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];
static unsigned int nr_swapper_spaces[MAX_SWAPFILES];
bool swap_vma_readahead = 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);
	spin_lock_irq(&address_space->tree_lock);
	for (i = 0; i < nr; i++) {
		set_page_private(page + i, entry.val + i);
		error = radix_tree_insert(&address_space->page_tree,
					  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->page_tree, idx + i);
			set_page_private(page + i, 0UL);
		}
		ClearPageSwapCache(page);
		page_ref_sub(page, nr);
	}
	spin_unlock_irq(&address_space->tree_lock);

	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->page_tree, 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;

	if (mem_cgroup_try_charge_swap(page, entry))
		goto fail;

	/*
	 * 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);
	spin_lock_irq(&address_space->tree_lock);
	__delete_from_swap_cache(page);
	spin_unlock_irq(&address_space->tree_lock);

	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, false);
}

/*
 * 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;
	unsigned long ra_info;
	int win, hits, readahead;

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

	INC_CACHE_INFO(find_total);
	if (page) {
		INC_CACHE_INFO(find_success);
		if (unlikely(PageTransCompound(page)))
			return page;
		readahead = TestClearPageReadahead(page);
		if (vma) {
			ra_info = GET_SWAP_RA_VAL(vma);
			win = SWAP_RA_WIN(ra_info);
			hits = SWAP_RA_HITS(ra_info);
			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)
				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(prev_offset, offset, hits, max_pages,
				  atomic_read(&last_readahead_pages));
	if (!hits)
		prev_offset = offset;
	atomic_set(&last_readahead_pages, pages);

	return pages;
}

/**
 * swapin_readahead - swap in pages in hope we need them soon
 * @entry: swap entry of this memory
 * @gfp_mask: memory allocation flags
 * @vma: user vma this address belongs to
 * @addr: target address for mempolicy
 *
 * 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 vma is not NULL.
 */
struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
			struct vm_area_struct *vma, unsigned long addr)
{
	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 blk_plug plug;
	bool do_poll = true, page_allocated;

	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++;

	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 &&
			    likely(!PageTransCompound(page))) {
				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 = kvzalloc(sizeof(struct address_space) * nr, GFP_KERNEL);
	if (!spaces)
		return -ENOMEM;
	for (i = 0; i < nr; i++) {
		space = spaces + i;
		INIT_RADIX_TREE(&space->page_tree, 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);
		spin_lock_init(&space->tree_lock);
	}
	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));
}

struct page *swap_readahead_detect(struct vm_fault *vmf,
				   struct vma_swap_readahead *swap_ra)
{
	struct vm_area_struct *vma = vmf->vma;
	unsigned long swap_ra_info;
	struct page *page;
	swp_entry_t entry;
	unsigned long faddr, pfn, fpfn;
	unsigned long start, end;
	pte_t *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) {
		swap_ra->win = 1;
		return NULL;
	}

	faddr = vmf->address;
	entry = pte_to_swp_entry(vmf->orig_pte);
	if ((unlikely(non_swap_entry(entry))))
		return NULL;
	page = lookup_swap_cache(entry, vma, faddr);
	if (page)
		return page;

	fpfn = PFN_DOWN(faddr);
	swap_ra_info = GET_SWAP_RA_VAL(vma);
	pfn = PFN_DOWN(SWAP_RA_ADDR(swap_ra_info));
	prev_win = SWAP_RA_WIN(swap_ra_info);
	hits = SWAP_RA_HITS(swap_ra_info);
	swap_ra->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)
		return NULL;

	/* 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);
	}
	swap_ra->nr_pte = end - start;
	swap_ra->offset = fpfn - start;
	pte = vmf->pte - swap_ra->offset;
#ifdef CONFIG_64BIT
	swap_ra->ptes = pte;
#else
	tpte = swap_ra->ptes;
	for (pfn = start; pfn != end; pfn++)
		*tpte++ = *pte++;
#endif

	return NULL;
}

struct page *do_swap_page_readahead(swp_entry_t fentry, gfp_t gfp_mask,
				    struct vm_fault *vmf,
				    struct vma_swap_readahead *swap_ra)
{
	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;

	if (swap_ra->win == 1)
		goto skip;

	blk_start_plug(&plug);
	for (i = 0, pte = swap_ra->ptes; i < swap_ra->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 != swap_ra->offset &&
			    likely(!PageTransCompound(page))) {
				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,
				     swap_ra->win == 1);
}

#ifdef CONFIG_SYSFS
static ssize_t vma_ra_enabled_show(struct kobject *kobj,
				     struct kobj_attribute *attr, char *buf)
{
	return sprintf(buf, "%s\n", swap_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))
		swap_vma_readahead = true;
	else if (!strncmp(buf, "false", 5) || !strncmp(buf, "0", 1))
		swap_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