Linux 4.19.168

[linux/fpc-iii.git] / mm / swap_state.c

// SPDX-License-Identifier: GPL-2.0
/*
 *  linux/mm/swap_state.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *  Swap reorganised 29.12.95, Stephen Tweedie
 *
 *  Rewritten to use page cache, (C) 1998 Stephen Tweedie
 */
#include <linux/mm.h>
#include <linux/gfp.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/init.h>
#include <linux/pagemap.h>
#include <linux/backing-dev.h>
#include <linux/blkdev.h>
#include <linux/pagevec.h>
#include <linux/migrate.h>
#include <linux/vmalloc.h>
#include <linux/swap_slots.h>
#include <linux/huge_mm.h>

#include <asm/pgtable.h>
#include "internal.h"

/*
 * swapper_space is a fiction, retained to simplify the path through
 * vmscan's shrink_page_list.
 */
static const struct address_space_operations swap_aops = {
        .writepage      = swap_writepage,
        .set_page_dirty = swap_set_page_dirty,
#ifdef CONFIG_MIGRATION
        .migratepage    = migrate_page,
#endif
};

struct address_space *swapper_spaces[MAX_SWAPFILES] __read_mostly;
static unsigned int nr_swapper_spaces[MAX_SWAPFILES] __read_mostly;
static bool enable_vma_readahead __read_mostly = true;

#define SWAP_RA_WIN_SHIFT       (PAGE_SHIFT / 2)
#define SWAP_RA_HITS_MASK       ((1UL << SWAP_RA_WIN_SHIFT) - 1)
#define SWAP_RA_HITS_MAX        SWAP_RA_HITS_MASK
#define SWAP_RA_WIN_MASK        (~PAGE_MASK & ~SWAP_RA_HITS_MASK)

#define SWAP_RA_HITS(v)         ((v) & SWAP_RA_HITS_MASK)
#define SWAP_RA_WIN(v)          (((v) & SWAP_RA_WIN_MASK) >> SWAP_RA_WIN_SHIFT)
#define SWAP_RA_ADDR(v)         ((v) & PAGE_MASK)

#define SWAP_RA_VAL(addr, win, hits)                            \
        (((addr) & PAGE_MASK) |                                 \
         (((win) << SWAP_RA_WIN_SHIFT) & SWAP_RA_WIN_MASK) |    \
         ((hits) & SWAP_RA_HITS_MASK))

/* Initial readahead hits is 4 to start up with a small window */
#define GET_SWAP_RA_VAL(vma)                                    \
        (atomic_long_read(&(vma)->swap_readahead_info) ? : 4)

#define INC_CACHE_INFO(x)       do { swap_cache_info.x++; } while (0)
#define ADD_CACHE_INFO(x, nr)   do { swap_cache_info.x += (nr); } while (0)

static struct {
        unsigned long add_total;
        unsigned long del_total;
        unsigned long find_success;
        unsigned long find_total;
} swap_cache_info;

unsigned long total_swapcache_pages(void)
{
        unsigned int i, j, nr;
        unsigned long ret = 0;
        struct address_space *spaces;

        rcu_read_lock();
        for (i = 0; i < MAX_SWAPFILES; i++) {
                /*
                 * The corresponding entries in nr_swapper_spaces and
                 * swapper_spaces will be reused only after at least
                 * one grace period.  So it is impossible for them
                 * belongs to different usage.
                 */
                nr = nr_swapper_spaces[i];
                spaces = rcu_dereference(swapper_spaces[i]);
                if (!nr || !spaces)
                        continue;
                for (j = 0; j < nr; j++)
                        ret += spaces[j].nrpages;
        }
        rcu_read_unlock();
        return ret;
}

static atomic_t swapin_readahead_hits = ATOMIC_INIT(4);

void show_swap_cache_info(void)
{
        printk("%lu pages in swap cache\n", total_swapcache_pages());
        printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu\n",
                swap_cache_info.add_total, swap_cache_info.del_total,
                swap_cache_info.find_success, swap_cache_info.find_total);
        printk("Free swap  = %ldkB\n",
                get_nr_swap_pages() << (PAGE_SHIFT - 10));
        printk("Total swap = %lukB\n", total_swap_pages << (PAGE_SHIFT - 10));
}

/*
 * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
 * but sets SwapCache flag and private instead of mapping and index.
 */
int __add_to_swap_cache(struct page *page, swp_entry_t entry)
{
        int error, i, nr = hpage_nr_pages(page);
        struct address_space *address_space;
        pgoff_t idx = swp_offset(entry);

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(PageSwapCache(page), page);
        VM_BUG_ON_PAGE(!PageSwapBacked(page), page);

        page_ref_add(page, nr);
        SetPageSwapCache(page);

        address_space = swap_address_space(entry);
        xa_lock_irq(&address_space->i_pages);
        for (i = 0; i < nr; i++) {
                set_page_private(page + i, entry.val + i);
                error = radix_tree_insert(&address_space->i_pages,
                                          idx + i, page + i);
                if (unlikely(error))
                        break;
        }
        if (likely(!error)) {
                address_space->nrpages += nr;
                __mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, nr);
                ADD_CACHE_INFO(add_total, nr);
        } else {
                /*
                 * Only the context which have set SWAP_HAS_CACHE flag
                 * would call add_to_swap_cache().
                 * So add_to_swap_cache() doesn't returns -EEXIST.
                 */
                VM_BUG_ON(error == -EEXIST);
                set_page_private(page + i, 0UL);
                while (i--) {
                        radix_tree_delete(&address_space->i_pages, idx + i);
                        set_page_private(page + i, 0UL);
                }
                ClearPageSwapCache(page);
                page_ref_sub(page, nr);
        }
        xa_unlock_irq(&address_space->i_pages);

        return error;
}


int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
{
        int error;

        error = radix_tree_maybe_preload_order(gfp_mask, compound_order(page));
        if (!error) {
                error = __add_to_swap_cache(page, entry);
                radix_tree_preload_end();
        }
        return error;
}

/*
 * This must be called only on pages that have
 * been verified to be in the swap cache.
 */
void __delete_from_swap_cache(struct page *page)
{
        struct address_space *address_space;
        int i, nr = hpage_nr_pages(page);
        swp_entry_t entry;
        pgoff_t idx;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageSwapCache(page), page);
        VM_BUG_ON_PAGE(PageWriteback(page), page);

        entry.val = page_private(page);
        address_space = swap_address_space(entry);
        idx = swp_offset(entry);
        for (i = 0; i < nr; i++) {
                radix_tree_delete(&address_space->i_pages, idx + i);
                set_page_private(page + i, 0);
        }
        ClearPageSwapCache(page);
        address_space->nrpages -= nr;
        __mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, -nr);
        ADD_CACHE_INFO(del_total, nr);
}

/**
 * add_to_swap - allocate swap space for a page
 * @page: page we want to move to swap
 *
 * Allocate swap space for the page and add the page to the
 * swap cache.  Caller needs to hold the page lock. 
 */
int add_to_swap(struct page *page)
{
        swp_entry_t entry;
        int err;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageUptodate(page), page);

        entry = get_swap_page(page);
        if (!entry.val)
                return 0;

        /*
         * Radix-tree node allocations from PF_MEMALLOC contexts could
         * completely exhaust the page allocator. __GFP_NOMEMALLOC
         * stops emergency reserves from being allocated.
         *
         * TODO: this could cause a theoretical memory reclaim
         * deadlock in the swap out path.
         */
        /*
         * Add it to the swap cache.
         */
        err = add_to_swap_cache(page, entry,
                        __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
        /* -ENOMEM radix-tree allocation failure */
        if (err)
                /*
                 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
                 * clear SWAP_HAS_CACHE flag.
                 */
                goto fail;
        /*
         * Normally the page will be dirtied in unmap because its pte should be
         * dirty. A special case is MADV_FREE page. The page'e pte could have
         * dirty bit cleared but the page's SwapBacked bit is still set because
         * clearing the dirty bit and SwapBacked bit has no lock protected. For
         * such page, unmap will not set dirty bit for it, so page reclaim will
         * not write the page out. This can cause data corruption when the page
         * is swap in later. Always setting the dirty bit for the page solves
         * the problem.
         */
        set_page_dirty(page);

        return 1;

fail:
        put_swap_page(page, entry);
        return 0;
}

/*
 * This must be called only on pages that have
 * been verified to be in the swap cache and locked.
 * It will never put the page into the free list,
 * the caller has a reference on the page.
 */
void delete_from_swap_cache(struct page *page)
{
        swp_entry_t entry;
        struct address_space *address_space;

        entry.val = page_private(page);

        address_space = swap_address_space(entry);
        xa_lock_irq(&address_space->i_pages);
        __delete_from_swap_cache(page);
        xa_unlock_irq(&address_space->i_pages);

        put_swap_page(page, entry);
        page_ref_sub(page, hpage_nr_pages(page));
}

/* 
 * If we are the only user, then try to free up the swap cache. 
 * 
 * Its ok to check for PageSwapCache without the page lock
 * here because we are going to recheck again inside
 * try_to_free_swap() _with_ the lock.
 *                                      - Marcelo
 */
static inline void free_swap_cache(struct page *page)
{
        if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
                try_to_free_swap(page);
                unlock_page(page);
        }
}

/* 
 * Perform a free_page(), also freeing any swap cache associated with
 * this page if it is the last user of the page.
 */
void free_page_and_swap_cache(struct page *page)
{
        free_swap_cache(page);
        if (!is_huge_zero_page(page))
                put_page(page);
}

/*
 * Passed an array of pages, drop them all from swapcache and then release
 * them.  They are removed from the LRU and freed if this is their last use.
 */
void free_pages_and_swap_cache(struct page **pages, int nr)
{
        struct page **pagep = pages;
        int i;

        lru_add_drain();
        for (i = 0; i < nr; i++)
                free_swap_cache(pagep[i]);
        release_pages(pagep, nr);
}

static inline bool swap_use_vma_readahead(void)
{
        return READ_ONCE(enable_vma_readahead) && !atomic_read(&nr_rotate_swap);
}

/*
 * Lookup a swap entry in the swap cache. A found page will be returned
 * unlocked and with its refcount incremented - we rely on the kernel
 * lock getting page table operations atomic even if we drop the page
 * lock before returning.
 */
struct page *lookup_swap_cache(swp_entry_t entry, struct vm_area_struct *vma,
                               unsigned long addr)
{
        struct page *page;

        page = find_get_page(swap_address_space(entry), swp_offset(entry));

        INC_CACHE_INFO(find_total);
        if (page) {
                bool vma_ra = swap_use_vma_readahead();
                bool readahead;

                INC_CACHE_INFO(find_success);
                /*
                 * At the moment, we don't support PG_readahead for anon THP
                 * so let's bail out rather than confusing the readahead stat.
                 */
                if (unlikely(PageTransCompound(page)))
                        return page;

                readahead = TestClearPageReadahead(page);
                if (vma && vma_ra) {
                        unsigned long ra_val;
                        int win, hits;

                        ra_val = GET_SWAP_RA_VAL(vma);
                        win = SWAP_RA_WIN(ra_val);
                        hits = SWAP_RA_HITS(ra_val);
                        if (readahead)
                                hits = min_t(int, hits + 1, SWAP_RA_HITS_MAX);
                        atomic_long_set(&vma->swap_readahead_info,
                                        SWAP_RA_VAL(addr, win, hits));
                }

                if (readahead) {
                        count_vm_event(SWAP_RA_HIT);
                        if (!vma || !vma_ra)
                                atomic_inc(&swapin_readahead_hits);
                }
        }

        return page;
}

struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                        struct vm_area_struct *vma, unsigned long addr,
                        bool *new_page_allocated)
{
        struct page *found_page, *new_page = NULL;
        struct address_space *swapper_space = swap_address_space(entry);
        int err;
        *new_page_allocated = false;

        do {
                /*
                 * First check the swap cache.  Since this is normally
                 * called after lookup_swap_cache() failed, re-calling
                 * that would confuse statistics.
                 */
                found_page = find_get_page(swapper_space, swp_offset(entry));
                if (found_page)
                        break;

                /*
                 * Just skip read ahead for unused swap slot.
                 * During swap_off when swap_slot_cache is disabled,
                 * we have to handle the race between putting
                 * swap entry in swap cache and marking swap slot
                 * as SWAP_HAS_CACHE.  That's done in later part of code or
                 * else swap_off will be aborted if we return NULL.
                 */
                if (!__swp_swapcount(entry) && swap_slot_cache_enabled)
                        break;

                /*
                 * Get a new page to read into from swap.
                 */
                if (!new_page) {
                        new_page = alloc_page_vma(gfp_mask, vma, addr);
                        if (!new_page)
                                break;          /* Out of memory */
                }

                /*
                 * call radix_tree_preload() while we can wait.
                 */
                err = radix_tree_maybe_preload(gfp_mask & GFP_RECLAIM_MASK);
                if (err)
                        break;

                /*
                 * Swap entry may have been freed since our caller observed it.
                 */
                err = swapcache_prepare(entry);
                if (err == -EEXIST) {
                        radix_tree_preload_end();
                        /*
                         * We might race against get_swap_page() and stumble
                         * across a SWAP_HAS_CACHE swap_map entry whose page
                         * has not been brought into the swapcache yet.
                         */
                        cond_resched();
                        continue;
                }
                if (err) {              /* swp entry is obsolete ? */
                        radix_tree_preload_end();
                        break;
                }

                /* May fail (-ENOMEM) if radix-tree node allocation failed. */
                __SetPageLocked(new_page);
                __SetPageSwapBacked(new_page);
                err = __add_to_swap_cache(new_page, entry);
                if (likely(!err)) {
                        radix_tree_preload_end();
                        /*
                         * Initiate read into locked page and return.
                         */
                        lru_cache_add_anon(new_page);
                        *new_page_allocated = true;
                        return new_page;
                }
                radix_tree_preload_end();
                __ClearPageLocked(new_page);
                /*
                 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
                 * clear SWAP_HAS_CACHE flag.
                 */
                put_swap_page(new_page, entry);
        } while (err != -ENOMEM);

        if (new_page)
                put_page(new_page);
        return found_page;
}

/*
 * Locate a page of swap in physical memory, reserving swap cache space
 * and reading the disk if it is not already cached.
 * A failure return means that either the page allocation failed or that
 * the swap entry is no longer in use.
 */
struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                struct vm_area_struct *vma, unsigned long addr, bool do_poll)
{
        bool page_was_allocated;
        struct page *retpage = __read_swap_cache_async(entry, gfp_mask,
                        vma, addr, &page_was_allocated);

        if (page_was_allocated)
                swap_readpage(retpage, do_poll);

        return retpage;
}

static unsigned int __swapin_nr_pages(unsigned long prev_offset,
                                      unsigned long offset,
                                      int hits,
                                      int max_pages,
                                      int prev_win)
{
        unsigned int pages, last_ra;

        /*
         * This heuristic has been found to work well on both sequential and
         * random loads, swapping to hard disk or to SSD: please don't ask
         * what the "+ 2" means, it just happens to work well, that's all.
         */
        pages = hits + 2;
        if (pages == 2) {
                /*
                 * We can have no readahead hits to judge by: but must not get
                 * stuck here forever, so check for an adjacent offset instead
                 * (and don't even bother to check whether swap type is same).
                 */
                if (offset != prev_offset + 1 && offset != prev_offset - 1)
                        pages = 1;
        } else {
                unsigned int roundup = 4;
                while (roundup < pages)
                        roundup <<= 1;
                pages = roundup;
        }

        if (pages > max_pages)
                pages = max_pages;

        /* Don't shrink readahead too fast */
        last_ra = prev_win / 2;
        if (pages < last_ra)
                pages = last_ra;

        return pages;
}

static unsigned long swapin_nr_pages(unsigned long offset)
{
        static unsigned long prev_offset;
        unsigned int hits, pages, max_pages;
        static atomic_t last_readahead_pages;

        max_pages = 1 << READ_ONCE(page_cluster);
        if (max_pages <= 1)
                return 1;

        hits = atomic_xchg(&swapin_readahead_hits, 0);
        pages = __swapin_nr_pages(READ_ONCE(prev_offset), offset, hits,
                                  max_pages,
                                  atomic_read(&last_readahead_pages));
        if (!hits)
                WRITE_ONCE(prev_offset, offset);
        atomic_set(&last_readahead_pages, pages);

        return pages;
}

/**
 * swap_cluster_readahead - swap in pages in hope we need them soon
 * @entry: swap entry of this memory
 * @gfp_mask: memory allocation flags
 * @vmf: fault information
 *
 * Returns the struct page for entry and addr, after queueing swapin.
 *
 * Primitive swap readahead code. We simply read an aligned block of
 * (1 << page_cluster) entries in the swap area. This method is chosen
 * because it doesn't cost us any seek time.  We also make sure to queue
 * the 'original' request together with the readahead ones...
 *
 * This has been extended to use the NUMA policies from the mm triggering
 * the readahead.
 *
 * Caller must hold down_read on the vma->vm_mm if vmf->vma is not NULL.
 */
struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
                                struct vm_fault *vmf)
{
        struct page *page;
        unsigned long entry_offset = swp_offset(entry);
        unsigned long offset = entry_offset;
        unsigned long start_offset, end_offset;
        unsigned long mask;
        struct swap_info_struct *si = swp_swap_info(entry);
        struct blk_plug plug;
        bool do_poll = true, page_allocated;
        struct vm_area_struct *vma = vmf->vma;
        unsigned long addr = vmf->address;

        mask = swapin_nr_pages(offset) - 1;
        if (!mask)
                goto skip;

        do_poll = false;
        /* Read a page_cluster sized and aligned cluster around offset. */
        start_offset = offset & ~mask;
        end_offset = offset | mask;
        if (!start_offset)      /* First page is swap header. */
                start_offset++;
        if (end_offset >= si->max)
                end_offset = si->max - 1;

        blk_start_plug(&plug);
        for (offset = start_offset; offset <= end_offset ; offset++) {
                /* Ok, do the async read-ahead now */
                page = __read_swap_cache_async(
                        swp_entry(swp_type(entry), offset),
                        gfp_mask, vma, addr, &page_allocated);
                if (!page)
                        continue;
                if (page_allocated) {
                        swap_readpage(page, false);
                        if (offset != entry_offset) {
                                SetPageReadahead(page);
                                count_vm_event(SWAP_RA);
                        }
                }
                put_page(page);
        }
        blk_finish_plug(&plug);

        lru_add_drain();        /* Push any new pages onto the LRU now */
skip:
        return read_swap_cache_async(entry, gfp_mask, vma, addr, do_poll);
}

int init_swap_address_space(unsigned int type, unsigned long nr_pages)
{
        struct address_space *spaces, *space;
        unsigned int i, nr;

        nr = DIV_ROUND_UP(nr_pages, SWAP_ADDRESS_SPACE_PAGES);
        spaces = kvcalloc(nr, sizeof(struct address_space), GFP_KERNEL);
        if (!spaces)
                return -ENOMEM;
        for (i = 0; i < nr; i++) {
                space = spaces + i;
                INIT_RADIX_TREE(&space->i_pages, GFP_ATOMIC|__GFP_NOWARN);
                atomic_set(&space->i_mmap_writable, 0);
                space->a_ops = &swap_aops;
                /* swap cache doesn't use writeback related tags */
                mapping_set_no_writeback_tags(space);
        }
        nr_swapper_spaces[type] = nr;
        rcu_assign_pointer(swapper_spaces[type], spaces);

        return 0;
}

void exit_swap_address_space(unsigned int type)
{
        struct address_space *spaces;

        spaces = swapper_spaces[type];
        nr_swapper_spaces[type] = 0;
        rcu_assign_pointer(swapper_spaces[type], NULL);
        synchronize_rcu();
        kvfree(spaces);
}

static inline void swap_ra_clamp_pfn(struct vm_area_struct *vma,
                                     unsigned long faddr,
                                     unsigned long lpfn,
                                     unsigned long rpfn,
                                     unsigned long *start,
                                     unsigned long *end)
{
        *start = max3(lpfn, PFN_DOWN(vma->vm_start),
                      PFN_DOWN(faddr & PMD_MASK));
        *end = min3(rpfn, PFN_DOWN(vma->vm_end),
                    PFN_DOWN((faddr & PMD_MASK) + PMD_SIZE));
}

static void swap_ra_info(struct vm_fault *vmf,
                        struct vma_swap_readahead *ra_info)
{
        struct vm_area_struct *vma = vmf->vma;
        unsigned long ra_val;
        swp_entry_t entry;
        unsigned long faddr, pfn, fpfn;
        unsigned long start, end;
        pte_t *pte, *orig_pte;
        unsigned int max_win, hits, prev_win, win, left;
#ifndef CONFIG_64BIT
        pte_t *tpte;
#endif

        max_win = 1 << min_t(unsigned int, READ_ONCE(page_cluster),
                             SWAP_RA_ORDER_CEILING);
        if (max_win == 1) {
                ra_info->win = 1;
                return;
        }

        faddr = vmf->address;
        orig_pte = pte = pte_offset_map(vmf->pmd, faddr);
        entry = pte_to_swp_entry(*pte);
        if ((unlikely(non_swap_entry(entry)))) {
                pte_unmap(orig_pte);
                return;
        }

        fpfn = PFN_DOWN(faddr);
        ra_val = GET_SWAP_RA_VAL(vma);
        pfn = PFN_DOWN(SWAP_RA_ADDR(ra_val));
        prev_win = SWAP_RA_WIN(ra_val);
        hits = SWAP_RA_HITS(ra_val);
        ra_info->win = win = __swapin_nr_pages(pfn, fpfn, hits,
                                               max_win, prev_win);
        atomic_long_set(&vma->swap_readahead_info,
                        SWAP_RA_VAL(faddr, win, 0));

        if (win == 1) {
                pte_unmap(orig_pte);
                return;
        }

        /* Copy the PTEs because the page table may be unmapped */
        if (fpfn == pfn + 1)
                swap_ra_clamp_pfn(vma, faddr, fpfn, fpfn + win, &start, &end);
        else if (pfn == fpfn + 1)
                swap_ra_clamp_pfn(vma, faddr, fpfn - win + 1, fpfn + 1,
                                  &start, &end);
        else {
                left = (win - 1) / 2;
                swap_ra_clamp_pfn(vma, faddr, fpfn - left, fpfn + win - left,
                                  &start, &end);
        }
        ra_info->nr_pte = end - start;
        ra_info->offset = fpfn - start;
        pte -= ra_info->offset;
#ifdef CONFIG_64BIT
        ra_info->ptes = pte;
#else
        tpte = ra_info->ptes;
        for (pfn = start; pfn != end; pfn++)
                *tpte++ = *pte++;
#endif
        pte_unmap(orig_pte);
}

static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
                                       struct vm_fault *vmf)
{
        struct blk_plug plug;
        struct vm_area_struct *vma = vmf->vma;
        struct page *page;
        pte_t *pte, pentry;
        swp_entry_t entry;
        unsigned int i;
        bool page_allocated;
        struct vma_swap_readahead ra_info = {0,};

        swap_ra_info(vmf, &ra_info);
        if (ra_info.win == 1)
                goto skip;

        blk_start_plug(&plug);
        for (i = 0, pte = ra_info.ptes; i < ra_info.nr_pte;
             i++, pte++) {
                pentry = *pte;
                if (pte_none(pentry))
                        continue;
                if (pte_present(pentry))
                        continue;
                entry = pte_to_swp_entry(pentry);
                if (unlikely(non_swap_entry(entry)))
                        continue;
                page = __read_swap_cache_async(entry, gfp_mask, vma,
                                               vmf->address, &page_allocated);
                if (!page)
                        continue;
                if (page_allocated) {
                        swap_readpage(page, false);
                        if (i != ra_info.offset) {
                                SetPageReadahead(page);
                                count_vm_event(SWAP_RA);
                        }
                }
                put_page(page);
        }
        blk_finish_plug(&plug);
        lru_add_drain();
skip:
        return read_swap_cache_async(fentry, gfp_mask, vma, vmf->address,
                                     ra_info.win == 1);
}

/**
 * swapin_readahead - swap in pages in hope we need them soon
 * @entry: swap entry of this memory
 * @gfp_mask: memory allocation flags
 * @vmf: fault information
 *
 * Returns the struct page for entry and addr, after queueing swapin.
 *
 * It's a main entry function for swap readahead. By the configuration,
 * it will read ahead blocks by cluster-based(ie, physical disk based)
 * or vma-based(ie, virtual address based on faulty address) readahead.
 */
struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
                                struct vm_fault *vmf)
{
        return swap_use_vma_readahead() ?
                        swap_vma_readahead(entry, gfp_mask, vmf) :
                        swap_cluster_readahead(entry, gfp_mask, vmf);
}

#ifdef CONFIG_SYSFS
static ssize_t vma_ra_enabled_show(struct kobject *kobj,
                                     struct kobj_attribute *attr, char *buf)
{
        return sprintf(buf, "%s\n", enable_vma_readahead ? "true" : "false");
}
static ssize_t vma_ra_enabled_store(struct kobject *kobj,
                                      struct kobj_attribute *attr,
                                      const char *buf, size_t count)
{
        if (!strncmp(buf, "true", 4) || !strncmp(buf, "1", 1))
                enable_vma_readahead = true;
        else if (!strncmp(buf, "false", 5) || !strncmp(buf, "0", 1))
                enable_vma_readahead = false;
        else
                return -EINVAL;

        return count;
}
static struct kobj_attribute vma_ra_enabled_attr =
        __ATTR(vma_ra_enabled, 0644, vma_ra_enabled_show,
               vma_ra_enabled_store);

static struct attribute *swap_attrs[] = {
        &vma_ra_enabled_attr.attr,
        NULL,
};

static struct attribute_group swap_attr_group = {
        .attrs = swap_attrs,
};

static int __init swap_init_sysfs(void)
{
        int err;
        struct kobject *swap_kobj;

        swap_kobj = kobject_create_and_add("swap", mm_kobj);
        if (!swap_kobj) {
                pr_err("failed to create swap kobject\n");
                return -ENOMEM;
        }
        err = sysfs_create_group(swap_kobj, &swap_attr_group);
        if (err) {
                pr_err("failed to register swap group\n");
                goto delete_obj;
        }
        return 0;

delete_obj:
        kobject_put(swap_kobj);
        return err;
}
subsys_initcall(swap_init_sysfs);
#endif