/*
   *	linux/mm/filemap.c
   *
   * Copyright (C) 1994-1999  Linus Torvalds
   */
  
  /*
   * This file handles the generic file mmap semantics used by
   * most "normal" filesystems (but you don't /have/ to use this:
   * the NFS filesystem used to do this differently, for example)
   */

  #include <linux/export.h>

  #include <linux/compiler.h>

  #include <linux/dax.h>

  #include <linux/fs.h>

  #include <linux/uaccess.h>

  #include <linux/capability.h>

  #include <linux/kernel_stat.h>

  #include <linux/gfp.h>

  #include <linux/mm.h>
  #include <linux/swap.h>
  #include <linux/mman.h>
  #include <linux/pagemap.h>
  #include <linux/file.h>
  #include <linux/uio.h>
  #include <linux/hash.h>
  #include <linux/writeback.h>

  #include <linux/backing-dev.h>

  #include <linux/pagevec.h>
  #include <linux/blkdev.h>
  #include <linux/security.h>

  #include <linux/cpuset.h>

  #include <linux/hardirq.h> /* for BUG_ON(!in_atomic()) only */

  #include <linux/hugetlb.h>

  #include <linux/memcontrol.h>

  #include <linux/cleancache.h>

  #include <linux/rmap.h>

  #include "internal.h"

  #define CREATE_TRACE_POINTS
  #include <trace/events/filemap.h>

  /*

   * FIXME: remove all knowledge of the buffer layer from the core VM
   */

  #include <linux/buffer_head.h> /* for try_to_free_buffers */

  #include <asm/mman.h>
  
  /*
   * Shared mappings implemented 30.11.1994. It's not fully working yet,
   * though.
   *
   * Shared mappings now work. 15.8.1995  Bruno.
   *
   * finished 'unifying' the page and buffer cache and SMP-threaded the
   * page-cache, 21.05.1999, Ingo Molnar <mingo@redhat.com>
   *
   * SMP-threaded pagemap-LRU 1999, Andrea Arcangeli <andrea@suse.de>
   */
  
  /*
   * Lock ordering:
   *

   *  ->i_mmap_rwsem		(truncate_pagecache)

   *    ->private_lock		(__free_pte->__set_page_dirty_buffers)

   *      ->swap_lock		(exclusive_swap_page, others)
   *        ->mapping->tree_lock

   *

   *  ->i_mutex

   *    ->i_mmap_rwsem		(truncate->unmap_mapping_range)

   *
   *  ->mmap_sem

   *    ->i_mmap_rwsem

   *      ->page_table_lock or pte_lock	(various, mainly in memory.c)

   *        ->mapping->tree_lock	(arch-dependent flush_dcache_mmap_lock)
   *
   *  ->mmap_sem
   *    ->lock_page		(access_process_vm)
   *

   *  ->i_mutex			(generic_perform_write)

   *    ->mmap_sem		(fault_in_pages_readable->do_page_fault)

   *

   *  bdi->wb.list_lock

   *    sb_lock			(fs/fs-writeback.c)

   *    ->mapping->tree_lock	(__sync_single_inode)
   *

   *  ->i_mmap_rwsem

   *    ->anon_vma.lock		(vma_adjust)
   *
   *  ->anon_vma.lock

   *    ->page_table_lock or pte_lock	(anon_vma_prepare and various)

   *

   *  ->page_table_lock or pte_lock

   *    ->swap_lock		(try_to_unmap_one)

   *    ->private_lock		(try_to_unmap_one)
   *    ->tree_lock		(try_to_unmap_one)

   *    ->zone_lru_lock(zone)	(follow_page->mark_page_accessed)
   *    ->zone_lru_lock(zone)	(check_pte_range->isolate_lru_page)

   *    ->private_lock		(page_remove_rmap->set_page_dirty)
   *    ->tree_lock		(page_remove_rmap->set_page_dirty)

   *    bdi.wb->list_lock		(page_remove_rmap->set_page_dirty)

   *    ->inode->i_lock		(page_remove_rmap->set_page_dirty)

   *    ->memcg->move_lock	(page_remove_rmap->lock_page_memcg)

   *    bdi.wb->list_lock		(zap_pte_range->set_page_dirty)

   *    ->inode->i_lock		(zap_pte_range->set_page_dirty)

   *    ->private_lock		(zap_pte_range->__set_page_dirty_buffers)
   *

   * ->i_mmap_rwsem

   *   ->tasklist_lock            (memory_failure, collect_procs_ao)

   */

  static int page_cache_tree_insert(struct address_space *mapping,
  				  struct page *page, void **shadowp)
  {
  	struct radix_tree_node *node;
  	void **slot;
  	int error;
  
  	error = __radix_tree_create(&mapping->page_tree, page->index, 0,
  				    &node, &slot);
  	if (error)
  		return error;
  	if (*slot) {
  		void *p;
  
  		p = radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
  		if (!radix_tree_exceptional_entry(p))
  			return -EEXIST;
  
  		mapping->nrexceptional--;
  		if (!dax_mapping(mapping)) {
  			if (shadowp)
  				*shadowp = p;
  			if (node)
  				workingset_node_shadows_dec(node);
  		} else {
  			/* DAX can replace empty locked entry with a hole */
  			WARN_ON_ONCE(p !=
  				(void *)(RADIX_TREE_EXCEPTIONAL_ENTRY |
  					 RADIX_DAX_ENTRY_LOCK));
  			/* DAX accounts exceptional entries as normal pages */
  			if (node)
  				workingset_node_pages_dec(node);
  			/* Wakeup waiters for exceptional entry lock */
  			dax_wake_mapping_entry_waiter(mapping, page->index,

  						      true);

  		}
  	}
  	radix_tree_replace_slot(slot, page);
  	mapping->nrpages++;
  	if (node) {
  		workingset_node_pages_inc(node);
  		/*
  		 * Don't track node that contains actual pages.
  		 *
  		 * Avoid acquiring the list_lru lock if already
  		 * untracked.  The list_empty() test is safe as
  		 * node->private_list is protected by
  		 * mapping->tree_lock.
  		 */
  		if (!list_empty(&node->private_list))
  			list_lru_del(&workingset_shadow_nodes,
  				     &node->private_list);
  	}
  	return 0;
  }

  static void page_cache_tree_delete(struct address_space *mapping,
  				   struct page *page, void *shadow)
  {

  	int i, nr = PageHuge(page) ? 1 : hpage_nr_pages(page);

  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(PageTail(page), page);
  	VM_BUG_ON_PAGE(nr != 1 && shadow, page);

  	for (i = 0; i < nr; i++) {

  		struct radix_tree_node *node;
  		void **slot;
  
  		__radix_tree_lookup(&mapping->page_tree, page->index + i,
  				    &node, &slot);
  
  		radix_tree_clear_tags(&mapping->page_tree, node, slot);

  		if (!node) {
  			VM_BUG_ON_PAGE(nr != 1, page);

  			/*
  			 * We need a node to properly account shadow
  			 * entries. Don't plant any without. XXX
  			 */
  			shadow = NULL;

  		}

  		radix_tree_replace_slot(slot, shadow);
  
  		if (!node)
  			break;

  		workingset_node_pages_dec(node);
  		if (shadow)
  			workingset_node_shadows_inc(node);
  		else
  			if (__radix_tree_delete_node(&mapping->page_tree, node))
  				continue;
  
  		/*
  		 * Track node that only contains shadow entries. DAX mappings
  		 * contain no shadow entries and may contain other exceptional
  		 * entries so skip those.
  		 *
  		 * Avoid acquiring the list_lru lock if already tracked.
  		 * The list_empty() test is safe as node->private_list is
  		 * protected by mapping->tree_lock.
  		 */
  		if (!dax_mapping(mapping) && !workingset_node_pages(node) &&
  				list_empty(&node->private_list)) {
  			node->private_data = mapping;
  			list_lru_add(&workingset_shadow_nodes,
  					&node->private_list);
  		}

  	}

  
  	if (shadow) {
  		mapping->nrexceptional += nr;
  		/*
  		 * Make sure the nrexceptional update is committed before
  		 * the nrpages update so that final truncate racing
  		 * with reclaim does not see both counters 0 at the
  		 * same time and miss a shadow entry.
  		 */
  		smp_wmb();
  	}
  	mapping->nrpages -= nr;

  }

  /*

   * Delete a page from the page cache and free it. Caller has to make

   * sure the page is locked and that nobody else uses it - or that usage

   * is safe.  The caller must hold the mapping's tree_lock.

   */

  void __delete_from_page_cache(struct page *page, void *shadow)

  {
  	struct address_space *mapping = page->mapping;

  	int nr = hpage_nr_pages(page);

  	trace_mm_filemap_delete_from_page_cache(page);

  	/*
  	 * if we're uptodate, flush out into the cleancache, otherwise
  	 * invalidate any existing cleancache entries.  We can't leave
  	 * stale data around in the cleancache once our page is gone
  	 */
  	if (PageUptodate(page) && PageMappedToDisk(page))
  		cleancache_put_page(page);
  	else

  		cleancache_invalidate_page(mapping, page);

  	VM_BUG_ON_PAGE(PageTail(page), page);

  	VM_BUG_ON_PAGE(page_mapped(page), page);
  	if (!IS_ENABLED(CONFIG_DEBUG_VM) && unlikely(page_mapped(page))) {
  		int mapcount;
  
  		pr_alert("BUG: Bad page cache in process %s  pfn:%05lx
  ",
  			 current->comm, page_to_pfn(page));
  		dump_page(page, "still mapped when deleted");
  		dump_stack();
  		add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
  
  		mapcount = page_mapcount(page);
  		if (mapping_exiting(mapping) &&
  		    page_count(page) >= mapcount + 2) {
  			/*
  			 * All vmas have already been torn down, so it's
  			 * a good bet that actually the page is unmapped,
  			 * and we'd prefer not to leak it: if we're wrong,
  			 * some other bad page check should catch it later.
  			 */
  			page_mapcount_reset(page);

  			page_ref_sub(page, mapcount);

  		}
  	}

  	page_cache_tree_delete(mapping, page, shadow);

  	page->mapping = NULL;

  	/* Leave page->index set: truncation lookup relies upon it */

  	/* hugetlb pages do not participate in page cache accounting. */
  	if (!PageHuge(page))

  		__mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, -nr);

  	if (PageSwapBacked(page)) {

  		__mod_node_page_state(page_pgdat(page), NR_SHMEM, -nr);

  		if (PageTransHuge(page))

  			__dec_node_page_state(page, NR_SHMEM_THPS);

  	} else {
  		VM_BUG_ON_PAGE(PageTransHuge(page) && !PageHuge(page), page);
  	}

  
  	/*

  	 * At this point page must be either written or cleaned by truncate.
  	 * Dirty page here signals a bug and loss of unwritten data.

  	 *

  	 * This fixes dirty accounting after removing the page entirely but
  	 * leaves PageDirty set: it has no effect for truncated page and
  	 * anyway will be cleared before returning page into buddy allocator.

  	 */

  	if (WARN_ON_ONCE(PageDirty(page)))

  		account_page_cleaned(page, mapping, inode_to_wb(mapping->host));

  }

  /**
   * delete_from_page_cache - delete page from page cache
   * @page: the page which the kernel is trying to remove from page cache
   *
   * This must be called only on pages that have been verified to be in the page
   * cache and locked.  It will never put the page into the free list, the caller
   * has a reference on the page.
   */
  void delete_from_page_cache(struct page *page)

  {

  	struct address_space *mapping = page_mapping(page);

  	unsigned long flags;

  	void (*freepage)(struct page *);

  	BUG_ON(!PageLocked(page));

  	freepage = mapping->a_ops->freepage;

  	spin_lock_irqsave(&mapping->tree_lock, flags);

  	__delete_from_page_cache(page, NULL);

  	spin_unlock_irqrestore(&mapping->tree_lock, flags);

  
  	if (freepage)
  		freepage(page);

  
  	if (PageTransHuge(page) && !PageHuge(page)) {
  		page_ref_sub(page, HPAGE_PMD_NR);
  		VM_BUG_ON_PAGE(page_count(page) <= 0, page);
  	} else {
  		put_page(page);
  	}

  }
  EXPORT_SYMBOL(delete_from_page_cache);

  int filemap_check_errors(struct address_space *mapping)

  {
  	int ret = 0;
  	/* Check for outstanding write errors */

  	if (test_bit(AS_ENOSPC, &mapping->flags) &&
  	    test_and_clear_bit(AS_ENOSPC, &mapping->flags))

  		ret = -ENOSPC;

  	if (test_bit(AS_EIO, &mapping->flags) &&
  	    test_and_clear_bit(AS_EIO, &mapping->flags))

  		ret = -EIO;
  	return ret;
  }

  EXPORT_SYMBOL(filemap_check_errors);

  /**

   * __filemap_fdatawrite_range - start writeback on mapping dirty pages in range

   * @mapping:	address space structure to write
   * @start:	offset in bytes where the range starts

   * @end:	offset in bytes where the range ends (inclusive)

   * @sync_mode:	enable synchronous operation

   *

   * Start writeback against all of a mapping's dirty pages that lie
   * within the byte offsets <start, end> inclusive.
   *

   * If sync_mode is WB_SYNC_ALL then this is a "data integrity" operation, as

   * opposed to a regular memory cleansing writeback.  The difference between

   * these two operations is that if a dirty page/buffer is encountered, it must
   * be waited upon, and not just skipped over.
   */

  int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
  				loff_t end, int sync_mode)

  {
  	int ret;
  	struct writeback_control wbc = {
  		.sync_mode = sync_mode,

  		.nr_to_write = LONG_MAX,

  		.range_start = start,
  		.range_end = end,

  	};
  
  	if (!mapping_cap_writeback_dirty(mapping))
  		return 0;

  	wbc_attach_fdatawrite_inode(&wbc, mapping->host);

  	ret = do_writepages(mapping, &wbc);

  	wbc_detach_inode(&wbc);

  	return ret;
  }
  
  static inline int __filemap_fdatawrite(struct address_space *mapping,
  	int sync_mode)
  {

  	return __filemap_fdatawrite_range(mapping, 0, LLONG_MAX, sync_mode);

  }
  
  int filemap_fdatawrite(struct address_space *mapping)
  {
  	return __filemap_fdatawrite(mapping, WB_SYNC_ALL);
  }
  EXPORT_SYMBOL(filemap_fdatawrite);

  int filemap_fdatawrite_range(struct address_space *mapping, loff_t start,

  				loff_t end)

  {
  	return __filemap_fdatawrite_range(mapping, start, end, WB_SYNC_ALL);
  }

  EXPORT_SYMBOL(filemap_fdatawrite_range);

  /**
   * filemap_flush - mostly a non-blocking flush
   * @mapping:	target address_space
   *

   * This is a mostly non-blocking flush.  Not suitable for data-integrity
   * purposes - I/O may not be started against all dirty pages.
   */
  int filemap_flush(struct address_space *mapping)
  {
  	return __filemap_fdatawrite(mapping, WB_SYNC_NONE);
  }
  EXPORT_SYMBOL(filemap_flush);

  static int __filemap_fdatawait_range(struct address_space *mapping,
  				     loff_t start_byte, loff_t end_byte)

  {

  	pgoff_t index = start_byte >> PAGE_SHIFT;
  	pgoff_t end = end_byte >> PAGE_SHIFT;

  	struct pagevec pvec;
  	int nr_pages;

  	int ret = 0;

  	if (end_byte < start_byte)

  		goto out;

  
  	pagevec_init(&pvec, 0);

  	while ((index <= end) &&
  			(nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
  			PAGECACHE_TAG_WRITEBACK,
  			min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1)) != 0) {
  		unsigned i;
  
  		for (i = 0; i < nr_pages; i++) {
  			struct page *page = pvec.pages[i];
  
  			/* until radix tree lookup accepts end_index */
  			if (page->index > end)
  				continue;
  
  			wait_on_page_writeback(page);

  			if (TestClearPageError(page))

  				ret = -EIO;
  		}
  		pagevec_release(&pvec);
  		cond_resched();
  	}

  out:

  	return ret;
  }
  
  /**
   * filemap_fdatawait_range - wait for writeback to complete
   * @mapping:		address space structure to wait for
   * @start_byte:		offset in bytes where the range starts
   * @end_byte:		offset in bytes where the range ends (inclusive)
   *
   * Walk the list of under-writeback pages of the given address space
   * in the given range and wait for all of them.  Check error status of
   * the address space and return it.
   *
   * Since the error status of the address space is cleared by this function,
   * callers are responsible for checking the return value and handling and/or
   * reporting the error.
   */
  int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
  			    loff_t end_byte)
  {
  	int ret, ret2;
  
  	ret = __filemap_fdatawait_range(mapping, start_byte, end_byte);

  	ret2 = filemap_check_errors(mapping);
  	if (!ret)
  		ret = ret2;

  
  	return ret;
  }

  EXPORT_SYMBOL(filemap_fdatawait_range);
  
  /**

   * filemap_fdatawait_keep_errors - wait for writeback without clearing errors
   * @mapping: address space structure to wait for
   *
   * Walk the list of under-writeback pages of the given address space
   * and wait for all of them.  Unlike filemap_fdatawait(), this function
   * does not clear error status of the address space.
   *
   * Use this function if callers don't handle errors themselves.  Expected
   * call sites are system-wide / filesystem-wide data flushers: e.g. sync(2),
   * fsfreeze(8)
   */
  void filemap_fdatawait_keep_errors(struct address_space *mapping)
  {
  	loff_t i_size = i_size_read(mapping->host);
  
  	if (i_size == 0)
  		return;
  
  	__filemap_fdatawait_range(mapping, 0, i_size - 1);
  }
  
  /**

   * filemap_fdatawait - wait for all under-writeback pages to complete

   * @mapping: address space structure to wait for

   *
   * Walk the list of under-writeback pages of the given address space

   * and wait for all of them.  Check error status of the address space
   * and return it.
   *
   * Since the error status of the address space is cleared by this function,
   * callers are responsible for checking the return value and handling and/or
   * reporting the error.

   */
  int filemap_fdatawait(struct address_space *mapping)
  {
  	loff_t i_size = i_size_read(mapping->host);
  
  	if (i_size == 0)
  		return 0;

  	return filemap_fdatawait_range(mapping, 0, i_size - 1);

  }
  EXPORT_SYMBOL(filemap_fdatawait);
  
  int filemap_write_and_wait(struct address_space *mapping)
  {

  	int err = 0;

  	if ((!dax_mapping(mapping) && mapping->nrpages) ||
  	    (dax_mapping(mapping) && mapping->nrexceptional)) {

  		err = filemap_fdatawrite(mapping);
  		/*
  		 * Even if the above returned error, the pages may be
  		 * written partially (e.g. -ENOSPC), so we wait for it.
  		 * But the -EIO is special case, it may indicate the worst
  		 * thing (e.g. bug) happened, so we avoid waiting for it.
  		 */
  		if (err != -EIO) {
  			int err2 = filemap_fdatawait(mapping);
  			if (!err)
  				err = err2;
  		}

  	} else {
  		err = filemap_check_errors(mapping);

  	}

  	return err;

  }

  EXPORT_SYMBOL(filemap_write_and_wait);

  /**
   * filemap_write_and_wait_range - write out & wait on a file range
   * @mapping:	the address_space for the pages
   * @lstart:	offset in bytes where the range starts
   * @lend:	offset in bytes where the range ends (inclusive)
   *

   * Write out and wait upon file offsets lstart->lend, inclusive.
   *
   * Note that `lend' is inclusive (describes the last byte to be written) so
   * that this function can be used to write to the very end-of-file (end = -1).
   */

  int filemap_write_and_wait_range(struct address_space *mapping,
  				 loff_t lstart, loff_t lend)
  {

  	int err = 0;

  	if ((!dax_mapping(mapping) && mapping->nrpages) ||
  	    (dax_mapping(mapping) && mapping->nrexceptional)) {

  		err = __filemap_fdatawrite_range(mapping, lstart, lend,
  						 WB_SYNC_ALL);
  		/* See comment of filemap_write_and_wait() */
  		if (err != -EIO) {

  			int err2 = filemap_fdatawait_range(mapping,
  						lstart, lend);

  			if (!err)
  				err = err2;
  		}

  	} else {
  		err = filemap_check_errors(mapping);

  	}

  	return err;

  }

  EXPORT_SYMBOL(filemap_write_and_wait_range);

  /**

   * replace_page_cache_page - replace a pagecache page with a new one
   * @old:	page to be replaced
   * @new:	page to replace with
   * @gfp_mask:	allocation mode
   *
   * This function replaces a page in the pagecache with a new one.  On
   * success it acquires the pagecache reference for the new page and
   * drops it for the old page.  Both the old and new pages must be
   * locked.  This function does not add the new page to the LRU, the
   * caller must do that.
   *
   * The remove + add is atomic.  The only way this function can fail is
   * memory allocation failure.
   */
  int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
  {
  	int error;

  	VM_BUG_ON_PAGE(!PageLocked(old), old);
  	VM_BUG_ON_PAGE(!PageLocked(new), new);
  	VM_BUG_ON_PAGE(new->mapping, new);

  	error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
  	if (!error) {
  		struct address_space *mapping = old->mapping;
  		void (*freepage)(struct page *);

  		unsigned long flags;

  
  		pgoff_t offset = old->index;
  		freepage = mapping->a_ops->freepage;

  		get_page(new);

  		new->mapping = mapping;
  		new->index = offset;

  		spin_lock_irqsave(&mapping->tree_lock, flags);

  		__delete_from_page_cache(old, NULL);

  		error = page_cache_tree_insert(mapping, new, NULL);

  		BUG_ON(error);

  
  		/*
  		 * hugetlb pages do not participate in page cache accounting.
  		 */
  		if (!PageHuge(new))

  			__inc_node_page_state(new, NR_FILE_PAGES);

  		if (PageSwapBacked(new))

  			__inc_node_page_state(new, NR_SHMEM);

  		spin_unlock_irqrestore(&mapping->tree_lock, flags);

  		mem_cgroup_migrate(old, new);

  		radix_tree_preload_end();
  		if (freepage)
  			freepage(old);

  		put_page(old);

  	}
  
  	return error;
  }
  EXPORT_SYMBOL_GPL(replace_page_cache_page);

  static int __add_to_page_cache_locked(struct page *page,
  				      struct address_space *mapping,
  				      pgoff_t offset, gfp_t gfp_mask,
  				      void **shadowp)

  {

  	int huge = PageHuge(page);
  	struct mem_cgroup *memcg;

  	int error;

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

  	if (!huge) {
  		error = mem_cgroup_try_charge(page, current->mm,

  					      gfp_mask, &memcg, false);

  		if (error)
  			return error;
  	}

  	error = radix_tree_maybe_preload(gfp_mask & ~__GFP_HIGHMEM);

  	if (error) {

  		if (!huge)

  			mem_cgroup_cancel_charge(page, memcg, false);

  		return error;
  	}

  	get_page(page);

  	page->mapping = mapping;
  	page->index = offset;
  
  	spin_lock_irq(&mapping->tree_lock);

  	error = page_cache_tree_insert(mapping, page, shadowp);

  	radix_tree_preload_end();
  	if (unlikely(error))
  		goto err_insert;

  
  	/* hugetlb pages do not participate in page cache accounting. */
  	if (!huge)

  		__inc_node_page_state(page, NR_FILE_PAGES);

  	spin_unlock_irq(&mapping->tree_lock);

  	if (!huge)

  		mem_cgroup_commit_charge(page, memcg, false, false);

  	trace_mm_filemap_add_to_page_cache(page);
  	return 0;
  err_insert:
  	page->mapping = NULL;
  	/* Leave page->index set: truncation relies upon it */
  	spin_unlock_irq(&mapping->tree_lock);

  	if (!huge)

  		mem_cgroup_cancel_charge(page, memcg, false);

  	put_page(page);

  	return error;
  }

  
  /**
   * add_to_page_cache_locked - add a locked page to the pagecache
   * @page:	page to add
   * @mapping:	the page's address_space
   * @offset:	page index
   * @gfp_mask:	page allocation mode
   *
   * This function is used to add a page to the pagecache. It must be locked.
   * This function does not add the page to the LRU.  The caller must do that.
   */
  int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
  		pgoff_t offset, gfp_t gfp_mask)
  {
  	return __add_to_page_cache_locked(page, mapping, offset,
  					  gfp_mask, NULL);
  }

  EXPORT_SYMBOL(add_to_page_cache_locked);

  
  int add_to_page_cache_lru(struct page *page, struct address_space *mapping,

  				pgoff_t offset, gfp_t gfp_mask)

  {

  	void *shadow = NULL;

  	int ret;

  	__SetPageLocked(page);

  	ret = __add_to_page_cache_locked(page, mapping, offset,
  					 gfp_mask, &shadow);
  	if (unlikely(ret))

  		__ClearPageLocked(page);

  	else {
  		/*
  		 * The page might have been evicted from cache only
  		 * recently, in which case it should be activated like
  		 * any other repeatedly accessed page.

  		 * The exception is pages getting rewritten; evicting other
  		 * data from the working set, only to cache data that will
  		 * get overwritten with something else, is a waste of memory.

  		 */

  		if (!(gfp_mask & __GFP_WRITE) &&
  		    shadow && workingset_refault(shadow)) {

  			SetPageActive(page);
  			workingset_activation(page);
  		} else
  			ClearPageActive(page);
  		lru_cache_add(page);
  	}

  	return ret;
  }

  EXPORT_SYMBOL_GPL(add_to_page_cache_lru);

  #ifdef CONFIG_NUMA

  struct page *__page_cache_alloc(gfp_t gfp)

  {

  	int n;
  	struct page *page;

  	if (cpuset_do_page_mem_spread()) {

  		unsigned int cpuset_mems_cookie;
  		do {

  			cpuset_mems_cookie = read_mems_allowed_begin();

  			n = cpuset_mem_spread_node();

  			page = __alloc_pages_node(n, gfp, 0);

  		} while (!page && read_mems_allowed_retry(cpuset_mems_cookie));

  		return page;

  	}

  	return alloc_pages(gfp, 0);

  }

  EXPORT_SYMBOL(__page_cache_alloc);

  #endif

  /*
   * In order to wait for pages to become available there must be
   * waitqueues associated with pages. By using a hash table of
   * waitqueues where the bucket discipline is to maintain all
   * waiters on the same queue and wake all when any of the pages
   * become available, and for the woken contexts to check to be
   * sure the appropriate page became available, this saves space
   * at a cost of "thundering herd" phenomena during rare hash
   * collisions.
   */

  wait_queue_head_t *page_waitqueue(struct page *page)

  {

  	return bit_waitqueue(page, 0);

  }

  EXPORT_SYMBOL(page_waitqueue);

  void wait_on_page_bit(struct page *page, int bit_nr)

  {
  	DEFINE_WAIT_BIT(wait, &page->flags, bit_nr);
  
  	if (test_bit(bit_nr, &page->flags))

  		__wait_on_bit(page_waitqueue(page), &wait, bit_wait_io,

  							TASK_UNINTERRUPTIBLE);
  }
  EXPORT_SYMBOL(wait_on_page_bit);

  int wait_on_page_bit_killable(struct page *page, int bit_nr)
  {
  	DEFINE_WAIT_BIT(wait, &page->flags, bit_nr);
  
  	if (!test_bit(bit_nr, &page->flags))
  		return 0;
  
  	return __wait_on_bit(page_waitqueue(page), &wait,

  			     bit_wait_io, TASK_KILLABLE);

  }

  int wait_on_page_bit_killable_timeout(struct page *page,
  				       int bit_nr, unsigned long timeout)
  {
  	DEFINE_WAIT_BIT(wait, &page->flags, bit_nr);
  
  	wait.key.timeout = jiffies + timeout;
  	if (!test_bit(bit_nr, &page->flags))
  		return 0;
  	return __wait_on_bit(page_waitqueue(page), &wait,
  			     bit_wait_io_timeout, TASK_KILLABLE);
  }
  EXPORT_SYMBOL_GPL(wait_on_page_bit_killable_timeout);

  /**

   * add_page_wait_queue - Add an arbitrary waiter to a page's wait queue

   * @page: Page defining the wait queue of interest
   * @waiter: Waiter to add to the queue

   *
   * Add an arbitrary @waiter to the wait queue for the nominated @page.
   */
  void add_page_wait_queue(struct page *page, wait_queue_t *waiter)
  {
  	wait_queue_head_t *q = page_waitqueue(page);
  	unsigned long flags;
  
  	spin_lock_irqsave(&q->lock, flags);
  	__add_wait_queue(q, waiter);
  	spin_unlock_irqrestore(&q->lock, flags);
  }
  EXPORT_SYMBOL_GPL(add_page_wait_queue);
  
  /**

   * unlock_page - unlock a locked page

   * @page: the page
   *
   * Unlocks the page and wakes up sleepers in ___wait_on_page_locked().
   * Also wakes sleepers in wait_on_page_writeback() because the wakeup

   * mechanism between PageLocked pages and PageWriteback pages is shared.

   * But that's OK - sleepers in wait_on_page_writeback() just go back to sleep.
   *

   * The mb is necessary to enforce ordering between the clear_bit and the read
   * of the waitqueue (to avoid SMP races with a parallel wait_on_page_locked()).

   */

  void unlock_page(struct page *page)

  {

  	page = compound_head(page);

  	VM_BUG_ON_PAGE(!PageLocked(page), page);

  	clear_bit_unlock(PG_locked, &page->flags);

  	smp_mb__after_atomic();

  	wake_up_page(page, PG_locked);
  }
  EXPORT_SYMBOL(unlock_page);

  /**
   * end_page_writeback - end writeback against a page
   * @page: the page

   */
  void end_page_writeback(struct page *page)
  {

  	/*
  	 * TestClearPageReclaim could be used here but it is an atomic
  	 * operation and overkill in this particular case. Failing to
  	 * shuffle a page marked for immediate reclaim is too mild to
  	 * justify taking an atomic operation penalty at the end of
  	 * ever page writeback.
  	 */
  	if (PageReclaim(page)) {
  		ClearPageReclaim(page);

  		rotate_reclaimable_page(page);

  	}

  
  	if (!test_clear_page_writeback(page))
  		BUG();

  	smp_mb__after_atomic();

  	wake_up_page(page, PG_writeback);
  }
  EXPORT_SYMBOL(end_page_writeback);

  /*
   * After completing I/O on a page, call this routine to update the page
   * flags appropriately
   */

  void page_endio(struct page *page, bool is_write, int err)

  {

  	if (!is_write) {

  		if (!err) {
  			SetPageUptodate(page);
  		} else {
  			ClearPageUptodate(page);
  			SetPageError(page);
  		}
  		unlock_page(page);

  	} else {

  		if (err) {
  			SetPageError(page);
  			if (page->mapping)
  				mapping_set_error(page->mapping, err);
  		}
  		end_page_writeback(page);
  	}
  }
  EXPORT_SYMBOL_GPL(page_endio);

  /**
   * __lock_page - get a lock on the page, assuming we need to sleep to get it
   * @page: the page to lock

   */

  void __lock_page(struct page *page)

  {

  	struct page *page_head = compound_head(page);
  	DEFINE_WAIT_BIT(wait, &page_head->flags, PG_locked);

  	__wait_on_bit_lock(page_waitqueue(page_head), &wait, bit_wait_io,

  							TASK_UNINTERRUPTIBLE);
  }
  EXPORT_SYMBOL(__lock_page);

  int __lock_page_killable(struct page *page)

  {

  	struct page *page_head = compound_head(page);
  	DEFINE_WAIT_BIT(wait, &page_head->flags, PG_locked);

  	return __wait_on_bit_lock(page_waitqueue(page_head), &wait,

  					bit_wait_io, TASK_KILLABLE);

  }

  EXPORT_SYMBOL_GPL(__lock_page_killable);

  /*
   * Return values:
   * 1 - page is locked; mmap_sem is still held.
   * 0 - page is not locked.
   *     mmap_sem has been released (up_read()), unless flags had both
   *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in
   *     which case mmap_sem is still held.
   *
   * If neither ALLOW_RETRY nor KILLABLE are set, will always return 1
   * with the page locked and the mmap_sem unperturbed.
   */

  int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
  			 unsigned int flags)
  {

  	if (flags & FAULT_FLAG_ALLOW_RETRY) {
  		/*
  		 * CAUTION! In this case, mmap_sem is not released
  		 * even though return 0.
  		 */
  		if (flags & FAULT_FLAG_RETRY_NOWAIT)
  			return 0;
  
  		up_read(&mm->mmap_sem);
  		if (flags & FAULT_FLAG_KILLABLE)
  			wait_on_page_locked_killable(page);
  		else

  			wait_on_page_locked(page);

  		return 0;

  	} else {
  		if (flags & FAULT_FLAG_KILLABLE) {
  			int ret;
  
  			ret = __lock_page_killable(page);
  			if (ret) {
  				up_read(&mm->mmap_sem);
  				return 0;
  			}
  		} else
  			__lock_page(page);
  		return 1;

  	}
  }

  /**

   * page_cache_next_hole - find the next hole (not-present entry)
   * @mapping: mapping
   * @index: index
   * @max_scan: maximum range to search
   *
   * Search the set [index, min(index+max_scan-1, MAX_INDEX)] for the
   * lowest indexed hole.
   *
   * Returns: the index of the hole if found, otherwise returns an index
   * outside of the set specified (in which case 'return - index >=
   * max_scan' will be true). In rare cases of index wrap-around, 0 will
   * be returned.
   *
   * page_cache_next_hole may be called under rcu_read_lock. However,
   * like radix_tree_gang_lookup, this will not atomically search a
   * snapshot of the tree at a single point in time. For example, if a
   * hole is created at index 5, then subsequently a hole is created at
   * index 10, page_cache_next_hole covering both indexes may return 10
   * if called under rcu_read_lock.
   */
  pgoff_t page_cache_next_hole(struct address_space *mapping,
  			     pgoff_t index, unsigned long max_scan)
  {
  	unsigned long i;
  
  	for (i = 0; i < max_scan; i++) {

  		struct page *page;
  
  		page = radix_tree_lookup(&mapping->page_tree, index);
  		if (!page || radix_tree_exceptional_entry(page))

  			break;
  		index++;
  		if (index == 0)
  			break;
  	}
  
  	return index;
  }
  EXPORT_SYMBOL(page_cache_next_hole);
  
  /**
   * page_cache_prev_hole - find the prev hole (not-present entry)
   * @mapping: mapping
   * @index: index
   * @max_scan: maximum range to search
   *
   * Search backwards in the range [max(index-max_scan+1, 0), index] for
   * the first hole.
   *
   * Returns: the index of the hole if found, otherwise returns an index
   * outside of the set specified (in which case 'index - return >=
   * max_scan' will be true). In rare cases of wrap-around, ULONG_MAX
   * will be returned.
   *
   * page_cache_prev_hole may be called under rcu_read_lock. However,
   * like radix_tree_gang_lookup, this will not atomically search a
   * snapshot of the tree at a single point in time. For example, if a
   * hole is created at index 10, then subsequently a hole is created at
   * index 5, page_cache_prev_hole covering both indexes may return 5 if
   * called under rcu_read_lock.
   */
  pgoff_t page_cache_prev_hole(struct address_space *mapping,
  			     pgoff_t index, unsigned long max_scan)
  {
  	unsigned long i;
  
  	for (i = 0; i < max_scan; i++) {

  		struct page *page;
  
  		page = radix_tree_lookup(&mapping->page_tree, index);
  		if (!page || radix_tree_exceptional_entry(page))

  			break;
  		index--;
  		if (index == ULONG_MAX)
  			break;
  	}
  
  	return index;
  }
  EXPORT_SYMBOL(page_cache_prev_hole);
  
  /**

   * find_get_entry - find and get a page cache entry

   * @mapping: the address_space to search

   * @offset: the page cache index
   *
   * Looks up the page cache slot at @mapping & @offset.  If there is a
   * page cache page, it is returned with an increased refcount.

   *

   * If the slot holds a shadow entry of a previously evicted page, or a
   * swap entry from shmem/tmpfs, it is returned.

   *
   * Otherwise, %NULL is returned.

   */

  struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)

  {

  	void **pagep;

  	struct page *head, *page;

  	rcu_read_lock();
  repeat:
  	page = NULL;
  	pagep = radix_tree_lookup_slot(&mapping->page_tree, offset);
  	if (pagep) {
  		page = radix_tree_deref_slot(pagep);

  		if (unlikely(!page))
  			goto out;

  		if (radix_tree_exception(page)) {

  			if (radix_tree_deref_retry(page))
  				goto repeat;
  			/*

  			 * A shadow entry of a recently evicted page,
  			 * or a swap entry from shmem/tmpfs.  Return
  			 * it without attempting to raise page count.

  			 */
  			goto out;

  		}

  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);

  			goto repeat;

  		}

  
  		/*
  		 * Has the page moved?
  		 * This is part of the lockless pagecache protocol. See
  		 * include/linux/pagemap.h for details.
  		 */
  		if (unlikely(page != *pagep)) {

  			put_page(head);

  			goto repeat;
  		}
  	}

  out:

  	rcu_read_unlock();

  	return page;
  }

  EXPORT_SYMBOL(find_get_entry);

  /**

   * find_lock_entry - locate, pin and lock a page cache entry
   * @mapping: the address_space to search
   * @offset: the page cache index
   *
   * Looks up the page cache slot at @mapping & @offset.  If there is a
   * page cache page, it is returned locked and with an increased
   * refcount.
   *

   * If the slot holds a shadow entry of a previously evicted page, or a
   * swap entry from shmem/tmpfs, it is returned.

   *
   * Otherwise, %NULL is returned.
   *
   * find_lock_entry() may sleep.
   */
  struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset)

  {
  	struct page *page;

  repeat:

  	page = find_get_entry(mapping, offset);

  	if (page && !radix_tree_exception(page)) {

  		lock_page(page);
  		/* Has the page been truncated? */

  		if (unlikely(page_mapping(page) != mapping)) {

  			unlock_page(page);

  			put_page(page);

  			goto repeat;

  		}

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

  	}

  	return page;
  }

  EXPORT_SYMBOL(find_lock_entry);
  
  /**

   * pagecache_get_page - find and get a page reference

   * @mapping: the address_space to search
   * @offset: the page index

   * @fgp_flags: PCG flags

   * @gfp_mask: gfp mask to use for the page cache data page allocation

   *

   * Looks up the page cache slot at @mapping & @offset.

   *

   * PCG flags modify how the page is returned.

   *

   * FGP_ACCESSED: the page will be marked accessed
   * FGP_LOCK: Page is return locked
   * FGP_CREAT: If page is not present then a new page is allocated using

   *		@gfp_mask and added to the page cache and the VM's LRU
   *		list. The page is returned locked and with an increased
   *		refcount. Otherwise, %NULL is returned.

   *

   * If FGP_LOCK or FGP_CREAT are specified then the function may sleep even
   * if the GFP flags specified for FGP_CREAT are atomic.

   *

   * If there is a page cache page, it is returned with an increased refcount.

   */

  struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,

  	int fgp_flags, gfp_t gfp_mask)

  {

  	struct page *page;

  repeat:

  	page = find_get_entry(mapping, offset);
  	if (radix_tree_exceptional_entry(page))
  		page = NULL;
  	if (!page)
  		goto no_page;
  
  	if (fgp_flags & FGP_LOCK) {
  		if (fgp_flags & FGP_NOWAIT) {
  			if (!trylock_page(page)) {

  				put_page(page);

  				return NULL;
  			}
  		} else {
  			lock_page(page);
  		}
  
  		/* Has the page been truncated? */
  		if (unlikely(page->mapping != mapping)) {
  			unlock_page(page);

  			put_page(page);

  			goto repeat;
  		}
  		VM_BUG_ON_PAGE(page->index != offset, page);
  	}
  
  	if (page && (fgp_flags & FGP_ACCESSED))
  		mark_page_accessed(page);
  
  no_page:
  	if (!page && (fgp_flags & FGP_CREAT)) {
  		int err;
  		if ((fgp_flags & FGP_WRITE) && mapping_cap_account_dirty(mapping))

  			gfp_mask |= __GFP_WRITE;
  		if (fgp_flags & FGP_NOFS)
  			gfp_mask &= ~__GFP_FS;

  		page = __page_cache_alloc(gfp_mask);

  		if (!page)
  			return NULL;

  
  		if (WARN_ON_ONCE(!(fgp_flags & FGP_LOCK)))
  			fgp_flags |= FGP_LOCK;

  		/* Init accessed so avoid atomic mark_page_accessed later */

  		if (fgp_flags & FGP_ACCESSED)

  			__SetPageReferenced(page);

  		err = add_to_page_cache_lru(page, mapping, offset,
  				gfp_mask & GFP_RECLAIM_MASK);

  		if (unlikely(err)) {

  			put_page(page);

  			page = NULL;
  			if (err == -EEXIST)
  				goto repeat;

  		}

  	}

  	return page;
  }

  EXPORT_SYMBOL(pagecache_get_page);

  
  /**

   * find_get_entries - gang pagecache lookup
   * @mapping:	The address_space to search
   * @start:	The starting page cache index
   * @nr_entries:	The maximum number of entries
   * @entries:	Where the resulting entries are placed
   * @indices:	The cache indices corresponding to the entries in @entries
   *
   * find_get_entries() will search for and return a group of up to
   * @nr_entries entries in the mapping.  The entries are placed at
   * @entries.  find_get_entries() takes a reference against any actual
   * pages it returns.
   *
   * The search returns a group of mapping-contiguous page cache entries
   * with ascending indexes.  There may be holes in the indices due to
   * not-present pages.
   *

   * Any shadow entries of evicted pages, or swap entries from
   * shmem/tmpfs, are included in the returned array.

   *
   * find_get_entries() returns the number of pages and shadow entries
   * which were found.
   */
  unsigned find_get_entries(struct address_space *mapping,
  			  pgoff_t start, unsigned int nr_entries,
  			  struct page **entries, pgoff_t *indices)
  {
  	void **slot;
  	unsigned int ret = 0;
  	struct radix_tree_iter iter;
  
  	if (!nr_entries)
  		return 0;
  
  	rcu_read_lock();

  	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {

  		struct page *head, *page;

  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			continue;
  		if (radix_tree_exception(page)) {

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

  			/*

  			 * A shadow entry of a recently evicted page, a swap
  			 * entry from shmem/tmpfs or a DAX entry.  Return it
  			 * without attempting to raise page count.

  			 */
  			goto export;
  		}

  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);

  			goto repeat;

  		}

  
  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {

  			put_page(head);

  			goto repeat;
  		}
  export:
  		indices[ret] = iter.index;
  		entries[ret] = page;
  		if (++ret == nr_entries)
  			break;
  	}
  	rcu_read_unlock();
  	return ret;
  }
  
  /**

   * find_get_pages - gang pagecache lookup
   * @mapping:	The address_space to search
   * @start:	The starting page index
   * @nr_pages:	The maximum number of pages
   * @pages:	Where the resulting pages are placed
   *
   * find_get_pages() will search for and return a group of up to
   * @nr_pages pages in the mapping.  The pages are placed at @pages.
   * find_get_pages() takes a reference against the returned pages.
   *
   * The search returns a group of mapping-contiguous pages with ascending
   * indexes.  There may be holes in the indices due to not-present pages.
   *
   * find_get_pages() returns the number of pages which were found.
   */
  unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
  			    unsigned int nr_pages, struct page **pages)
  {

  	struct radix_tree_iter iter;
  	void **slot;
  	unsigned ret = 0;
  
  	if (unlikely(!nr_pages))
  		return 0;

  
  	rcu_read_lock();

  	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {

  		struct page *head, *page;

  repeat:

  		page = radix_tree_deref_slot(slot);

  		if (unlikely(!page))
  			continue;

  		if (radix_tree_exception(page)) {

  			if (radix_tree_deref_retry(page)) {

  				slot = radix_tree_iter_retry(&iter);
  				continue;

  			}

  			/*

  			 * A shadow entry of a recently evicted page,
  			 * or a swap entry from shmem/tmpfs.  Skip
  			 * over it.

  			 */

  			continue;

  		}

  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);

  			goto repeat;

  		}

  
  		/* Has the page moved? */

  		if (unlikely(page != *slot)) {

  			put_page(head);

  			goto repeat;
  		}

  		pages[ret] = page;

  		if (++ret == nr_pages)
  			break;

  	}

  	rcu_read_unlock();

  	return ret;
  }

  /**
   * find_get_pages_contig - gang contiguous pagecache lookup
   * @mapping:	The address_space to search
   * @index:	The starting page index
   * @nr_pages:	The maximum number of pages
   * @pages:	Where the resulting pages are placed
   *
   * find_get_pages_contig() works exactly like find_get_pages(), except
   * that the returned number of pages are guaranteed to be contiguous.
   *
   * find_get_pages_contig() returns the number of pages which were found.
   */
  unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
  			       unsigned int nr_pages, struct page **pages)
  {

  	struct radix_tree_iter iter;
  	void **slot;
  	unsigned int ret = 0;
  
  	if (unlikely(!nr_pages))
  		return 0;

  
  	rcu_read_lock();

  	radix_tree_for_each_contig(slot, &mapping->page_tree, &iter, index) {

  		struct page *head, *page;

  repeat:

  		page = radix_tree_deref_slot(slot);
  		/* The hole, there no reason to continue */

  		if (unlikely(!page))

  			break;

  		if (radix_tree_exception(page)) {

  			if (radix_tree_deref_retry(page)) {

  				slot = radix_tree_iter_retry(&iter);
  				continue;

  			}

  			/*

  			 * A shadow entry of a recently evicted page,
  			 * or a swap entry from shmem/tmpfs.  Stop
  			 * looking for contiguous pages.

  			 */

  			break;

  		}

  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))
  			goto repeat;
  
  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);

  			goto repeat;

  		}

  
  		/* Has the page moved? */

  		if (unlikely(page != *slot)) {

  			put_page(head);

  			goto repeat;
  		}

  		/*
  		 * must check mapping and index after taking the ref.
  		 * otherwise we can get both false positives and false
  		 * negatives, which is just confusing to the caller.
  		 */

  		if (page->mapping == NULL || page_to_pgoff(page) != iter.index) {

  			put_page(page);

  			break;
  		}

  		pages[ret] = page;

  		if (++ret == nr_pages)
  			break;

  	}

  	rcu_read_unlock();
  	return ret;

  }

  EXPORT_SYMBOL(find_get_pages_contig);

  /**
   * find_get_pages_tag - find and return pages that match @tag
   * @mapping:	the address_space to search
   * @index:	the starting page index
   * @tag:	the tag index
   * @nr_pages:	the maximum number of pages
   * @pages:	where the resulting pages are placed
   *

   * Like find_get_pages, except we only return pages which are tagged with

   * @tag.   We update @index to index the next page for the traversal.

   */
  unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
  			int tag, unsigned int nr_pages, struct page **pages)
  {

  	struct radix_tree_iter iter;
  	void **slot;
  	unsigned ret = 0;
  
  	if (unlikely(!nr_pages))
  		return 0;

  
  	rcu_read_lock();

  	radix_tree_for_each_tagged(slot, &mapping->page_tree,
  				   &iter, *index, tag) {

  		struct page *head, *page;

  repeat:

  		page = radix_tree_deref_slot(slot);

  		if (unlikely(!page))
  			continue;

  		if (radix_tree_exception(page)) {

  			if (radix_tree_deref_retry(page)) {

  				slot = radix_tree_iter_retry(&iter);
  				continue;

  			}

  			/*

  			 * A shadow entry of a recently evicted page.
  			 *
  			 * Those entries should never be tagged, but
  			 * this tree walk is lockless and the tags are
  			 * looked up in bulk, one radix tree node at a
  			 * time, so there is a sizable window for page
  			 * reclaim to evict a page we saw tagged.
  			 *
  			 * Skip over it.

  			 */

  			continue;

  		}

  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))

  			goto repeat;

  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}

  		/* Has the page moved? */

  		if (unlikely(page != *slot)) {

  			put_page(head);

  			goto repeat;
  		}
  
  		pages[ret] = page;

  		if (++ret == nr_pages)
  			break;

  	}

  	rcu_read_unlock();

  	if (ret)
  		*index = pages[ret - 1]->index + 1;

  	return ret;
  }

  EXPORT_SYMBOL(find_get_pages_tag);

  /**
   * find_get_entries_tag - find and return entries that match @tag
   * @mapping:	the address_space to search
   * @start:	the starting page cache index
   * @tag:	the tag index
   * @nr_entries:	the maximum number of entries
   * @entries:	where the resulting entries are placed
   * @indices:	the cache indices corresponding to the entries in @entries
   *
   * Like find_get_entries, except we only return entries which are tagged with
   * @tag.
   */
  unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
  			int tag, unsigned int nr_entries,
  			struct page **entries, pgoff_t *indices)
  {
  	void **slot;
  	unsigned int ret = 0;
  	struct radix_tree_iter iter;
  
  	if (!nr_entries)
  		return 0;
  
  	rcu_read_lock();

  	radix_tree_for_each_tagged(slot, &mapping->page_tree,
  				   &iter, start, tag) {

  		struct page *head, *page;

  repeat:
  		page = radix_tree_deref_slot(slot);
  		if (unlikely(!page))
  			continue;
  		if (radix_tree_exception(page)) {
  			if (radix_tree_deref_retry(page)) {

  				slot = radix_tree_iter_retry(&iter);
  				continue;

  			}
  
  			/*
  			 * A shadow entry of a recently evicted page, a swap
  			 * entry from shmem/tmpfs or a DAX entry.  Return it
  			 * without attempting to raise page count.
  			 */
  			goto export;
  		}

  
  		head = compound_head(page);
  		if (!page_cache_get_speculative(head))

  			goto repeat;

  		/* The page was split under us? */
  		if (compound_head(page) != head) {
  			put_page(head);
  			goto repeat;
  		}

  		/* Has the page moved? */
  		if (unlikely(page != *slot)) {

  			put_page(head);

  			goto repeat;
  		}
  export:
  		indices[ret] = iter.index;
  		entries[ret] = page;
  		if (++ret == nr_entries)
  			break;
  	}
  	rcu_read_unlock();
  	return ret;
  }
  EXPORT_SYMBOL(find_get_entries_tag);

  /*
   * CD/DVDs are error prone. When a medium error occurs, the driver may fail
   * a _large_ part of the i/o request. Imagine the worst scenario:
   *
   *      ---R__________________________________________B__________
   *         ^ reading here                             ^ bad block(assume 4k)
   *
   * read(R) => miss => readahead(R...B) => media error => frustrating retries
   * => failing the whole request => read(R) => read(R+1) =>
   * readahead(R+1...B+1) => bang => read(R+2) => read(R+3) =>
   * readahead(R+3...B+2) => bang => read(R+3) => read(R+4) =>
   * readahead(R+4...B+3) => bang => read(R+4) => read(R+5) => ......
   *
   * It is going insane. Fix it by quickly scaling down the readahead size.
   */
  static void shrink_readahead_size_eio(struct file *filp,
  					struct file_ra_state *ra)
  {

  	ra->ra_pages /= 4;

  }

  /**

   * do_generic_file_read - generic file read routine

   * @filp:	the file to read
   * @ppos:	current file position

   * @iter:	data destination
   * @written:	already copied

   *

   * This is a generic file read routine, and uses the

   * mapping->a_ops->readpage() function for the actual low-level stuff.

   *
   * This is really ugly. But the goto's actually try to clarify some
   * of the logic when it comes to error handling etc.

   */

  static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
  		struct iov_iter *iter, ssize_t written)

  {

  	struct address_space *mapping = filp->f_mapping;

  	struct inode *inode = mapping->host;

  	struct file_ra_state *ra = &filp->f_ra;

  	pgoff_t index;
  	pgoff_t last_index;
  	pgoff_t prev_index;
  	unsigned long offset;      /* offset into pagecache page */

  	unsigned int prev_offset;

  	int error = 0;

  	if (unlikely(*ppos >= inode->i_sb->s_maxbytes))

  		return 0;

  	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);

  	index = *ppos >> PAGE_SHIFT;
  	prev_index = ra->prev_pos >> PAGE_SHIFT;
  	prev_offset = ra->prev_pos & (PAGE_SIZE-1);
  	last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
  	offset = *ppos & ~PAGE_MASK;

  	for (;;) {
  		struct page *page;

  		pgoff_t end_index;

  		loff_t isize;

  		unsigned long nr, ret;

  		cond_resched();

  find_page:

  		if (fatal_signal_pending(current)) {
  			error = -EINTR;
  			goto out;
  		}

  		page = find_get_page(mapping, index);

  		if (!page) {

  			page_cache_sync_readahead(mapping,

  					ra, filp,

  					index, last_index - index);
  			page = find_get_page(mapping, index);
  			if (unlikely(page == NULL))
  				goto no_cached_page;
  		}
  		if (PageReadahead(page)) {

  			page_cache_async_readahead(mapping,

  					ra, filp, page,

  					index, last_index - index);

  		}

  		if (!PageUptodate(page)) {

  			/*
  			 * See comment in do_read_cache_page on why
  			 * wait_on_page_locked is used to avoid unnecessarily
  			 * serialisations and why it's safe.
  			 */

  			error = wait_on_page_locked_killable(page);
  			if (unlikely(error))
  				goto readpage_error;

  			if (PageUptodate(page))
  				goto page_ok;

  			if (inode->i_blkbits == PAGE_SHIFT ||

  					!mapping->a_ops->is_partially_uptodate)
  				goto page_not_up_to_date;

  			/* pipes can't handle partially uptodate pages */
  			if (unlikely(iter->type & ITER_PIPE))
  				goto page_not_up_to_date;

  			if (!trylock_page(page))

  				goto page_not_up_to_date;