Merge tag 'trace-printf-v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/trace...

[drm/drm-misc.git] / mm / pagewalk.c

// SPDX-License-Identifier: GPL-2.0
#include <linux/pagewalk.h>
#include <linux/highmem.h>
#include <linux/sched.h>
#include <linux/hugetlb.h>
#include <linux/mmu_context.h>
#include <linux/swap.h>
#include <linux/swapops.h>

#include <asm/tlbflush.h>

#include "internal.h"

/*
 * We want to know the real level where a entry is located ignoring any
 * folding of levels which may be happening. For example if p4d is folded then
 * a missing entry found at level 1 (p4d) is actually at level 0 (pgd).
 */
static int real_depth(int depth)
{
        if (depth == 3 && PTRS_PER_PMD == 1)
                depth = 2;
        if (depth == 2 && PTRS_PER_PUD == 1)
                depth = 1;
        if (depth == 1 && PTRS_PER_P4D == 1)
                depth = 0;
        return depth;
}

static int walk_pte_range_inner(pte_t *pte, unsigned long addr,
                                unsigned long end, struct mm_walk *walk)
{
        const struct mm_walk_ops *ops = walk->ops;
        int err = 0;

        for (;;) {
                if (ops->install_pte && pte_none(ptep_get(pte))) {
                        pte_t new_pte;

                        err = ops->install_pte(addr, addr + PAGE_SIZE, &new_pte,
                                               walk);
                        if (err)
                                break;

                        set_pte_at(walk->mm, addr, pte, new_pte);
                        /* Non-present before, so for arches that need it. */
                        if (!WARN_ON_ONCE(walk->no_vma))
                                update_mmu_cache(walk->vma, addr, pte);
                } else {
                        err = ops->pte_entry(pte, addr, addr + PAGE_SIZE, walk);
                        if (err)
                                break;
                }
                if (addr >= end - PAGE_SIZE)
                        break;
                addr += PAGE_SIZE;
                pte++;
        }
        return err;
}

static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
                          struct mm_walk *walk)
{
        pte_t *pte;
        int err = 0;
        spinlock_t *ptl;

        if (walk->no_vma) {
                /*
                 * pte_offset_map() might apply user-specific validation.
                 * Indeed, on x86_64 the pmd entries set up by init_espfix_ap()
                 * fit its pmd_bad() check (_PAGE_NX set and _PAGE_RW clear),
                 * and CONFIG_EFI_PGT_DUMP efi_mm goes so far as to walk them.
                 */
                if (walk->mm == &init_mm || addr >= TASK_SIZE)
                        pte = pte_offset_kernel(pmd, addr);
                else
                        pte = pte_offset_map(pmd, addr);
                if (pte) {
                        err = walk_pte_range_inner(pte, addr, end, walk);
                        if (walk->mm != &init_mm && addr < TASK_SIZE)
                                pte_unmap(pte);
                }
        } else {
                pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
                if (pte) {
                        err = walk_pte_range_inner(pte, addr, end, walk);
                        pte_unmap_unlock(pte, ptl);
                }
        }
        if (!pte)
                walk->action = ACTION_AGAIN;
        return err;
}

static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
                          struct mm_walk *walk)
{
        pmd_t *pmd;
        unsigned long next;
        const struct mm_walk_ops *ops = walk->ops;
        bool has_handler = ops->pte_entry;
        bool has_install = ops->install_pte;
        int err = 0;
        int depth = real_depth(3);

        pmd = pmd_offset(pud, addr);
        do {
again:
                next = pmd_addr_end(addr, end);
                if (pmd_none(*pmd)) {
                        if (has_install)
                                err = __pte_alloc(walk->mm, pmd);
                        else if (ops->pte_hole)
                                err = ops->pte_hole(addr, next, depth, walk);
                        if (err)
                                break;
                        if (!has_install)
                                continue;
                }

                walk->action = ACTION_SUBTREE;

                /*
                 * This implies that each ->pmd_entry() handler
                 * needs to know about pmd_trans_huge() pmds
                 */
                if (ops->pmd_entry)
                        err = ops->pmd_entry(pmd, addr, next, walk);
                if (err)
                        break;

                if (walk->action == ACTION_AGAIN)
                        goto again;
                if (walk->action == ACTION_CONTINUE)
                        continue;

                if (!has_handler) { /* No handlers for lower page tables. */
                        if (!has_install)
                                continue; /* Nothing to do. */
                        /*
                         * We are ONLY installing, so avoid unnecessarily
                         * splitting a present huge page.
                         */
                        if (pmd_present(*pmd) &&
                            (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)))
                                continue;
                }

                if (walk->vma)
                        split_huge_pmd(walk->vma, pmd, addr);
                else if (pmd_leaf(*pmd) || !pmd_present(*pmd))
                        continue; /* Nothing to do. */

                err = walk_pte_range(pmd, addr, next, walk);
                if (err)
                        break;

                if (walk->action == ACTION_AGAIN)
                        goto again;

        } while (pmd++, addr = next, addr != end);

        return err;
}

static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
                          struct mm_walk *walk)
{
        pud_t *pud;
        unsigned long next;
        const struct mm_walk_ops *ops = walk->ops;
        bool has_handler = ops->pmd_entry || ops->pte_entry;
        bool has_install = ops->install_pte;
        int err = 0;
        int depth = real_depth(2);

        pud = pud_offset(p4d, addr);
        do {
 again:
                next = pud_addr_end(addr, end);
                if (pud_none(*pud)) {
                        if (has_install)
                                err = __pmd_alloc(walk->mm, pud, addr);
                        else if (ops->pte_hole)
                                err = ops->pte_hole(addr, next, depth, walk);
                        if (err)
                                break;
                        if (!has_install)
                                continue;
                }

                walk->action = ACTION_SUBTREE;

                if (ops->pud_entry)
                        err = ops->pud_entry(pud, addr, next, walk);
                if (err)
                        break;

                if (walk->action == ACTION_AGAIN)
                        goto again;
                if (walk->action == ACTION_CONTINUE)
                        continue;

                if (!has_handler) { /* No handlers for lower page tables. */
                        if (!has_install)
                                continue; /* Nothing to do. */
                        /*
                         * We are ONLY installing, so avoid unnecessarily
                         * splitting a present huge page.
                         */
                        if (pud_present(*pud) &&
                            (pud_trans_huge(*pud) || pud_devmap(*pud)))
                                continue;
                }

                if (walk->vma)
                        split_huge_pud(walk->vma, pud, addr);
                else if (pud_leaf(*pud) || !pud_present(*pud))
                        continue; /* Nothing to do. */

                if (pud_none(*pud))
                        goto again;

                err = walk_pmd_range(pud, addr, next, walk);
                if (err)
                        break;
        } while (pud++, addr = next, addr != end);

        return err;
}

static int walk_p4d_range(pgd_t *pgd, unsigned long addr, unsigned long end,
                          struct mm_walk *walk)
{
        p4d_t *p4d;
        unsigned long next;
        const struct mm_walk_ops *ops = walk->ops;
        bool has_handler = ops->pud_entry || ops->pmd_entry || ops->pte_entry;
        bool has_install = ops->install_pte;
        int err = 0;
        int depth = real_depth(1);

        p4d = p4d_offset(pgd, addr);
        do {
                next = p4d_addr_end(addr, end);
                if (p4d_none_or_clear_bad(p4d)) {
                        if (has_install)
                                err = __pud_alloc(walk->mm, p4d, addr);
                        else if (ops->pte_hole)
                                err = ops->pte_hole(addr, next, depth, walk);
                        if (err)
                                break;
                        if (!has_install)
                                continue;
                }
                if (ops->p4d_entry) {
                        err = ops->p4d_entry(p4d, addr, next, walk);
                        if (err)
                                break;
                }
                if (has_handler || has_install)
                        err = walk_pud_range(p4d, addr, next, walk);
                if (err)
                        break;
        } while (p4d++, addr = next, addr != end);

        return err;
}

static int walk_pgd_range(unsigned long addr, unsigned long end,
                          struct mm_walk *walk)
{
        pgd_t *pgd;
        unsigned long next;
        const struct mm_walk_ops *ops = walk->ops;
        bool has_handler = ops->p4d_entry || ops->pud_entry || ops->pmd_entry ||
                ops->pte_entry;
        bool has_install = ops->install_pte;
        int err = 0;

        if (walk->pgd)
                pgd = walk->pgd + pgd_index(addr);
        else
                pgd = pgd_offset(walk->mm, addr);
        do {
                next = pgd_addr_end(addr, end);
                if (pgd_none_or_clear_bad(pgd)) {
                        if (has_install)
                                err = __p4d_alloc(walk->mm, pgd, addr);
                        else if (ops->pte_hole)
                                err = ops->pte_hole(addr, next, 0, walk);
                        if (err)
                                break;
                        if (!has_install)
                                continue;
                }
                if (ops->pgd_entry) {
                        err = ops->pgd_entry(pgd, addr, next, walk);
                        if (err)
                                break;
                }
                if (has_handler || has_install)
                        err = walk_p4d_range(pgd, addr, next, walk);
                if (err)
                        break;
        } while (pgd++, addr = next, addr != end);

        return err;
}

#ifdef CONFIG_HUGETLB_PAGE
static unsigned long hugetlb_entry_end(struct hstate *h, unsigned long addr,
                                       unsigned long end)
{
        unsigned long boundary = (addr & huge_page_mask(h)) + huge_page_size(h);
        return boundary < end ? boundary : end;
}

static int walk_hugetlb_range(unsigned long addr, unsigned long end,
                              struct mm_walk *walk)
{
        struct vm_area_struct *vma = walk->vma;
        struct hstate *h = hstate_vma(vma);
        unsigned long next;
        unsigned long hmask = huge_page_mask(h);
        unsigned long sz = huge_page_size(h);
        pte_t *pte;
        const struct mm_walk_ops *ops = walk->ops;
        int err = 0;

        hugetlb_vma_lock_read(vma);
        do {
                next = hugetlb_entry_end(h, addr, end);
                pte = hugetlb_walk(vma, addr & hmask, sz);
                if (pte)
                        err = ops->hugetlb_entry(pte, hmask, addr, next, walk);
                else if (ops->pte_hole)
                        err = ops->pte_hole(addr, next, -1, walk);
                if (err)
                        break;
        } while (addr = next, addr != end);
        hugetlb_vma_unlock_read(vma);

        return err;
}

#else /* CONFIG_HUGETLB_PAGE */
static int walk_hugetlb_range(unsigned long addr, unsigned long end,
                              struct mm_walk *walk)
{
        return 0;
}

#endif /* CONFIG_HUGETLB_PAGE */

/*
 * Decide whether we really walk over the current vma on [@start, @end)
 * or skip it via the returned value. Return 0 if we do walk over the
 * current vma, and return 1 if we skip the vma. Negative values means
 * error, where we abort the current walk.
 */
static int walk_page_test(unsigned long start, unsigned long end,
                        struct mm_walk *walk)
{
        struct vm_area_struct *vma = walk->vma;
        const struct mm_walk_ops *ops = walk->ops;

        if (ops->test_walk)
                return ops->test_walk(start, end, walk);

        /*
         * vma(VM_PFNMAP) doesn't have any valid struct pages behind VM_PFNMAP
         * range, so we don't walk over it as we do for normal vmas. However,
         * Some callers are interested in handling hole range and they don't
         * want to just ignore any single address range. Such users certainly
         * define their ->pte_hole() callbacks, so let's delegate them to handle
         * vma(VM_PFNMAP).
         */
        if (vma->vm_flags & VM_PFNMAP) {
                int err = 1;
                if (ops->pte_hole)
                        err = ops->pte_hole(start, end, -1, walk);
                return err ? err : 1;
        }
        return 0;
}

static int __walk_page_range(unsigned long start, unsigned long end,
                        struct mm_walk *walk)
{
        int err = 0;
        struct vm_area_struct *vma = walk->vma;
        const struct mm_walk_ops *ops = walk->ops;
        bool is_hugetlb = is_vm_hugetlb_page(vma);

        /* We do not support hugetlb PTE installation. */
        if (ops->install_pte && is_hugetlb)
                return -EINVAL;

        if (ops->pre_vma) {
                err = ops->pre_vma(start, end, walk);
                if (err)
                        return err;
        }

        if (is_hugetlb) {
                if (ops->hugetlb_entry)
                        err = walk_hugetlb_range(start, end, walk);
        } else
                err = walk_pgd_range(start, end, walk);

        if (ops->post_vma)
                ops->post_vma(walk);

        return err;
}

static inline void process_mm_walk_lock(struct mm_struct *mm,
                                        enum page_walk_lock walk_lock)
{
        if (walk_lock == PGWALK_RDLOCK)
                mmap_assert_locked(mm);
        else
                mmap_assert_write_locked(mm);
}

static inline void process_vma_walk_lock(struct vm_area_struct *vma,
                                         enum page_walk_lock walk_lock)
{
#ifdef CONFIG_PER_VMA_LOCK
        switch (walk_lock) {
        case PGWALK_WRLOCK:
                vma_start_write(vma);
                break;
        case PGWALK_WRLOCK_VERIFY:
                vma_assert_write_locked(vma);
                break;
        case PGWALK_RDLOCK:
                /* PGWALK_RDLOCK is handled by process_mm_walk_lock */
                break;
        }
#endif
}

/*
 * See the comment for walk_page_range(), this performs the heavy lifting of the
 * operation, only sets no restrictions on how the walk proceeds.
 *
 * We usually restrict the ability to install PTEs, but this functionality is
 * available to internal memory management code and provided in mm/internal.h.
 */
int walk_page_range_mm(struct mm_struct *mm, unsigned long start,
                unsigned long end, const struct mm_walk_ops *ops,
                void *private)
{
        int err = 0;
        unsigned long next;
        struct vm_area_struct *vma;
        struct mm_walk walk = {
                .ops            = ops,
                .mm             = mm,
                .private        = private,
        };

        if (start >= end)
                return -EINVAL;

        if (!walk.mm)
                return -EINVAL;

        process_mm_walk_lock(walk.mm, ops->walk_lock);

        vma = find_vma(walk.mm, start);
        do {
                if (!vma) { /* after the last vma */
                        walk.vma = NULL;
                        next = end;
                        if (ops->pte_hole)
                                err = ops->pte_hole(start, next, -1, &walk);
                } else if (start < vma->vm_start) { /* outside vma */
                        walk.vma = NULL;
                        next = min(end, vma->vm_start);
                        if (ops->pte_hole)
                                err = ops->pte_hole(start, next, -1, &walk);
                } else { /* inside vma */
                        process_vma_walk_lock(vma, ops->walk_lock);
                        walk.vma = vma;
                        next = min(end, vma->vm_end);
                        vma = find_vma(mm, vma->vm_end);

                        err = walk_page_test(start, next, &walk);
                        if (err > 0) {
                                /*
                                 * positive return values are purely for
                                 * controlling the pagewalk, so should never
                                 * be passed to the callers.
                                 */
                                err = 0;
                                continue;
                        }
                        if (err < 0)
                                break;
                        err = __walk_page_range(start, next, &walk);
                }
                if (err)
                        break;
        } while (start = next, start < end);
        return err;
}

/*
 * Determine if the walk operations specified are permitted to be used for a
 * page table walk.
 *
 * This check is performed on all functions which are parameterised by walk
 * operations and exposed in include/linux/pagewalk.h.
 *
 * Internal memory management code can use the walk_page_range_mm() function to
 * be able to use all page walking operations.
 */
static bool check_ops_valid(const struct mm_walk_ops *ops)
{
        /*
         * The installation of PTEs is solely under the control of memory
         * management logic and subject to many subtle locking, security and
         * cache considerations so we cannot permit other users to do so, and
         * certainly not for exported symbols.
         */
        if (ops->install_pte)
                return false;

        return true;
}

/**
 * walk_page_range - walk page table with caller specific callbacks
 * @mm:         mm_struct representing the target process of page table walk
 * @start:      start address of the virtual address range
 * @end:        end address of the virtual address range
 * @ops:        operation to call during the walk
 * @private:    private data for callbacks' usage
 *
 * Recursively walk the page table tree of the process represented by @mm
 * within the virtual address range [@start, @end). During walking, we can do
 * some caller-specific works for each entry, by setting up pmd_entry(),
 * pte_entry(), and/or hugetlb_entry(). If you don't set up for some of these
 * callbacks, the associated entries/pages are just ignored.
 * The return values of these callbacks are commonly defined like below:
 *
 *  - 0  : succeeded to handle the current entry, and if you don't reach the
 *         end address yet, continue to walk.
 *  - >0 : succeeded to handle the current entry, and return to the caller
 *         with caller specific value.
 *  - <0 : failed to handle the current entry, and return to the caller
 *         with error code.
 *
 * Before starting to walk page table, some callers want to check whether
 * they really want to walk over the current vma, typically by checking
 * its vm_flags. walk_page_test() and @ops->test_walk() are used for this
 * purpose.
 *
 * If operations need to be staged before and committed after a vma is walked,
 * there are two callbacks, pre_vma() and post_vma(). Note that post_vma(),
 * since it is intended to handle commit-type operations, can't return any
 * errors.
 *
 * struct mm_walk keeps current values of some common data like vma and pmd,
 * which are useful for the access from callbacks. If you want to pass some
 * caller-specific data to callbacks, @private should be helpful.
 *
 * Locking:
 *   Callers of walk_page_range() and walk_page_vma() should hold @mm->mmap_lock,
 *   because these function traverse vma list and/or access to vma's data.
 */
int walk_page_range(struct mm_struct *mm, unsigned long start,
                unsigned long end, const struct mm_walk_ops *ops,
                void *private)
{
        if (!check_ops_valid(ops))
                return -EINVAL;

        return walk_page_range_mm(mm, start, end, ops, private);
}

/**
 * walk_page_range_novma - walk a range of pagetables not backed by a vma
 * @mm:         mm_struct representing the target process of page table walk
 * @start:      start address of the virtual address range
 * @end:        end address of the virtual address range
 * @ops:        operation to call during the walk
 * @pgd:        pgd to walk if different from mm->pgd
 * @private:    private data for callbacks' usage
 *
 * Similar to walk_page_range() but can walk any page tables even if they are
 * not backed by VMAs. Because 'unusual' entries may be walked this function
 * will also not lock the PTEs for the pte_entry() callback. This is useful for
 * walking the kernel pages tables or page tables for firmware.
 *
 * Note: Be careful to walk the kernel pages tables, the caller may be need to
 * take other effective approaches (mmap lock may be insufficient) to prevent
 * the intermediate kernel page tables belonging to the specified address range
 * from being freed (e.g. memory hot-remove).
 */
int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
                          unsigned long end, const struct mm_walk_ops *ops,
                          pgd_t *pgd,
                          void *private)
{
        struct mm_walk walk = {
                .ops            = ops,
                .mm             = mm,
                .pgd            = pgd,
                .private        = private,
                .no_vma         = true
        };

        if (start >= end || !walk.mm)
                return -EINVAL;
        if (!check_ops_valid(ops))
                return -EINVAL;

        /*
         * 1) For walking the user virtual address space:
         *
         * The mmap lock protects the page walker from changes to the page
         * tables during the walk.  However a read lock is insufficient to
         * protect those areas which don't have a VMA as munmap() detaches
         * the VMAs before downgrading to a read lock and actually tearing
         * down PTEs/page tables. In which case, the mmap write lock should
         * be hold.
         *
         * 2) For walking the kernel virtual address space:
         *
         * The kernel intermediate page tables usually do not be freed, so
         * the mmap map read lock is sufficient. But there are some exceptions.
         * E.g. memory hot-remove. In which case, the mmap lock is insufficient
         * to prevent the intermediate kernel pages tables belonging to the
         * specified address range from being freed. The caller should take
         * other actions to prevent this race.
         */
        if (mm == &init_mm)
                mmap_assert_locked(walk.mm);
        else
                mmap_assert_write_locked(walk.mm);

        return walk_pgd_range(start, end, &walk);
}

int walk_page_range_vma(struct vm_area_struct *vma, unsigned long start,
                        unsigned long end, const struct mm_walk_ops *ops,
                        void *private)
{
        struct mm_walk walk = {
                .ops            = ops,
                .mm             = vma->vm_mm,
                .vma            = vma,
                .private        = private,
        };

        if (start >= end || !walk.mm)
                return -EINVAL;
        if (start < vma->vm_start || end > vma->vm_end)
                return -EINVAL;
        if (!check_ops_valid(ops))
                return -EINVAL;

        process_mm_walk_lock(walk.mm, ops->walk_lock);
        process_vma_walk_lock(vma, ops->walk_lock);
        return __walk_page_range(start, end, &walk);
}

int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops,
                void *private)
{
        struct mm_walk walk = {
                .ops            = ops,
                .mm             = vma->vm_mm,
                .vma            = vma,
                .private        = private,
        };

        if (!walk.mm)
                return -EINVAL;
        if (!check_ops_valid(ops))
                return -EINVAL;

        process_mm_walk_lock(walk.mm, ops->walk_lock);
        process_vma_walk_lock(vma, ops->walk_lock);
        return __walk_page_range(vma->vm_start, vma->vm_end, &walk);
}

/**
 * walk_page_mapping - walk all memory areas mapped into a struct address_space.
 * @mapping: Pointer to the struct address_space
 * @first_index: First page offset in the address_space
 * @nr: Number of incremental page offsets to cover
 * @ops:        operation to call during the walk
 * @private:    private data for callbacks' usage
 *
 * This function walks all memory areas mapped into a struct address_space.
 * The walk is limited to only the given page-size index range, but if
 * the index boundaries cross a huge page-table entry, that entry will be
 * included.
 *
 * Also see walk_page_range() for additional information.
 *
 * Locking:
 *   This function can't require that the struct mm_struct::mmap_lock is held,
 *   since @mapping may be mapped by multiple processes. Instead
 *   @mapping->i_mmap_rwsem must be held. This might have implications in the
 *   callbacks, and it's up tho the caller to ensure that the
 *   struct mm_struct::mmap_lock is not needed.
 *
 *   Also this means that a caller can't rely on the struct
 *   vm_area_struct::vm_flags to be constant across a call,
 *   except for immutable flags. Callers requiring this shouldn't use
 *   this function.
 *
 * Return: 0 on success, negative error code on failure, positive number on
 * caller defined premature termination.
 */
int walk_page_mapping(struct address_space *mapping, pgoff_t first_index,
                      pgoff_t nr, const struct mm_walk_ops *ops,
                      void *private)
{
        struct mm_walk walk = {
                .ops            = ops,
                .private        = private,
        };
        struct vm_area_struct *vma;
        pgoff_t vba, vea, cba, cea;
        unsigned long start_addr, end_addr;
        int err = 0;

        if (!check_ops_valid(ops))
                return -EINVAL;

        lockdep_assert_held(&mapping->i_mmap_rwsem);
        vma_interval_tree_foreach(vma, &mapping->i_mmap, first_index,
                                  first_index + nr - 1) {
                /* Clip to the vma */
                vba = vma->vm_pgoff;
                vea = vba + vma_pages(vma);
                cba = first_index;
                cba = max(cba, vba);
                cea = first_index + nr;
                cea = min(cea, vea);

                start_addr = ((cba - vba) << PAGE_SHIFT) + vma->vm_start;
                end_addr = ((cea - vba) << PAGE_SHIFT) + vma->vm_start;
                if (start_addr >= end_addr)
                        continue;

                walk.vma = vma;
                walk.mm = vma->vm_mm;

                err = walk_page_test(vma->vm_start, vma->vm_end, &walk);
                if (err > 0) {
                        err = 0;
                        break;
                } else if (err < 0)
                        break;

                err = __walk_page_range(start_addr, end_addr, &walk);
                if (err)
                        break;
        }

        return err;
}

/**
 * folio_walk_start - walk the page tables to a folio
 * @fw: filled with information on success.
 * @vma: the VMA.
 * @addr: the virtual address to use for the page table walk.
 * @flags: flags modifying which folios to walk to.
 *
 * Walk the page tables using @addr in a given @vma to a mapped folio and
 * return the folio, making sure that the page table entry referenced by
 * @addr cannot change until folio_walk_end() was called.
 *
 * As default, this function returns only folios that are not special (e.g., not
 * the zeropage) and never returns folios that are supposed to be ignored by the
 * VM as documented by vm_normal_page(). If requested, zeropages will be
 * returned as well.
 *
 * As default, this function only considers present page table entries.
 * If requested, it will also consider migration entries.
 *
 * If this function returns NULL it might either indicate "there is nothing" or
 * "there is nothing suitable".
 *
 * On success, @fw is filled and the function returns the folio while the PTL
 * is still held and folio_walk_end() must be called to clean up,
 * releasing any held locks. The returned folio must *not* be used after the
 * call to folio_walk_end(), unless a short-term folio reference is taken before
 * that call.
 *
 * @fw->page will correspond to the page that is effectively referenced by
 * @addr. However, for migration entries and shared zeropages @fw->page is
 * set to NULL. Note that large folios might be mapped by multiple page table
 * entries, and this function will always only lookup a single entry as
 * specified by @addr, which might or might not cover more than a single page of
 * the returned folio.
 *
 * This function must *not* be used as a naive replacement for
 * get_user_pages() / pin_user_pages(), especially not to perform DMA or
 * to carelessly modify page content. This function may *only* be used to grab
 * short-term folio references, never to grab long-term folio references.
 *
 * Using the page table entry pointers in @fw for reading or modifying the
 * entry should be avoided where possible: however, there might be valid
 * use cases.
 *
 * WARNING: Modifying page table entries in hugetlb VMAs requires a lot of care.
 * For example, PMD page table sharing might require prior unsharing. Also,
 * logical hugetlb entries might span multiple physical page table entries,
 * which *must* be modified in a single operation (set_huge_pte_at(),
 * huge_ptep_set_*, ...). Note that the page table entry stored in @fw might
 * not correspond to the first physical entry of a logical hugetlb entry.
 *
 * The mmap lock must be held in read mode.
 *
 * Return: folio pointer on success, otherwise NULL.
 */
struct folio *folio_walk_start(struct folio_walk *fw,
                struct vm_area_struct *vma, unsigned long addr,
                folio_walk_flags_t flags)
{
        unsigned long entry_size;
        bool expose_page = true;
        struct page *page;
        pud_t *pudp, pud;
        pmd_t *pmdp, pmd;
        pte_t *ptep, pte;
        spinlock_t *ptl;
        pgd_t *pgdp;
        p4d_t *p4dp;

        mmap_assert_locked(vma->vm_mm);
        vma_pgtable_walk_begin(vma);

        if (WARN_ON_ONCE(addr < vma->vm_start || addr >= vma->vm_end))
                goto not_found;

        pgdp = pgd_offset(vma->vm_mm, addr);
        if (pgd_none_or_clear_bad(pgdp))
                goto not_found;

        p4dp = p4d_offset(pgdp, addr);
        if (p4d_none_or_clear_bad(p4dp))
                goto not_found;

        pudp = pud_offset(p4dp, addr);
        pud = pudp_get(pudp);
        if (pud_none(pud))
                goto not_found;
        if (IS_ENABLED(CONFIG_PGTABLE_HAS_HUGE_LEAVES) &&
            (!pud_present(pud) || pud_leaf(pud))) {
                ptl = pud_lock(vma->vm_mm, pudp);
                pud = pudp_get(pudp);

                entry_size = PUD_SIZE;
                fw->level = FW_LEVEL_PUD;
                fw->pudp = pudp;
                fw->pud = pud;

                /*
                 * TODO: FW_MIGRATION support for PUD migration entries
                 * once there are relevant users.
                 */
                if (!pud_present(pud) || pud_devmap(pud) || pud_special(pud)) {
                        spin_unlock(ptl);
                        goto not_found;
                } else if (!pud_leaf(pud)) {
                        spin_unlock(ptl);
                        goto pmd_table;
                }
                /*
                 * TODO: vm_normal_page_pud() will be handy once we want to
                 * support PUD mappings in VM_PFNMAP|VM_MIXEDMAP VMAs.
                 */
                page = pud_page(pud);
                goto found;
        }

pmd_table:
        VM_WARN_ON_ONCE(!pud_present(pud) || pud_leaf(pud));
        pmdp = pmd_offset(pudp, addr);
        pmd = pmdp_get_lockless(pmdp);
        if (pmd_none(pmd))
                goto not_found;
        if (IS_ENABLED(CONFIG_PGTABLE_HAS_HUGE_LEAVES) &&
            (!pmd_present(pmd) || pmd_leaf(pmd))) {
                ptl = pmd_lock(vma->vm_mm, pmdp);
                pmd = pmdp_get(pmdp);

                entry_size = PMD_SIZE;
                fw->level = FW_LEVEL_PMD;
                fw->pmdp = pmdp;
                fw->pmd = pmd;

                if (pmd_none(pmd)) {
                        spin_unlock(ptl);
                        goto not_found;
                } else if (pmd_present(pmd) && !pmd_leaf(pmd)) {
                        spin_unlock(ptl);
                        goto pte_table;
                } else if (pmd_present(pmd)) {
                        page = vm_normal_page_pmd(vma, addr, pmd);
                        if (page) {
                                goto found;
                        } else if ((flags & FW_ZEROPAGE) &&
                                    is_huge_zero_pmd(pmd)) {
                                page = pfn_to_page(pmd_pfn(pmd));
                                expose_page = false;
                                goto found;
                        }
                } else if ((flags & FW_MIGRATION) &&
                           is_pmd_migration_entry(pmd)) {
                        swp_entry_t entry = pmd_to_swp_entry(pmd);

                        page = pfn_swap_entry_to_page(entry);
                        expose_page = false;
                        goto found;
                }
                spin_unlock(ptl);
                goto not_found;
        }

pte_table:
        VM_WARN_ON_ONCE(!pmd_present(pmd) || pmd_leaf(pmd));
        ptep = pte_offset_map_lock(vma->vm_mm, pmdp, addr, &ptl);
        if (!ptep)
                goto not_found;
        pte = ptep_get(ptep);

        entry_size = PAGE_SIZE;
        fw->level = FW_LEVEL_PTE;
        fw->ptep = ptep;
        fw->pte = pte;

        if (pte_present(pte)) {
                page = vm_normal_page(vma, addr, pte);
                if (page)
                        goto found;
                if ((flags & FW_ZEROPAGE) &&
                    is_zero_pfn(pte_pfn(pte))) {
                        page = pfn_to_page(pte_pfn(pte));
                        expose_page = false;
                        goto found;
                }
        } else if (!pte_none(pte)) {
                swp_entry_t entry = pte_to_swp_entry(pte);

                if ((flags & FW_MIGRATION) &&
                    is_migration_entry(entry)) {
                        page = pfn_swap_entry_to_page(entry);
                        expose_page = false;
                        goto found;
                }
        }
        pte_unmap_unlock(ptep, ptl);
not_found:
        vma_pgtable_walk_end(vma);
        return NULL;
found:
        if (expose_page)
                /* Note: Offset from the mapped page, not the folio start. */
                fw->page = nth_page(page, (addr & (entry_size - 1)) >> PAGE_SHIFT);
        else
                fw->page = NULL;
        fw->ptl = ptl;
        return page_folio(page);
}