shmem.c

/*
 * Resizable virtual memory filesystem for Linux.
 *
 * Copyright (C) 2000 Linus Torvalds.
 *		 2000 Transmeta Corp.
 *		 2000-2001 Christoph Rohland
 *		 2000-2001 SAP AG
 *		 2002 Red Hat Inc.
 * Copyright (C) 2002-2011 Hugh Dickins.
 * Copyright (C) 2011 Google Inc.
 * Copyright (C) 2002-2005 VERITAS Software Corporation.
 * Copyright (C) 2004 Andi Kleen, SuSE Labs
 *
 * Extended attribute support for tmpfs:
 * Copyright (c) 2004, Luke Kenneth Casson Leighton <lkcl@lkcl.net>
 * Copyright (c) 2004 Red Hat, Inc., James Morris <jmorris@redhat.com>
 *
 * tiny-shmem:
 * Copyright (c) 2004, 2008 Matt Mackall <mpm@selenic.com>
 *
 * This file is released under the GPL.
 */

#include <linux/fs.h>
#include <linux/init.h>
#include <linux/vfs.h>
#include <linux/mount.h>
#include <linux/ramfs.h>
#include <linux/pagemap.h>
#include <linux/file.h>
#include <linux/mm.h>
#include <linux/random.h>
#include <linux/sched/signal.h>
#include <linux/export.h>
#include <linux/swap.h>
#include <linux/uio.h>
#include <linux/khugepaged.h>
#include <linux/hugetlb.h>
#include <linux/frontswap.h>
#include <linux/fs_parser.h>

#include <asm/tlbflush.h> /* for arch/microblaze update_mmu_cache() */

static struct vfsmount *shm_mnt;

#ifdef CONFIG_SHMEM
/*
 * This virtual memory filesystem is heavily based on the ramfs. It
 * extends ramfs by the ability to use swap and honor resource limits
 * which makes it a completely usable filesystem.
 */

#include <linux/xattr.h>
#include <linux/exportfs.h>
#include <linux/posix_acl.h>
#include <linux/posix_acl_xattr.h>
#include <linux/mman.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/backing-dev.h>
#include <linux/shmem_fs.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/pagevec.h>
#include <linux/percpu_counter.h>
#include <linux/falloc.h>
#include <linux/splice.h>
#include <linux/security.h>
#include <linux/swapops.h>
#include <linux/mempolicy.h>
#include <linux/namei.h>
#include <linux/ctype.h>
#include <linux/migrate.h>
#include <linux/highmem.h>
#include <linux/seq_file.h>
#include <linux/magic.h>
#include <linux/syscalls.h>
#include <linux/fcntl.h>
#include <uapi/linux/memfd.h>
#include <linux/userfaultfd_k.h>
#include <linux/rmap.h>
#include <linux/uuid.h>

#include <linux/uaccess.h>

#include "internal.h"

#define BLOCKS_PER_PAGE  (PAGE_SIZE/512)
#define VM_ACCT(size)    (PAGE_ALIGN(size) >> PAGE_SHIFT)

/* Pretend that each entry is of this size in directory's i_size */
#define BOGO_DIRENT_SIZE 20

/* Symlink up to this size is kmalloc'ed instead of using a swappable page */
#define SHORT_SYMLINK_LEN 128

/*
 * shmem_fallocate communicates with shmem_fault or shmem_writepage via
 * inode->i_private (with i_mutex making sure that it has only one user at
 * a time): we would prefer not to enlarge the shmem inode just for that.
 */
struct shmem_falloc {
	wait_queue_head_t *waitq; /* faults into hole wait for punch to end */
	pgoff_t start;		/* start of range currently being fallocated */
	pgoff_t next;		/* the next page offset to be fallocated */
	pgoff_t nr_falloced;	/* how many new pages have been fallocated */
	pgoff_t nr_unswapped;	/* how often writepage refused to swap out */
};

struct shmem_options {
	unsigned long long blocks;
	unsigned long long inodes;
	struct mempolicy *mpol;
	kuid_t uid;
	kgid_t gid;
	umode_t mode;
	int huge;
	int seen;
#define SHMEM_SEEN_BLOCKS 1
#define SHMEM_SEEN_INODES 2
#define SHMEM_SEEN_HUGE 4
};

#ifdef CONFIG_TMPFS
static unsigned long shmem_default_max_blocks(void)
{
	return totalram_pages() / 2;
}

static unsigned long shmem_default_max_inodes(void)
{
	unsigned long nr_pages = totalram_pages();

	return min(nr_pages - totalhigh_pages(), nr_pages / 2);
}
#endif

static bool shmem_should_replace_page(struct page *page, gfp_t gfp);
static int shmem_replace_page(struct page **pagep, gfp_t gfp,
				struct shmem_inode_info *info, pgoff_t index);
static int shmem_swapin_page(struct inode *inode, pgoff_t index,
			     struct page **pagep, enum sgp_type sgp,
			     gfp_t gfp, struct vm_area_struct *vma,
			     vm_fault_t *fault_type);
static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
		struct page **pagep, enum sgp_type sgp,
		gfp_t gfp, struct vm_area_struct *vma,
		struct vm_fault *vmf, vm_fault_t *fault_type);

int shmem_getpage(struct inode *inode, pgoff_t index,
		struct page **pagep, enum sgp_type sgp)
{
	return shmem_getpage_gfp(inode, index, pagep, sgp,
		mapping_gfp_mask(inode->i_mapping), NULL, NULL, NULL);
}

static inline struct shmem_sb_info *SHMEM_SB(struct super_block *sb)
{
	return sb->s_fs_info;
}

/*
 * shmem_file_setup pre-accounts the whole fixed size of a VM object,
 * for shared memory and for shared anonymous (/dev/zero) mappings
 * (unless MAP_NORESERVE and sysctl_overcommit_memory <= 1),
 * consistent with the pre-accounting of private mappings ...
 */
static inline int shmem_acct_size(unsigned long flags, loff_t size)
{
	return (flags & VM_NORESERVE) ?
		0 : security_vm_enough_memory_mm(current->mm, VM_ACCT(size));
}

static inline void shmem_unacct_size(unsigned long flags, loff_t size)
{
	if (!(flags & VM_NORESERVE))
		vm_unacct_memory(VM_ACCT(size));
}

static inline int shmem_reacct_size(unsigned long flags,
		loff_t oldsize, loff_t newsize)
{
	if (!(flags & VM_NORESERVE)) {
		if (VM_ACCT(newsize) > VM_ACCT(oldsize))
			return security_vm_enough_memory_mm(current->mm,
					VM_ACCT(newsize) - VM_ACCT(oldsize));
		else if (VM_ACCT(newsize) < VM_ACCT(oldsize))
			vm_unacct_memory(VM_ACCT(oldsize) - VM_ACCT(newsize));
	}
	return 0;
}

/*
 * ... whereas tmpfs objects are accounted incrementally as
 * pages are allocated, in order to allow large sparse files.
 * shmem_getpage reports shmem_acct_block failure as -ENOSPC not -ENOMEM,
 * so that a failure on a sparse tmpfs mapping will give SIGBUS not OOM.
 */
static inline int shmem_acct_block(unsigned long flags, long pages)
{
	if (!(flags & VM_NORESERVE))
		return 0;

	return security_vm_enough_memory_mm(current->mm,
			pages * VM_ACCT(PAGE_SIZE));
}

static inline void shmem_unacct_blocks(unsigned long flags, long pages)
{
	if (flags & VM_NORESERVE)
		vm_unacct_memory(pages * VM_ACCT(PAGE_SIZE));
}

static inline bool shmem_inode_acct_block(struct inode *inode, long pages)
{
	struct shmem_inode_info *info = SHMEM_I(inode);
	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);

	if (shmem_acct_block(info->flags, pages))
		return false;

	if (sbinfo->max_blocks) {
		if (percpu_counter_compare(&sbinfo->used_blocks,
					   sbinfo->max_blocks - pages) > 0)
			goto unacct;
		percpu_counter_add(&sbinfo->used_blocks, pages);
	}

	return true;

unacct:
	shmem_unacct_blocks(info->flags, pages);
	return false;
}

static inline void shmem_inode_unacct_blocks(struct inode *inode, long pages)
{
	struct shmem_inode_info *info = SHMEM_I(inode);
	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);

	if (sbinfo->max_blocks)
		percpu_counter_sub(&sbinfo->used_blocks, pages);
	shmem_unacct_blocks(info->flags, pages);
}

static const struct super_operations shmem_ops;
static const struct address_space_operations shmem_aops;
static const struct file_operations shmem_file_operations;
static const struct inode_operations shmem_inode_operations;
static const struct inode_operations shmem_dir_inode_operations;
static const struct inode_operations shmem_special_inode_operations;
static const struct vm_operations_struct shmem_vm_ops;
static struct file_system_type shmem_fs_type;

bool vma_is_shmem(struct vm_area_struct *vma)
{
	return vma->vm_ops == &shmem_vm_ops;
}

static LIST_HEAD(shmem_swaplist);
static DEFINE_MUTEX(shmem_swaplist_mutex);

static int shmem_reserve_inode(struct super_block *sb)
{
	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
	if (sbinfo->max_inodes) {
		spin_lock(&sbinfo->stat_lock);
		if (!sbinfo->free_inodes) {
			spin_unlock(&sbinfo->stat_lock);
			return -ENOSPC;
		}
		sbinfo->free_inodes--;
		spin_unlock(&sbinfo->stat_lock);
	}
	return 0;
}

static void shmem_free_inode(struct super_block *sb)
{
	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
	if (sbinfo->max_inodes) {
		spin_lock(&sbinfo->stat_lock);
		sbinfo->free_inodes++;
		spin_unlock(&sbinfo->stat_lock);
	}
}

/**
 * shmem_recalc_inode - recalculate the block usage of an inode
 * @inode: inode to recalc
 *
 * We have to calculate the free blocks since the mm can drop
 * undirtied hole pages behind our back.
 *
 * But normally   info->alloced == inode->i_mapping->nrpages + info->swapped
 * So mm freed is info->alloced - (inode->i_mapping->nrpages + info->swapped)
 *
 * It has to be called with the spinlock held.
 */
static void shmem_recalc_inode(struct inode *inode)
{
	struct shmem_inode_info *info = SHMEM_I(inode);
	long freed;

	freed = info->alloced - info->swapped - inode->i_mapping->nrpages;
	if (freed > 0) {
		info->alloced -= freed;
		inode->i_blocks -= freed * BLOCKS_PER_PAGE;
		shmem_inode_unacct_blocks(inode, freed);
	}
}

bool shmem_charge(struct inode *inode, long pages)
{
	struct shmem_inode_info *info = SHMEM_I(inode);
	unsigned long flags;

	if (!shmem_inode_acct_block(inode, pages))
		return false;

	/* nrpages adjustment first, then shmem_recalc_inode() when balanced */
	inode->i_mapping->nrpages += pages;

	spin_lock_irqsave(&info->lock, flags);
	info->alloced += pages;
	inode->i_blocks += pages * BLOCKS_PER_PAGE;
	shmem_recalc_inode(inode);
	spin_unlock_irqrestore(&info->lock, flags);

	return true;
}

void shmem_uncharge(struct inode *inode, long pages)
{
	struct shmem_inode_info *info = SHMEM_I(inode);
	unsigned long flags;

	/* nrpages adjustment done by __delete_from_page_cache() or caller */

	spin_lock_irqsave(&info->lock, flags);
	info->alloced -= pages;
	inode->i_blocks -= pages * BLOCKS_PER_PAGE;
	shmem_recalc_inode(inode);
	spin_unlock_irqrestore(&info->lock, flags);

	shmem_inode_unacct_blocks(inode, pages);
}

/*
 * Replace item expected in xarray by a new item, while holding xa_lock.
 */
static int shmem_replace_entry(struct address_space *mapping,
			pgoff_t index, void *expected, void *replacement)
{
	XA_STATE(xas, &mapping->i_pages, index);
	void *item;

	VM_BUG_ON(!expected);
	VM_BUG_ON(!replacement);
	item = xas_load(&xas);
	if (item != expected)
		return -ENOENT;
	xas_store(&xas, replacement);
	return 0;
}

/*
 * Sometimes, before we decide whether to proceed or to fail, we must check
 * that an entry was not already brought back from swap by a racing thread.
 *
 * Checking page is not enough: by the time a SwapCache page is locked, it
 * might be reused, and again be SwapCache, using the same swap as before.
 */
static bool shmem_confirm_swap(struct address_space *mapping,
			       pgoff_t index, swp_entry_t swap)
{
	return xa_load(&mapping->i_pages, index) == swp_to_radix_entry(swap);
}

/*
 * Definitions for "huge tmpfs": tmpfs mounted with the huge= option
 *
 * SHMEM_HUGE_NEVER:
 *	disables huge pages for the mount;
 * SHMEM_HUGE_ALWAYS:
 *	enables huge pages for the mount;
 * SHMEM_HUGE_WITHIN_SIZE:
 *	only allocate huge pages if the page will be fully within i_size,
 *	also respect fadvise()/madvise() hints;
 * SHMEM_HUGE_ADVISE:
 *	only allocate huge pages if requested with fadvise()/madvise();
 */

#define SHMEM_HUGE_NEVER	0
#define SHMEM_HUGE_ALWAYS	1
#define SHMEM_HUGE_WITHIN_SIZE	2
#define SHMEM_HUGE_ADVISE	3

/*
 * Special values.
 * Only can be set via /sys/kernel/mm/transparent_hugepage/shmem_enabled:
 *
 * SHMEM_HUGE_DENY:
 *	disables huge on shm_mnt and all mounts, for emergency use;
 * SHMEM_HUGE_FORCE:
 *	enables huge on shm_mnt and all mounts, w/o needing option, for testing;
 *
 */
#define SHMEM_HUGE_DENY		(-1)
#define SHMEM_HUGE_FORCE	(-2)

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
/* ifdef here to avoid bloating shmem.o when not necessary */

static int shmem_huge __read_mostly;

#if defined(CONFIG_SYSFS)
static int shmem_parse_huge(const char *str)
{
	if (!strcmp(str, "never"))
		return SHMEM_HUGE_NEVER;
	if (!strcmp(str, "always"))
		return SHMEM_HUGE_ALWAYS;
	if (!strcmp(str, "within_size"))
		return SHMEM_HUGE_WITHIN_SIZE;
	if (!strcmp(str, "advise"))
		return SHMEM_HUGE_ADVISE;
	if (!strcmp(str, "deny"))
		return SHMEM_HUGE_DENY;
	if (!strcmp(str, "force"))
		return SHMEM_HUGE_FORCE;
	return -EINVAL;
}
#endif

#if defined(CONFIG_SYSFS) || defined(CONFIG_TMPFS)
static const char *shmem_format_huge(int huge)
{
	switch (huge) {
	case SHMEM_HUGE_NEVER:
		return "never";
	case SHMEM_HUGE_ALWAYS:
		return "always";
	case SHMEM_HUGE_WITHIN_SIZE:
		return "within_size";
	case SHMEM_HUGE_ADVISE:
		return "advise";
	case SHMEM_HUGE_DENY:
		return "deny";
	case SHMEM_HUGE_FORCE:
		return "force";
	default:
		VM_BUG_ON(1);
		return "bad_val";
	}
}
#endif

static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
		struct shrink_control *sc, unsigned long nr_to_split)
{
	LIST_HEAD(list), *pos, *next;
	LIST_HEAD(to_remove);
	struct inode *inode;
	struct shmem_inode_info *info;
	struct page *page;
	unsigned long batch = sc ? sc->nr_to_scan : 128;
	int removed = 0, split = 0;

	if (list_empty(&sbinfo->shrinklist))
		return SHRINK_STOP;

	spin_lock(&sbinfo->shrinklist_lock);
	list_for_each_safe(pos, next, &sbinfo->shrinklist) {
		info = list_entry(pos, struct shmem_inode_info, shrinklist);

		/* pin the inode */
		inode = igrab(&info->vfs_inode);

		/* inode is about to be evicted */
		if (!inode) {
			list_del_init(&info->shrinklist);
			removed++;
			goto next;
		}

		/* Check if there's anything to gain */
		if (round_up(inode->i_size, PAGE_SIZE) ==
				round_up(inode->i_size, HPAGE_PMD_SIZE)) {
			list_move(&info->shrinklist, &to_remove);
			removed++;
			goto next;
		}

		list_move(&info->shrinklist, &list);
next:
		if (!--batch)
			break;
	}
	spin_unlock(&sbinfo->shrinklist_lock);

	list_for_each_safe(pos, next, &to_remove) {
		info = list_entry(pos, struct shmem_inode_info, shrinklist);
		inode = &info->vfs_inode;
		list_del_init(&info->shrinklist);
		iput(inode);
	}

	list_for_each_safe(pos, next, &list) {
		int ret;

		info = list_entry(pos, struct shmem_inode_info, shrinklist);
		inode = &info->vfs_inode;

		if (nr_to_split && split >= nr_to_split)
			goto leave;

		page = find_get_page(inode->i_mapping,
				(inode->i_size & HPAGE_PMD_MASK) >> PAGE_SHIFT);
		if (!page)
			goto drop;

		/* No huge page at the end of the file: nothing to split */
		if (!PageTransHuge(page)) {
			put_page(page);
			goto drop;
		}

		/*
		 * Leave the inode on the list if we failed to lock
		 * the page at this time.
		 *
		 * Waiting for the lock may lead to deadlock in the
		 * reclaim path.
		 */
		if (!trylock_page(page)) {
			put_page(page);
			goto leave;
		}

		ret = split_huge_page(page);
		unlock_page(page);
		put_page(page);

		/* If split failed leave the inode on the list */
		if (ret)
			goto leave;

		split++;
drop:
		list_del_init(&info->shrinklist);
		removed++;
leave:
		iput(inode);
	}

	spin_lock(&sbinfo->shrinklist_lock);
	list_splice_tail(&list, &sbinfo->shrinklist);
	sbinfo->shrinklist_len -= removed;
	spin_unlock(&sbinfo->shrinklist_lock);

	return split;
}

static long shmem_unused_huge_scan(struct super_block *sb,
		struct shrink_control *sc)
{
	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);

	if (!READ_ONCE(sbinfo->shrinklist_len))
		return SHRINK_STOP;

	return shmem_unused_huge_shrink(sbinfo, sc, 0);
}

static long shmem_unused_huge_count(struct super_block *sb,
		struct shrink_control *sc)
{
	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
	return READ_ONCE(sbinfo->shrinklist_len);
}
#else /* !CONFIG_TRANSPARENT_HUGEPAGE */

#define shmem_huge SHMEM_HUGE_DENY

static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
		struct shrink_control *sc, unsigned long nr_to_split)
{
	return 0;
}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

static inline bool is_huge_enabled(struct shmem_sb_info *sbinfo)
{
	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
	    (shmem_huge == SHMEM_HUGE_FORCE || sbinfo->huge) &&
	    shmem_huge != SHMEM_HUGE_DENY)
		return true;
	return false;
}

/*
 * Like add_to_page_cache_locked, but error if expected item has gone.
 */
static int shmem_add_to_page_cache(struct page *page,
				   struct address_space *mapping,
				   pgoff_t index, void *expected, gfp_t gfp,
				   struct mm_struct *charge_mm)
{
	XA_STATE_ORDER(xas, &mapping->i_pages, index, compound_order(page));
	unsigned long i = 0;
	unsigned long nr = compound_nr(page);
	int error;

	VM_BUG_ON_PAGE(PageTail(page), page);
	VM_BUG_ON_PAGE(index != round_down(index, nr), page);
	VM_BUG_ON_PAGE(!PageLocked(page), page);
	VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
	VM_BUG_ON(expected && PageTransHuge(page));

	page_ref_add(page, nr);
	page->mapping = mapping;
	page->index = index;

	if (!PageSwapCache(page)) {
		error = mem_cgroup_charge(page, charge_mm, gfp);
		if (error) {
			if (PageTransHuge(page)) {
				count_vm_event(THP_FILE_FALLBACK);
				count_vm_event(THP_FILE_FALLBACK_CHARGE);
			}
			goto error;
		}
	}
	cgroup_throttle_swaprate(page, gfp);

	do {
		void *entry;
		xas_lock_irq(&xas);
		entry = xas_find_conflict(&xas);
		if (entry != expected)
			xas_set_err(&xas, -EEXIST);
		xas_create_range(&xas);
		if (xas_error(&xas))
			goto unlock;
next:
		xas_store(&xas, page);
		if (++i < nr) {
			xas_next(&xas);
			goto next;
		}
		if (PageTransHuge(page)) {
			count_vm_event(THP_FILE_ALLOC);
			__inc_node_page_state(page, NR_SHMEM_THPS);
		}
		mapping->nrpages += nr;
		__mod_lruvec_page_state(page, NR_FILE_PAGES, nr);
		__mod_lruvec_page_state(page, NR_SHMEM, nr);
unlock:
		xas_unlock_irq(&xas);
	} while (xas_nomem(&xas, gfp));

	if (xas_error(&xas)) {
		error = xas_error(&xas);
		goto error;
	}

	return 0;
error:
	page->mapping = NULL;
	page_ref_sub(page, nr);
	return error;
}

/*
 * Like delete_from_page_cache, but substitutes swap for page.
 */
static void shmem_delete_from_page_cache(struct page *page, void *radswap)
{
	struct address_space *mapping = page->mapping;
	int error;

	VM_BUG_ON_PAGE(PageCompound(page), page);

	xa_lock_irq(&mapping->i_pages);
	error = shmem_replace_entry(mapping, page->index, page, radswap);
	page->mapping = NULL;
	mapping->nrpages--;
	__dec_lruvec_page_state(page, NR_FILE_PAGES);
	__dec_lruvec_page_state(page, NR_SHMEM);
	xa_unlock_irq(&mapping->i_pages);
	put_page(page);
	BUG_ON(error);
}

/*
 * Remove swap entry from page cache, free the swap and its page cache.
 */
static int shmem_free_swap(struct address_space *mapping,
			   pgoff_t index, void *radswap)
{
	void *old;

	old = xa_cmpxchg_irq(&mapping->i_pages, index, radswap, NULL, 0);
	if (old != radswap)
		return -ENOENT;
	free_swap_and_cache(radix_to_swp_entry(radswap));
	return 0;
}

/*
 * Determine (in bytes) how many of the shmem object's pages mapped by the
 * given offsets are swapped out.
 *
 * This is safe to call without i_mutex or the i_pages lock thanks to RCU,
 * as long as the inode doesn't go away and racy results are not a problem.
 */
unsigned long shmem_partial_swap_usage(struct address_space *mapping,
						pgoff_t start, pgoff_t end)
{
	XA_STATE(xas, &mapping->i_pages, start);
	struct page *page;
	unsigned long swapped = 0;

	rcu_read_lock();
	xas_for_each(&xas, page, end - 1) {
		if (xas_retry(&xas, page))
			continue;
		if (xa_is_value(page))
			swapped++;

		if (need_resched()) {
			xas_pause(&xas);
			cond_resched_rcu();
		}
	}

	rcu_read_unlock();

	return swapped << PAGE_SHIFT;
}

/*
 * Determine (in bytes) how many of the shmem object's pages mapped by the
 * given vma is swapped out.
 *
 * This is safe to call without i_mutex or the i_pages lock thanks to RCU,
 * as long as the inode doesn't go away and racy results are not a problem.
 */
unsigned long shmem_swap_usage(struct vm_area_struct *vma)
{
	struct inode *inode = file_inode(vma->vm_file);
	struct shmem_inode_info *info = SHMEM_I(inode);
	struct address_space *mapping = inode->i_mapping;
	unsigned long swapped;

	/* Be careful as we don't hold info->lock */
	swapped = READ_ONCE(info->swapped);

	/*
	 * The easier cases are when the shmem object has nothing in swap, or
	 * the vma maps it whole. Then we can simply use the stats that we
	 * already track.
	 */
	if (!swapped)
		return 0;

	if (!vma->vm_pgoff && vma->vm_end - vma->vm_start >= inode->i_size)
		return swapped << PAGE_SHIFT;

	/* Here comes the more involved part */
	return shmem_partial_swap_usage(mapping,
			linear_page_index(vma, vma->vm_start),
			linear_page_index(vma, vma->vm_end));
}

/*
 * SysV IPC SHM_UNLOCK restore Unevictable pages to their evictable lists.
 */
void shmem_unlock_mapping(struct address_space *mapping)
{
	struct pagevec pvec;
	pgoff_t indices[PAGEVEC_SIZE];
	pgoff_t index = 0;

	pagevec_init(&pvec);
	/*
	 * Minor point, but we might as well stop if someone else SHM_LOCKs it.
	 */
	while (!mapping_unevictable(mapping)) {
		/*
		 * Avoid pagevec_lookup(): find_get_pages() returns 0 as if it
		 * has finished, if it hits a row of PAGEVEC_SIZE swap entries.
		 */
		pvec.nr = find_get_entries(mapping, index,
					   PAGEVEC_SIZE, pvec.pages, indices);
		if (!pvec.nr)
			break;
		index = indices[pvec.nr - 1] + 1;
		pagevec_remove_exceptionals(&pvec);
		check_move_unevictable_pages(&pvec);
		pagevec_release(&pvec);
		cond_resched();
	}
}

/*
 * Check whether a hole-punch or truncation needs to split a huge page,
 * returning true if no split was required, or the split has been successful.
 *
 * Eviction (or truncation to 0 size) should never need to split a huge page;
 * but in rare cases might do so, if shmem_undo_range() failed to trylock on
 * head, and then succeeded to trylock on tail.
 *
 * A split can only succeed when there are no additional references on the
 * huge page: so the split below relies upon find_get_entries() having stopped
 * when it found a subpage of the huge page, without getting further references.
 */
static bool shmem_punch_compound(struct page *page, pgoff_t start, pgoff_t end)
{
	if (!PageTransCompound(page))
		return true;

	/* Just proceed to delete a huge page wholly within the range punched */
	if (PageHead(page) &&
	    page->index >= start && page->index + HPAGE_PMD_NR <= end)
		return true;

	/* Try to split huge page, so we can truly punch the hole or truncate */
	return split_huge_page(page) >= 0;
}

/*
 * Remove range of pages and swap entries from page cache, and free them.
 * If !unfalloc, truncate or punch hole; if unfalloc, undo failed fallocate.
 */
static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
								 bool unfalloc)
{
	struct address_space *mapping = inode->i_mapping;
	struct shmem_inode_info *info = SHMEM_I(inode);
	pgoff_t start = (lstart + PAGE_SIZE - 1) >> PAGE_SHIFT;
	pgoff_t end = (lend + 1) >> PAGE_SHIFT;
	unsigned int partial_start = lstart & (PAGE_SIZE - 1);
	unsigned int partial_end = (lend + 1) & (PAGE_SIZE - 1);
	struct pagevec pvec;
	pgoff_t indices[PAGEVEC_SIZE];
	long nr_swaps_freed = 0;
	pgoff_t index;
	int i;

	if (lend == -1)
		end = -1;	/* unsigned, so actually very big */

	pagevec_init(&pvec);
	index = start;
	while (index < end) {
		pvec.nr = find_get_entries(mapping, index,
			min(end - index, (pgoff_t)PAGEVEC_SIZE),
			pvec.pages, indices);
		if (!pvec.nr)
			break;
		for (i = 0; i < pagevec_count(&pvec); i++) {
			struct page *page = pvec.pages[i];

			index = indices[i];
			if (index >= end)
				break;

			if (xa_is_value(page)) {
				if (unfalloc)
					continue;
				nr_swaps_freed += !shmem_free_swap(mapping,
								index, page);
				continue;
			}

			VM_BUG_ON_PAGE(page_to_pgoff(page) != index, page);

			if (!trylock_page(page))
				continue;

			if ((!unfalloc || !PageUptodate(page)) &&
			    page_mapping(page) == mapping) {
				VM_BUG_ON_PAGE(PageWriteback(page), page);
				if (shmem_punch_compound(page, start, end))
					truncate_inode_page(mapping, page);
			}
			unlock_page(page);
		}
		pagevec_remove_exceptionals(&pvec);
		pagevec_release(&pvec);
		cond_resched();
		index++;
	}

	if (partial_start) {
		struct page *page = NULL;
		shmem_getpage(inode, start - 1, &page, SGP_READ);
		if (page) {
			unsigned int top = PAGE_SIZE;
			if (start > end) {
				top = partial_end;
				partial_end = 0;
			}
			zero_user_segment(page, partial_start, top);
			set_page_dirty(page);
			unlock_page(page);
			put_page(page);
		}
	}
	if (partial_end) {
		struct page *page = NULL;
		shmem_getpage(inode, end, &page, SGP_READ);
		if (page) {
			zero_user_segment(page, 0, partial_end);
			set_page_dirty(page);
			unlock_page(page);
			put_page(page);
		}
	}
	if (start >= end)
		return;

	index = start;
	while (index < end) {
		cond_resched();

		pvec.nr = find_get_entries(mapping, index,
				min(end - index, (pgoff_t)PAGEVEC_SIZE),
				pvec.pages, indices);
		if (!pvec.nr) {
			/* If all gone or hole-punch or unfalloc, we're done */
			if (index == start || end != -1)
				break;
			/* But if truncating, restart to make sure all gone */
			index = start;
			continue;
		}
		for (i = 0; i < pagevec_count(&pvec); i++) {
			struct page *page = pvec.pages[i];

			index = indices[i];
			if (index >= end)
				break;

			if (xa_is_value(page)) {
				if (unfalloc)
					continue;
				if (shmem_free_swap(mapping, index, page)) {
					/* Swap was replaced by page: retry */
					index--;
					break;
				}
				nr_swaps_freed++;
				continue;
			}

			lock_page(page);

			if (!unfalloc || !PageUptodate(page)) {
				if (page_mapping(page) != mapping) {
					/* Page was replaced by swap: retry */
					unlock_page(page);
					index--;
					break;
				}
				VM_BUG_ON_PAGE(PageWriteback(page), page);
				if (shmem_punch_compound(page, start, end))
					truncate_inode_page(mapping, page);
				else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
					/* Wipe the page and don't get stuck */
					clear_highpage(page);
					flush_dcache_page(page);
					set_page_dirty(page);
					if (index <
					    round_up(start, HPAGE_PMD_NR))
						start = index + 1;
				}
			}
			unlock_page(page);
		}
		pagevec_remove_exceptionals(&pvec);
		pagevec_release(&pvec);
		index++;
	}

	spin_lock_irq(&info->lock);
	info->swapped -= nr_swaps_freed;
	shmem_recalc_inode(inode);
	spin_unlock_irq(&info->lock);
}

void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
{
	shmem_undo_range(inode, lstart, lend, false);
	inode->i_ctime = inode->i_mtime = current_time(inode);
}
EXPORT_SYMBOL_GPL(shmem_truncate_range);