/*
   * 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/sched/signal.h>

  #include <linux/export.h>

  #include <linux/swap.h>

  #include <linux/uio.h>

  #include <linux/khugepaged.h>

  #include <linux/hugetlb.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 <asm/pgtable.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 */
  };

  #ifdef CONFIG_TMPFS

  static unsigned long shmem_default_max_blocks(void)
  {
  	return totalram_pages / 2;
  }
  
  static unsigned long shmem_default_max_inodes(void)
  {
  	return min(totalram_pages - totalhigh_pages, totalram_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_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, int *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;

  	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);

  	inode->i_mapping->nrpages += pages;

  	return true;
  }
  
  void shmem_uncharge(struct inode *inode, long pages)
  {
  	struct shmem_inode_info *info = SHMEM_I(inode);

  	unsigned long flags;

  	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 radix tree by a new item, while holding tree lock.
   */
  static int shmem_radix_tree_replace(struct address_space *mapping,
  			pgoff_t index, void *expected, void *replacement)
  {

  	struct radix_tree_node *node;

  	void **pslot;

  	void *item;

  
  	VM_BUG_ON(!expected);

  	VM_BUG_ON(!replacement);

  	item = __radix_tree_lookup(&mapping->page_tree, index, &node, &pslot);
  	if (!item)

  		return -ENOENT;

  	if (item != expected)
  		return -ENOENT;

  	__radix_tree_replace(&mapping->page_tree, node, pslot,
  			     replacement, NULL, NULL);

  	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)
  {
  	void *item;
  
  	rcu_read_lock();
  	item = radix_tree_lookup(&mapping->page_tree, index);
  	rcu_read_unlock();
  	return item == 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_HUGE_PAGECACHE

  /* ifdef here to avoid bloating shmem.o when not necessary */
  
  int shmem_huge __read_mostly;

  #if defined(CONFIG_SYSFS) || defined(CONFIG_TMPFS)

  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;
  }
  
  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_HUGE_PAGECACHE */

  
  #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_HUGE_PAGECACHE */

  
  /*

   * 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)

  {

  	int error, nr = hpage_nr_pages(page);

  	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;
  
  	spin_lock_irq(&mapping->tree_lock);

  	if (PageTransHuge(page)) {
  		void __rcu **results;
  		pgoff_t idx;
  		int i;
  
  		error = 0;
  		if (radix_tree_gang_lookup_slot(&mapping->page_tree,
  					&results, &idx, index, 1) &&
  				idx < index + HPAGE_PMD_NR) {
  			error = -EEXIST;
  		}
  
  		if (!error) {
  			for (i = 0; i < HPAGE_PMD_NR; i++) {
  				error = radix_tree_insert(&mapping->page_tree,
  						index + i, page + i);
  				VM_BUG_ON(error);
  			}
  			count_vm_event(THP_FILE_ALLOC);
  		}
  	} else if (!expected) {

  		error = radix_tree_insert(&mapping->page_tree, index, page);

  	} else {

  		error = shmem_radix_tree_replace(mapping, index, expected,
  								 page);

  	}

  	if (!error) {

  		mapping->nrpages += nr;
  		if (PageTransHuge(page))

  			__inc_node_page_state(page, NR_SHMEM_THPS);
  		__mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, nr);
  		__mod_node_page_state(page_pgdat(page), NR_SHMEM, nr);

  		spin_unlock_irq(&mapping->tree_lock);
  	} else {
  		page->mapping = NULL;
  		spin_unlock_irq(&mapping->tree_lock);

  		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);

  	spin_lock_irq(&mapping->tree_lock);
  	error = shmem_radix_tree_replace(mapping, page->index, page, radswap);
  	page->mapping = NULL;
  	mapping->nrpages--;

  	__dec_node_page_state(page, NR_FILE_PAGES);
  	__dec_node_page_state(page, NR_SHMEM);

  	spin_unlock_irq(&mapping->tree_lock);

  	put_page(page);

  	BUG_ON(error);
  }
  
  /*

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

  	void *old;

  
  	spin_lock_irq(&mapping->tree_lock);

  	old = radix_tree_delete_item(&mapping->page_tree, index, radswap);

  	spin_unlock_irq(&mapping->tree_lock);

  	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 mapping->tree_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)

  {

  	struct radix_tree_iter iter;
  	void **slot;
  	struct page *page;

  	unsigned long swapped = 0;

  
  	rcu_read_lock();

  	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
  		if (iter.index >= end)
  			break;
  
  		page = radix_tree_deref_slot(slot);

  		if (radix_tree_deref_retry(page)) {
  			slot = radix_tree_iter_retry(&iter);
  			continue;
  		}

  
  		if (radix_tree_exceptional_entry(page))
  			swapped++;
  
  		if (need_resched()) {

  			slot = radix_tree_iter_resume(slot, &iter);

  			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 mapping->tree_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, 0);
  	/*
  	 * 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.pages, pvec.nr);
  		pagevec_release(&pvec);
  		cond_resched();
  	}

  }
  
  /*
   * Remove range of pages and swap entries from radix tree, 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, 0);
  	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 (radix_tree_exceptional_entry(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 (PageTransTail(page)) {
  				/* Middle of THP: zero out the page */
  				clear_highpage(page);
  				unlock_page(page);
  				continue;
  			} else if (PageTransHuge(page)) {
  				if (index == round_down(end, HPAGE_PMD_NR)) {
  					/*
  					 * Range ends in the middle of THP:
  					 * zero out the page
  					 */
  					clear_highpage(page);
  					unlock_page(page);
  					continue;
  				}
  				index += HPAGE_PMD_NR - 1;
  				i += HPAGE_PMD_NR - 1;
  			}

  			if (!unfalloc || !PageUptodate(page)) {

  				VM_BUG_ON_PAGE(PageTail(page), page);
  				if (page_mapping(page) == mapping) {

  					VM_BUG_ON_PAGE(PageWriteback(page), page);

  					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 (radix_tree_exceptional_entry(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 (PageTransTail(page)) {
  				/* Middle of THP: zero out the page */
  				clear_highpage(page);
  				unlock_page(page);
  				/*
  				 * Partial thp truncate due 'start' in middle
  				 * of THP: don't need to look on these pages
  				 * again on !pvec.nr restart.
  				 */
  				if (index != round_down(end, HPAGE_PMD_NR))
  					start++;
  				continue;
  			} else if (PageTransHuge(page)) {
  				if (index == round_down(end, HPAGE_PMD_NR)) {
  					/*
  					 * Range ends in the middle of THP:
  					 * zero out the page
  					 */
  					clear_highpage(page);
  					unlock_page(page);
  					continue;
  				}
  				index += HPAGE_PMD_NR - 1;
  				i += HPAGE_PMD_NR - 1;
  			}

  			if (!unfalloc || !PageUptodate(page)) {

  				VM_BUG_ON_PAGE(PageTail(page), page);
  				if (page_mapping(page) == mapping) {

  					VM_BUG_ON_PAGE(PageWriteback(page), page);

  					truncate_inode_page(mapping, page);

  				} else {
  					/* Page was replaced by swap: retry */
  					unlock_page(page);
  					index--;
  					break;

  				}

  			}

  			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);

  static int shmem_getattr(const struct path *path, struct kstat *stat,
  			 u32 request_mask, unsigned int query_flags)

  {

  	struct inode *inode = path->dentry->d_inode;

  	struct shmem_inode_info *info = SHMEM_I(inode);

  	if (info->alloced - info->swapped != inode->i_mapping->nrpages) {

  		spin_lock_irq(&info->lock);

  		shmem_recalc_inode(inode);

  		spin_unlock_irq(&info->lock);

  	}

  	generic_fillattr(inode, stat);

  	return 0;
  }

  static int shmem_setattr(struct dentry *dentry, struct iattr *attr)

  {

  	struct inode *inode = d_inode(dentry);

  	struct shmem_inode_info *info = SHMEM_I(inode);

  	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);

  	int error;

  	error = setattr_prepare(dentry, attr);

  	if (error)
  		return error;

  	if (S_ISREG(inode->i_mode) && (attr->ia_valid & ATTR_SIZE)) {
  		loff_t oldsize = inode->i_size;
  		loff_t newsize = attr->ia_size;

  		/* protected by i_mutex */
  		if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
  		    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
  			return -EPERM;

  		if (newsize != oldsize) {

  			error = shmem_reacct_size(SHMEM_I(inode)->flags,
  					oldsize, newsize);
  			if (error)
  				return error;

  			i_size_write(inode, newsize);

  			inode->i_ctime = inode->i_mtime = current_time(inode);

  		}

  		if (newsize <= oldsize) {

  			loff_t holebegin = round_up(newsize, PAGE_SIZE);

  			if (oldsize > holebegin)
  				unmap_mapping_range(inode->i_mapping,
  							holebegin, 0, 1);
  			if (info->alloced)
  				shmem_truncate_range(inode,
  							newsize, (loff_t)-1);

  			/* unmap again to remove racily COWed private pages */

  			if (oldsize > holebegin)
  				unmap_mapping_range(inode->i_mapping,
  							holebegin, 0, 1);

  
  			/*
  			 * Part of the huge page can be beyond i_size: subject
  			 * to shrink under memory pressure.
  			 */
  			if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE)) {
  				spin_lock(&sbinfo->shrinklist_lock);

  				/*
  				 * _careful to defend against unlocked access to
  				 * ->shrink_list in shmem_unused_huge_shrink()
  				 */
  				if (list_empty_careful(&info->shrinklist)) {

  					list_add_tail(&info->shrinklist,
  							&sbinfo->shrinklist);
  					sbinfo->shrinklist_len++;
  				}
  				spin_unlock(&sbinfo->shrinklist_lock);
  			}

  		}

  	}

  	setattr_copy(inode, attr);

  	if (attr->ia_valid & ATTR_MODE)

  		error = posix_acl_chmod(inode, inode->i_mode);

  	return error;
  }

  static void shmem_evict_inode(struct inode *inode)

  {

  	struct shmem_inode_info *info = SHMEM_I(inode);

  	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);

  	if (inode->i_mapping->a_ops == &shmem_aops) {

  		shmem_unacct_size(info->flags, inode->i_size);
  		inode->i_size = 0;

  		shmem_truncate_range(inode, 0, (loff_t)-1);

  		if (!list_empty(&info->shrinklist)) {
  			spin_lock(&sbinfo->shrinklist_lock);
  			if (!list_empty(&info->shrinklist)) {
  				list_del_init(&info->shrinklist);
  				sbinfo->shrinklist_len--;
  			}
  			spin_unlock(&sbinfo->shrinklist_lock);
  		}

  		if (!list_empty(&info->swaplist)) {

  			mutex_lock(&shmem_swaplist_mutex);

  			list_del_init(&info->swaplist);

  			mutex_unlock(&shmem_swaplist_mutex);

  		}

  	}

  	simple_xattrs_free(&info->xattrs);

  	WARN_ON(inode->i_blocks);

  	shmem_free_inode(inode->i_sb);

  	clear_inode(inode);

  }

  static unsigned long find_swap_entry(struct radix_tree_root *root, void *item)
  {
  	struct radix_tree_iter iter;
  	void **slot;
  	unsigned long found = -1;
  	unsigned int checked = 0;
  
  	rcu_read_lock();
  	radix_tree_for_each_slot(slot, root, &iter, 0) {
  		if (*slot == item) {
  			found = iter.index;
  			break;
  		}
  		checked++;
  		if ((checked % 4096) != 0)
  			continue;
  		slot = radix_tree_iter_resume(slot, &iter);
  		cond_resched_rcu();
  	}
  
  	rcu_read_unlock();
  	return found;
  }

  /*
   * If swap found in inode, free it and move page from swapcache to filecache.
   */

  static int shmem_unuse_inode(struct shmem_inode_info *info,

  			     swp_entry_t swap, struct page **pagep)

  {

  	struct address_space *mapping = info->vfs_inode.i_mapping;

  	void *radswap;

  	pgoff_t index;

  	gfp_t gfp;
  	int error = 0;

  	radswap = swp_to_radix_entry(swap);

  	index = find_swap_entry(&mapping->page_tree, radswap);

  	if (index == -1)

  		return -EAGAIN;	/* tell shmem_unuse we found nothing */

  	/*
  	 * Move _head_ to start search for next from here.

  	 * But be careful: shmem_evict_inode checks list_empty without taking

  	 * mutex, and there's an instant in list_move_tail when info->swaplist

  	 * would appear empty, if it were the only one on shmem_swaplist.

  	 */
  	if (shmem_swaplist.next != &info->swaplist)
  		list_move_tail(&shmem_swaplist, &info->swaplist);

  	gfp = mapping_gfp_mask(mapping);
  	if (shmem_should_replace_page(*pagep, gfp)) {
  		mutex_unlock(&shmem_swaplist_mutex);
  		error = shmem_replace_page(pagep, gfp, info, index);
  		mutex_lock(&shmem_swaplist_mutex);
  		/*
  		 * We needed to drop mutex to make that restrictive page

  		 * allocation, but the inode might have been freed while we
  		 * dropped it: although a racing shmem_evict_inode() cannot
  		 * complete without emptying the radix_tree, our page lock
  		 * on this swapcache page is not enough to prevent that -
  		 * free_swap_and_cache() of our swap entry will only
  		 * trylock_page(), removing swap from radix_tree whatever.
  		 *
  		 * We must not proceed to shmem_add_to_page_cache() if the
  		 * inode has been freed, but of course we cannot rely on
  		 * inode or mapping or info to check that.  However, we can
  		 * safely check if our swap entry is still in use (and here
  		 * it can't have got reused for another page): if it's still
  		 * in use, then the inode cannot have been freed yet, and we
  		 * can safely proceed (if it's no longer in use, that tells
  		 * nothing about the inode, but we don't need to unuse swap).

  		 */
  		if (!page_swapcount(*pagep))
  			error = -ENOENT;
  	}

  	/*

  	 * We rely on shmem_swaplist_mutex, not only to protect the swaplist,
  	 * but also to hold up shmem_evict_inode(): so inode cannot be freed
  	 * beneath us (pagelock doesn't help until the page is in pagecache).

  	 */

  	if (!error)
  		error = shmem_add_to_page_cache(*pagep, mapping, index,

  						radswap);

  	if (error != -ENOMEM) {

  		/*
  		 * Truncation and eviction use free_swap_and_cache(), which
  		 * only does trylock page: if we raced, best clean up here.
  		 */

  		delete_from_swap_cache(*pagep);
  		set_page_dirty(*pagep);

  		if (!error) {

  			spin_lock_irq(&info->lock);

  			info->swapped--;

  			spin_unlock_irq(&info->lock);

  			swap_free(swap);
  		}

  	}

  	return error;

  }
  
  /*

   * Search through swapped inodes to find and replace swap by page.

   */

  int shmem_unuse(swp_entry_t swap, struct page *page)

  {

  	struct list_head *this, *next;

  	struct shmem_inode_info *info;

  	struct mem_cgroup *memcg;

  	int error = 0;
  
  	/*
  	 * There's a faint possibility that swap page was replaced before

  	 * caller locked it: caller will come back later with the right page.

  	 */

  	if (unlikely(!PageSwapCache(page) || page_private(page) != swap.val))

  		goto out;

  
  	/*
  	 * Charge page using GFP_KERNEL while we can wait, before taking
  	 * the shmem_swaplist_mutex which might hold up shmem_writepage().
  	 * Charged back to the user (not to caller) when swap account is used.

  	 */

  	error = mem_cgroup_try_charge(page, current->mm, GFP_KERNEL, &memcg,
  			false);

  	if (error)
  		goto out;

  	/* No radix_tree_preload: swap entry keeps a place for page in tree */

  	error = -EAGAIN;

  	mutex_lock(&shmem_swaplist_mutex);

  	list_for_each_safe(this, next, &shmem_swaplist) {
  		info = list_entry(this, struct shmem_inode_info, swaplist);

  		if (info->swapped)

  			error = shmem_unuse_inode(info, swap, &page);

  		else
  			list_del_init(&info->swaplist);

  		cond_resched();

  		if (error != -EAGAIN)

  			break;

  		/* found nothing in this: move on to search the next */

  	}

  	mutex_unlock(&shmem_swaplist_mutex);

  	if (error) {
  		if (error != -ENOMEM)
  			error = 0;

  		mem_cgroup_cancel_charge(page, memcg, false);

  	} else

  		mem_cgroup_commit_charge(page, memcg, true, false);

  out:

  	unlock_page(page);

  	put_page(page);

  	return error;

  }
  
  /*
   * Move the page from the page cache to the swap cache.
   */
  static int shmem_writepage(struct page *page, struct writeback_control *wbc)
  {
  	struct shmem_inode_info *info;

  	struct address_space *mapping;

  	struct inode *inode;

  	swp_entry_t swap;
  	pgoff_t index;

  	VM_BUG_ON_PAGE(PageCompound(page), page);

  	BUG_ON(!PageLocked(page));

  	mapping = page->mapping;
  	index = page->index;
  	inode = mapping->host;
  	info = SHMEM_I(inode);
  	if (info->flags & VM_LOCKED)
  		goto redirty;

  	if (!total_swap_pages)

  		goto redirty;

  	/*

  	 * Our capabilities prevent regular writeback or sync from ever calling
  	 * shmem_writepage; but a stacking filesystem might use ->writepage of
  	 * its underlying filesystem, in which case tmpfs should write out to
  	 * swap only in response to memory pressure, and not for the writeback
  	 * threads or sync.

  	 */

  	if (!wbc->for_reclaim) {
  		WARN_ON_ONCE(1);	/* Still happens? Tell us about it! */
  		goto redirty;
  	}

  
  	/*
  	 * This is somewhat ridiculous, but without plumbing a SWAP_MAP_FALLOC
  	 * value into swapfile.c, the only way we can correctly account for a
  	 * fallocated page arriving here is now to initialize it and write it.

  	 *
  	 * That's okay for a page already fallocated earlier, but if we have
  	 * not yet completed the fallocation, then (a) we want to keep track
  	 * of this page in case we have to undo it, and (b) it may not be a
  	 * good idea to continue anyway, once we're pushing into swap.  So
  	 * reactivate the page, and let shmem_fallocate() quit when too many.

  	 */
  	if (!PageUptodate(page)) {

  		if (inode->i_private) {
  			struct shmem_falloc *shmem_falloc;
  			spin_lock(&inode->i_lock);
  			shmem_falloc = inode->i_private;
  			if (shmem_falloc &&

  			    !shmem_falloc->waitq &&

  			    index >= shmem_falloc->start &&
  			    index < shmem_falloc->next)
  				shmem_falloc->nr_unswapped++;
  			else
  				shmem_falloc = NULL;
  			spin_unlock(&inode->i_lock);
  			if (shmem_falloc)
  				goto redirty;
  		}

  		clear_highpage(page);
  		flush_dcache_page(page);
  		SetPageUptodate(page);
  	}

  	swap = get_swap_page(page);

  	if (!swap.val)
  		goto redirty;

  	if (mem_cgroup_try_charge_swap(page, swap))
  		goto free_swap;

  	/*
  	 * Add inode to shmem_unuse()'s list of swapped-out inodes,

  	 * if it's not already there.  Do it now before the page is
  	 * moved to swap cache, when its pagelock no longer protects

  	 * the inode from eviction.  But don't unlock the mutex until

  	 * we've incremented swapped, because shmem_unuse_inode() will
  	 * prune a !swapped inode from the swaplist under this mutex.

  	 */

  	mutex_lock(&shmem_swaplist_mutex);
  	if (list_empty(&info->swaplist))
  		list_add_tail(&info->swaplist, &shmem_swaplist);

  	if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) {

  		spin_lock_irq(&info->lock);

  		shmem_recalc_inode(inode);

  		info->swapped++;

  		spin_unlock_irq(&info->lock);

  		swap_shmem_alloc(swap);
  		shmem_delete_from_page_cache(page, swp_to_radix_entry(swap));

  		mutex_unlock(&shmem_swaplist_mutex);

  		BUG_ON(page_mapped(page));

  		swap_writepage(page, wbc);

  		return 0;
  	}

  	mutex_unlock(&shmem_swaplist_mutex);

  free_swap:

  	put_swap_page(page, swap);

  redirty:
  	set_page_dirty(page);

  	if (wbc->for_reclaim)
  		return AOP_WRITEPAGE_ACTIVATE;	/* Return with page locked */
  	unlock_page(page);
  	return 0;

  }

  #if defined(CONFIG_NUMA) && defined(CONFIG_TMPFS)

  static void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)

  {

  	char buffer[64];

  	if (!mpol || mpol->mode == MPOL_DEFAULT)

  		return;		/* show nothing */

  	mpol_to_str(buffer, sizeof(buffer), mpol);

  
  	seq_printf(seq, ",mpol=%s", buffer);

  }

  
  static struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
  {
  	struct mempolicy *mpol = NULL;
  	if (sbinfo->mpol) {
  		spin_lock(&sbinfo->stat_lock);	/* prevent replace/use races */
  		mpol = sbinfo->mpol;
  		mpol_get(mpol);
  		spin_unlock(&sbinfo->stat_lock);
  	}
  	return mpol;
  }

  #else /* !CONFIG_NUMA || !CONFIG_TMPFS */
  static inline void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
  {
  }
  static inline struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
  {
  	return NULL;
  }
  #endif /* CONFIG_NUMA && CONFIG_TMPFS */
  #ifndef CONFIG_NUMA
  #define vm_policy vm_private_data
  #endif

  static void shmem_pseudo_vma_init(struct vm_area_struct *vma,
  		struct shmem_inode_info *info, pgoff_t index)
  {
  	/* Create a pseudo vma that just contains the policy */
  	vma->vm_start = 0;
  	/* Bias interleave by inode number to distribute better across nodes */
  	vma->vm_pgoff = index + info->vfs_inode.i_ino;
  	vma->vm_ops = NULL;
  	vma->vm_policy = mpol_shared_policy_lookup(&info->policy, index);
  }
  
  static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
  {
  	/* Drop reference taken by mpol_shared_policy_lookup() */
  	mpol_cond_put(vma->vm_policy);
  }

  static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
  			struct shmem_inode_info *info, pgoff_t index)

  {

  	struct vm_area_struct pvma;

  	struct page *page;

  	shmem_pseudo_vma_init(&pvma, info, index);

  	page = swapin_readahead(swap, gfp, &pvma, 0);

  	shmem_pseudo_vma_destroy(&pvma);

  	return page;
  }
  
  static struct page *shmem_alloc_hugepage(gfp_t gfp,
  		struct shmem_inode_info *info, pgoff_t index)
  {
  	struct vm_area_struct pvma;
  	struct inode *inode = &info->vfs_inode;
  	struct address_space *mapping = inode->i_mapping;

  	pgoff_t idx, hindex;

  	void __rcu **results;
  	struct page *page;

  	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))

  		return NULL;

  	hindex = round_down(index, HPAGE_PMD_NR);

  	rcu_read_lock();
  	if (radix_tree_gang_lookup_slot(&mapping->page_tree, &results, &idx,
  				hindex, 1) && idx < hindex + HPAGE_PMD_NR) {
  		rcu_read_unlock();
  		return NULL;
  	}
  	rcu_read_unlock();

  	shmem_pseudo_vma_init(&pvma, info, hindex);
  	page = alloc_pages_vma(gfp | __GFP_COMP | __GFP_NORETRY | __GFP_NOWARN,
  			HPAGE_PMD_ORDER, &pvma, 0, numa_node_id(), true);
  	shmem_pseudo_vma_destroy(&pvma);
  	if (page)
  		prep_transhuge_page(page);

  	return page;

  }

  static struct page *shmem_alloc_page(gfp_t gfp,

  			struct shmem_inode_info *info, pgoff_t index)

  {
  	struct vm_area_struct pvma;

  	struct page *page;

  	shmem_pseudo_vma_init(&pvma, info, index);
  	page = alloc_page_vma(gfp, &pvma, 0);
  	shmem_pseudo_vma_destroy(&pvma);
  
  	return page;
  }
  
  static struct page *shmem_alloc_and_acct_page(gfp_t gfp,

  		struct inode *inode,

  		pgoff_t index, bool huge)
  {

  	struct shmem_inode_info *info = SHMEM_I(inode);

  	struct page *page;
  	int nr;
  	int err = -ENOSPC;

  	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))

  		huge = false;
  	nr = huge ? HPAGE_PMD_NR : 1;

  	if (!shmem_inode_acct_block(inode, nr))

  		goto failed;

  
  	if (huge)
  		page = shmem_alloc_hugepage(gfp, info, index);
  	else
  		page = shmem_alloc_page(gfp, info, index);

  	if (page) {
  		__SetPageLocked(page);
  		__SetPageSwapBacked(page);

  		return page;

  	}

  	err = -ENOMEM;

  	shmem_inode_unacct_blocks(inode, nr);

  failed:
  	return ERR_PTR(err);

  }

  /*

   * When a page is moved from swapcache to shmem filecache (either by the
   * usual swapin of shmem_getpage_gfp(), or by the less common swapoff of
   * shmem_unuse_inode()), it may have been read in earlier from swap, in
   * ignorance of the mapping it belongs to.  If that mapping has special
   * constraints (like the gma500 GEM driver, which requires RAM below 4GB),
   * we may need to copy to a suitable page before moving to filecache.
   *
   * In a future release, this may well be extended to respect cpuset and
   * NUMA mempolicy, and applied also to anonymous pages in do_swap_page();
   * but for now it is a simple matter of zone.
   */
  static bool shmem_should_replace_page(struct page *page, gfp_t gfp)
  {
  	return page_zonenum(page) > gfp_zone(gfp);
  }
  
  static int shmem_replace_page(struct page **pagep, gfp_t gfp,
  				struct shmem_inode_info *info, pgoff_t index)
  {
  	struct page *oldpage, *newpage;
  	struct address_space *swap_mapping;
  	pgoff_t swap_index;
  	int error;
  
  	oldpage = *pagep;
  	swap_index = page_private(oldpage);
  	swap_mapping = page_mapping(oldpage);
  
  	/*
  	 * We have arrived here because our zones are constrained, so don't
  	 * limit chance of success by further cpuset and node constraints.
  	 */
  	gfp &= ~GFP_CONSTRAINT_MASK;
  	newpage = shmem_alloc_page(gfp, info, index);
  	if (!newpage)
  		return -ENOMEM;

  	get_page(newpage);

  	copy_highpage(newpage, oldpage);

  	flush_dcache_page(newpage);

  	__SetPageLocked(newpage);
  	__SetPageSwapBacked(newpage);

  	SetPageUptodate(newpage);

  	set_page_private(newpage, swap_index);

  	SetPageSwapCache(newpage);
  
  	/*
  	 * Our caller will very soon move newpage out of swapcache, but it's
  	 * a nice clean interface for us to replace oldpage by newpage there.
  	 */
  	spin_lock_irq(&swap_mapping->tree_lock);
  	error = shmem_radix_tree_replace(swap_mapping, swap_index, oldpage,
  								   newpage);

  	if (!error) {

  		__inc_node_page_state(newpage, NR_FILE_PAGES);
  		__dec_node_page_state(oldpage, NR_FILE_PAGES);

  	}

  	spin_unlock_irq(&swap_mapping->tree_lock);

  	if (unlikely(error)) {
  		/*
  		 * Is this possible?  I think not, now that our callers check
  		 * both PageSwapCache and page_private after getting page lock;
  		 * but be defensive.  Reverse old to newpage for clear and free.
  		 */
  		oldpage = newpage;
  	} else {

  		mem_cgroup_migrate(oldpage, newpage);

  		lru_cache_add_anon(newpage);
  		*pagep = newpage;
  	}

  
  	ClearPageSwapCache(oldpage);
  	set_page_private(oldpage, 0);
  
  	unlock_page(oldpage);

  	put_page(oldpage);
  	put_page(oldpage);

  	return error;

  }
  
  /*

   * shmem_getpage_gfp - find page in cache, or get from swap, or allocate

   *
   * If we allocate a new one we do not mark it dirty. That's up to the
   * vm. If we swap it in we mark it dirty since we also free the swap

   * entry since a page cannot live in both the swap and page cache.
   *
   * fault_mm and fault_type are only supplied by shmem_fault:
   * otherwise they are NULL.

   */

  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, int *fault_type)

  {
  	struct address_space *mapping = inode->i_mapping;

  	struct shmem_inode_info *info = SHMEM_I(inode);

  	struct shmem_sb_info *sbinfo;

  	struct mm_struct *charge_mm;

  	struct mem_cgroup *memcg;

  	struct page *page;

  	swp_entry_t swap;

  	enum sgp_type sgp_huge = sgp;

  	pgoff_t hindex = index;

  	int error;

  	int once = 0;

  	int alloced = 0;

  	if (index > (MAX_LFS_FILESIZE >> PAGE_SHIFT))

  		return -EFBIG;

  	if (sgp == SGP_NOHUGE || sgp == SGP_HUGE)
  		sgp = SGP_CACHE;

  repeat:

  	swap.val = 0;

  	page = find_lock_entry(mapping, index);

  	if (radix_tree_exceptional_entry(page)) {
  		swap = radix_to_swp_entry(page);
  		page = NULL;
  	}

  	if (sgp <= SGP_CACHE &&

  	    ((loff_t)index << PAGE_SHIFT) >= i_size_read(inode)) {

  		error = -EINVAL;

  		goto unlock;

  	}

  	if (page && sgp == SGP_WRITE)
  		mark_page_accessed(page);

  	/* fallocated page? */
  	if (page && !PageUptodate(page)) {
  		if (sgp != SGP_READ)
  			goto clear;
  		unlock_page(page);

  		put_page(page);

  		page = NULL;
  	}

  	if (page || (sgp == SGP_READ && !swap.val)) {

  		*pagep = page;
  		return 0;

  	}
  
  	/*

  	 * Fast cache lookup did not find it:
  	 * bring it back from swap or allocate.

  	 */

  	sbinfo = SHMEM_SB(inode->i_sb);

  	charge_mm = vma ? vma->vm_mm : current->mm;

  	if (swap.val) {
  		/* Look it up and read it in.. */

  		page = lookup_swap_cache(swap, NULL, 0);

  		if (!page) {

  			/* Or update major stats only when swapin succeeds?? */
  			if (fault_type) {

  				*fault_type |= VM_FAULT_MAJOR;

  				count_vm_event(PGMAJFAULT);

  				count_memcg_event_mm(charge_mm, PGMAJFAULT);

  			}
  			/* Here we actually start the io */

  			page = shmem_swapin(swap, gfp, info, index);

  			if (!page) {

  				error = -ENOMEM;
  				goto failed;

  			}

  		}
  
  		/* We have to do this with page locked to prevent races */

  		lock_page(page);

  		if (!PageSwapCache(page) || page_private(page) != swap.val ||

  		    !shmem_confirm_swap(mapping, index, swap)) {

  			error = -EEXIST;	/* try again */

  			goto unlock;

  		}