UFFD_CTX);
                if (prev) {
                        vma = prev;
                        goto next;
                }
                if (vma->vm_start < start) {
                        ret = split_vma(mm, vma, start, 1);
                        if (ret)
                                break;
                }
                if (vma->vm_end > end) {
                        ret = split_vma(mm, vma, end, 0);
                        if (ret)
                                break;
                }
        next:
                /*
                 * In the vma_merge() successful mprotect-like case 8:
                 * the next vma was merged into the current one and
                 * the current one has not been updated yet.
                 */
                vma->vm_flags = new_flags;
                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;

        skip:
                prev = vma;
                start = vma->vm_end;
                vma = vma->vm_next;
        } while (vma && vma->vm_start < end);
out_unlock:
        up_write(&mm->mmap_sem);
        mmput(mm);
out:
        return ret;
}

/*
 * userfaultfd_wake may be used in combination with the
 * UFFDIO_*_MODE_DONTWAKE to wakeup userfaults in batches.
 */
static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
                            unsigned long arg)
{
        int ret;
        struct uffdio_range uffdio_wake;
        struct userfaultfd_wake_range range;
        const void __user *buf = (void __user *)arg;

        ret = -EFAULT;
        if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
                goto out;

        ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
        if (ret)
                goto out;

        range.start = uffdio_wake.start;
        range.len = uffdio_wake.len;

        /*
         * len == 0 means wake all and we don't want to wake all here,
         * so check it again to be sure.
         */
        VM_BUG_ON(!range.len);

        wake_userfault(ctx, &range);
        ret = 0;

out:
        return ret;
}

static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
                            unsigned long arg)
{
        __s64 ret;
        struct uffdio_copy uffdio_copy;
        struct uffdio_copy __user *user_uffdio_copy;
        struct userfaultfd_wake_range range;

        user_uffdio_copy = (struct uffdio_copy __user *) arg;

        ret = -EAGAIN;
        if (READ_ONCE(ctx->mmap_changing))
                goto out;

        ret = -EFAULT;
        if (copy_from_user(&uffdio_copy, user_uffdio_copy,
                           /* don't copy "copy" last field */
                           sizeof(uffdio_copy)-sizeof(__s64)))
                goto out;

        ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
        if (ret)
                goto out;
        /*
         * double check for wraparound just in case. copy_from_user()
         * will later check uffdio_copy.src + uffdio_copy.len to fit
         * in the userland range.
         */
        ret = -EINVAL;
        if (uffdio_copy.src + uffdio_copy.len <= uffdio_copy.src)
                goto out;
        if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)
                goto out;
        if (mmget_not_zero(ctx->mm)) {
                ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
                                   uffdio_copy.len, &ctx->mmap_changing);
                mmput(ctx->mm);
        } else {
                return -ESRCH;
        }
        if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
                return -EFAULT;
        if (ret < 0)
                goto out;
        BUG_ON(!ret);
        /* len == 0 would wake all */
        range.len = ret;
        if (!(uffdio_copy.mode & UFFDIO_COPY_MODE_DONTWAKE)) {
                range.start = uffdio_copy.dst;
                wake_userfault(ctx, &range);
        }
        ret = range.len == uffdio_copy.len ? 0 : -EAGAIN;
out:
        return ret;
}

static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
                                unsigned long arg)
{
        __s64 ret;
        struct uffdio_zeropage uffdio_zeropage;
        struct uffdio_zeropage __user *user_uffdio_zeropage;
        struct userfaultfd_wake_range range;

        user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;

        ret = -EAGAIN;
        if (READ_ONCE(ctx->mmap_changing))
                goto out;

        ret = -EFAULT;
        if (copy_from_user(&uffdio_zeropage, user_uffdio_zeropage,
                           /* don't copy "zeropage" last field */
                           sizeof(uffdio_zeropage)-sizeof(__s64)))
                goto out;

        ret = validate_range(ctx->mm, uffdio_zeropage.range.start,
                             uffdio_zeropage.range.len);
        if (ret)
                goto out;
        ret = -EINVAL;
        if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
                goto out;

        if (mmget_not_zero(ctx->mm)) {
                ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
                                     uffdio_zeropage.range.len,
                                     &ctx->mmap_changing);
                mmput(ctx->mm);
        } else {
                return -ESRCH;
        }
        if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
                return -EFAULT;
        if (ret < 0)
                goto out;
        /* len == 0 would wake all */
        BUG_ON(!ret);
        range.len = ret;
        if (!(uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE)) {
                range.start = uffdio_zeropage.range.start;
                wake_userfault(ctx, &range);
        }
        ret = range.len == uffdio_zeropage.range.len ? 0 : -EAGAIN;
out:
        return ret;
}

static inline unsigned int uffd_ctx_features(__u64 user_features)
{
        /*
         * For the current set of features the bits just coincide
         */
        return (unsigned int)user_features;
}

/*
 * userland asks for a certain API version and we return which bits
 * and ioctl commands are implemented in this kernel for such API
 * version or -EINVAL if unknown.
 */
static int userfaultfd_api(struct userfaultfd_ctx *ctx,
                           unsigned long arg)
{
        struct uffdio_api uffdio_api;
        void __user *buf = (void __user *)arg;
        int ret;
        __u64 features;

        ret = -EINVAL;
        if (ctx->state != UFFD_STATE_WAIT_API)
                goto out;
        ret = -EFAULT;
        if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
                goto out;
        features = uffdio_api.features;
        if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) {
                memset(&uffdio_api, 0, sizeof(uffdio_api));
                if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
                        goto out;
                ret = -EINVAL;
                goto out;
        }
        /* report all available features and ioctls to userland */
        uffdio_api.features = UFFD_API_FEATURES;
        uffdio_api.ioctls = UFFD_API_IOCTLS;
        ret = -EFAULT;
        if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
                goto out;
        ctx->state = UFFD_STATE_RUNNING;
        /* only enable the requested features for this uffd context */
        ctx->features = uffd_ctx_features(features);
        ret = 0;
out:
        return ret;
}

static long userfaultfd_ioctl(struct file *file, unsigned cmd,
                              unsigned long arg)
{
        int ret = -EINVAL;
        struct userfaultfd_ctx *ctx = file->private_data;

        if (cmd != UFFDIO_API && ctx->state == UFFD_STATE_WAIT_API)
                return -EINVAL;

        switch(cmd) {
        case UFFDIO_API:
                ret = userfaultfd_api(ctx, arg);
                break;
        case UFFDIO_REGISTER:
                ret = userfaultfd_register(ctx, arg);
                break;
        case UFFDIO_UNREGISTER:
                ret = userfaultfd_unregister(ctx, arg);
                break;
        case UFFDIO_WAKE:
                ret = userfaultfd_wake(ctx, arg);
                break;
        case UFFDIO_COPY:
                ret = userfaultfd_copy(ctx, arg);
                break;
        case UFFDIO_ZEROPAGE:
                ret = userfaultfd_zeropage(ctx, arg);
                break;
        }
        return ret;
}

#ifdef CONFIG_PROC_FS
static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
{
        struct userfaultfd_ctx *ctx = f->private_data;
        wait_queue_entry_t *wq;
        unsigned long pending = 0, total = 0;

        spin_lock_irq(&ctx->fault_pending_wqh.lock);
        list_for_each_entry(wq, &ctx->fault_pending_wqh.head, entry) {
                pending++;
                total++;
        }
        list_for_each_entry(wq, &ctx->fault_wqh.head, entry) {
                total++;
        }
        spin_unlock_irq(&ctx->fault_pending_wqh.lock);

        /*
         * If more protocols will be added, there will be all shown
         * separated by a space. Like this:
         *      protocols: aa:... bb:...
         */
        seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
                   pending, total, UFFD_API, ctx->features,
                   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
}
#endif

static const struct file_operations userfaultfd_fops = {
#ifdef CONFIG_PROC_FS
        .show_fdinfo    = userfaultfd_show_fdinfo,
#endif
        .release        = userfaultfd_release,
        .poll           = userfaultfd_poll,
        .read           = userfaultfd_read,
        .unlocked_ioctl = userfaultfd_ioctl,
        .compat_ioctl   = userfaultfd_ioctl,
        .llseek         = noop_llseek,
};

static void init_once_userfaultfd_ctx(void *mem)
{
        struct userfaultfd_ctx *ctx = (struct userfaultfd_ctx *) mem;

        init_waitqueue_head(&ctx->fault_pending_wqh);
        init_waitqueue_head(&ctx->fault_wqh);
        init_waitqueue_head(&ctx->event_wqh);
        init_waitqueue_head(&ctx->fd_wqh);
        seqcount_init(&ctx->refile_seq);
}

SYSCALL_DEFINE1(userfaultfd, int, flags)
{
        struct userfaultfd_ctx *ctx;
        int fd;

        BUG_ON(!current->mm);

        /* Check the UFFD_* constants for consistency.  */
        BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
        BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);

        if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
                return -EINVAL;

        ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
        if (!ctx)
                return -ENOMEM;

        atomic_set(&ctx->refcount, 1);
        ctx->flags = flags;
        ctx->features = 0;
        ctx->state = UFFD_STATE_WAIT_API;
        ctx->released = false;
        ctx->mmap_changing = false;
        ctx->mm = current->mm;
        /* prevent the mm struct to be freed */
        mmgrab(ctx->mm);

        fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
                              O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
        if (fd < 0) {
                mmdrop(ctx->mm);
                kmem_cache_free(userfaultfd_ctx_cachep, ctx);
        }
        return fd;
}

static int __init userfaultfd_init(void)
{
        userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache",
                                                sizeof(struct userfaultfd_ctx),
                                                0,
                                                SLAB_HWCACHE_ALIGN|SLAB_PANIC,
                                                init_once_userfaultfd_ctx);
        return 0;
}
__initcall(userfaultfd_init);