/*
   * mm/truncate.c - code for taking down pages from address_spaces
   *
   * Copyright (C) 2002, Linus Torvalds
   *

   * 10Sep2002	Andrew Morton

   *		Initial version.
   */
  
  #include <linux/kernel.h>

  #include <linux/backing-dev.h>

  #include <linux/gfp.h>

  #include <linux/mm.h>

  #include <linux/swap.h>

  #include <linux/export.h>

  #include <linux/pagemap.h>

  #include <linux/highmem.h>

  #include <linux/pagevec.h>

  #include <linux/task_io_accounting_ops.h>

  #include <linux/buffer_head.h>	/* grr. try_to_release_page,

  				   do_invalidatepage */

  #include <linux/cleancache.h>

  #include "internal.h"

  static void clear_exceptional_entry(struct address_space *mapping,
  				    pgoff_t index, void *entry)
  {

  	struct radix_tree_node *node;
  	void **slot;

  	/* Handled by shmem itself */
  	if (shmem_mapping(mapping))
  		return;
  
  	spin_lock_irq(&mapping->tree_lock);
  	/*
  	 * Regular page slots are stabilized by the page lock even
  	 * without the tree itself locked.  These unlocked entries
  	 * need verification under the tree lock.
  	 */

  	if (!__radix_tree_lookup(&mapping->page_tree, index, &node, &slot))
  		goto unlock;
  	if (*slot != entry)
  		goto unlock;
  	radix_tree_replace_slot(slot, NULL);
  	mapping->nrshadows--;
  	if (!node)
  		goto unlock;
  	workingset_node_shadows_dec(node);
  	/*
  	 * Don't track node without shadow entries.
  	 *
  	 * 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 (!workingset_node_shadows(node) &&
  	    !list_empty(&node->private_list))
  		list_lru_del(&workingset_shadow_nodes, &node->private_list);
  	__radix_tree_delete_node(&mapping->page_tree, node);
  unlock:

  	spin_unlock_irq(&mapping->tree_lock);
  }

  /**

   * do_invalidatepage - invalidate part or all of a page

   * @page: the page which is affected

   * @offset: start of the range to invalidate
   * @length: length of the range to invalidate

   *
   * do_invalidatepage() is called when all or part of the page has become
   * invalidated by a truncate operation.
   *
   * do_invalidatepage() does not have to release all buffers, but it must
   * ensure that no dirty buffer is left outside @offset and that no I/O
   * is underway against any of the blocks which are outside the truncation
   * point.  Because the caller is about to free (and possibly reuse) those
   * blocks on-disk.
   */

  void do_invalidatepage(struct page *page, unsigned int offset,
  		       unsigned int length)

  {

  	void (*invalidatepage)(struct page *, unsigned int, unsigned int);

  	invalidatepage = page->mapping->a_ops->invalidatepage;

  #ifdef CONFIG_BLOCK

  	if (!invalidatepage)
  		invalidatepage = block_invalidatepage;

  #endif

  	if (invalidatepage)

  		(*invalidatepage)(page, offset, length);

  }

  /*
   * This cancels just the dirty bit on the kernel page itself, it
   * does NOT actually remove dirty bits on any mmap's that may be
   * around. It also leaves the page tagged dirty, so any sync
   * activity will still find it on the dirty lists, and in particular,
   * clear_page_dirty_for_io() will still look at the dirty bits in
   * the VM.
   *
   * Doing this should *normally* only ever be done when a page
   * is truncated, and is not actually mapped anywhere at all. However,
   * fs/buffer.c does this when it notices that somebody has cleaned
   * out all the buffers on a page without actually doing it through
   * the VM. Can you say "ext3 is horribly ugly"? Tought you could.
   */

  void cancel_dirty_page(struct page *page, unsigned int account_size)
  {

  	if (TestClearPageDirty(page)) {
  		struct address_space *mapping = page->mapping;
  		if (mapping && mapping_cap_account_dirty(mapping)) {
  			dec_zone_page_state(page, NR_FILE_DIRTY);

  			dec_bdi_stat(mapping->backing_dev_info,
  					BDI_RECLAIMABLE);

  			if (account_size)
  				task_io_account_cancelled_write(account_size);
  		}

  	}

  }

  EXPORT_SYMBOL(cancel_dirty_page);

  /*
   * If truncate cannot remove the fs-private metadata from the page, the page

   * becomes orphaned.  It will be left on the LRU and may even be mapped into

   * user pagetables if we're racing with filemap_fault().

   *
   * We need to bale out if page->mapping is no longer equal to the original
   * mapping.  This happens a) when the VM reclaimed the page while we waited on

   * its lock, b) when a concurrent invalidate_mapping_pages got there first and

   * c) when tmpfs swizzles a page between a tmpfs inode and swapper_space.
   */

  static int

  truncate_complete_page(struct address_space *mapping, struct page *page)
  {
  	if (page->mapping != mapping)

  		return -EIO;

  	if (page_has_private(page))

  		do_invalidatepage(page, 0, PAGE_CACHE_SIZE);

  	cancel_dirty_page(page, PAGE_CACHE_SIZE);

  	ClearPageMappedToDisk(page);

  	delete_from_page_cache(page);

  	return 0;

  }
  
  /*

   * This is for invalidate_mapping_pages().  That function can be called at

   * any time, and is not supposed to throw away dirty pages.  But pages can

   * be marked dirty at any time too, so use remove_mapping which safely
   * discards clean, unused pages.

   *
   * Returns non-zero if the page was successfully invalidated.
   */
  static int
  invalidate_complete_page(struct address_space *mapping, struct page *page)
  {

  	int ret;

  	if (page->mapping != mapping)
  		return 0;

  	if (page_has_private(page) && !try_to_release_page(page, 0))

  		return 0;

  	ret = remove_mapping(mapping, page);

  
  	return ret;

  }

  int truncate_inode_page(struct address_space *mapping, struct page *page)
  {
  	if (page_mapped(page)) {
  		unmap_mapping_range(mapping,
  				   (loff_t)page->index << PAGE_CACHE_SHIFT,
  				   PAGE_CACHE_SIZE, 0);
  	}
  	return truncate_complete_page(mapping, page);
  }

  /*

   * Used to get rid of pages on hardware memory corruption.
   */
  int generic_error_remove_page(struct address_space *mapping, struct page *page)
  {
  	if (!mapping)
  		return -EINVAL;
  	/*
  	 * Only punch for normal data pages for now.
  	 * Handling other types like directories would need more auditing.
  	 */
  	if (!S_ISREG(mapping->host->i_mode))
  		return -EIO;
  	return truncate_inode_page(mapping, page);
  }
  EXPORT_SYMBOL(generic_error_remove_page);
  
  /*

   * Safely invalidate one page from its pagecache mapping.
   * It only drops clean, unused pages. The page must be locked.
   *
   * Returns 1 if the page is successfully invalidated, otherwise 0.
   */
  int invalidate_inode_page(struct page *page)
  {
  	struct address_space *mapping = page_mapping(page);
  	if (!mapping)
  		return 0;
  	if (PageDirty(page) || PageWriteback(page))
  		return 0;
  	if (page_mapped(page))
  		return 0;
  	return invalidate_complete_page(mapping, page);
  }

  /**

   * truncate_inode_pages_range - truncate range of pages specified by start & end byte offsets

   * @mapping: mapping to truncate
   * @lstart: offset from which to truncate

   * @lend: offset to which to truncate (inclusive)

   *

   * Truncate the page cache, removing the pages that are between

   * specified offsets (and zeroing out partial pages
   * if lstart or lend + 1 is not page aligned).

   *
   * Truncate takes two passes - the first pass is nonblocking.  It will not
   * block on page locks and it will not block on writeback.  The second pass
   * will wait.  This is to prevent as much IO as possible in the affected region.
   * The first pass will remove most pages, so the search cost of the second pass
   * is low.
   *

   * We pass down the cache-hot hint to the page freeing code.  Even if the
   * mapping is large, it is probably the case that the final pages are the most
   * recently touched, and freeing happens in ascending file offset order.

   *
   * Note that since ->invalidatepage() accepts range to invalidate
   * truncate_inode_pages_range is able to handle cases where lend + 1 is not
   * page aligned properly.

   */

  void truncate_inode_pages_range(struct address_space *mapping,
  				loff_t lstart, loff_t lend)

  {

  	pgoff_t		start;		/* inclusive */
  	pgoff_t		end;		/* exclusive */
  	unsigned int	partial_start;	/* inclusive */
  	unsigned int	partial_end;	/* exclusive */
  	struct pagevec	pvec;

  	pgoff_t		indices[PAGEVEC_SIZE];

  	pgoff_t		index;
  	int		i;

  	cleancache_invalidate_inode(mapping);

  	if (mapping->nrpages == 0 && mapping->nrshadows == 0)

  		return;

  	/* Offsets within partial pages */
  	partial_start = lstart & (PAGE_CACHE_SIZE - 1);
  	partial_end = (lend + 1) & (PAGE_CACHE_SIZE - 1);
  
  	/*
  	 * 'start' and 'end' always covers the range of pages to be fully
  	 * truncated. Partial pages are covered with 'partial_start' at the
  	 * start of the range and 'partial_end' at the end of the range.
  	 * Note that 'end' is exclusive while 'lend' is inclusive.
  	 */
  	start = (lstart + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
  	if (lend == -1)
  		/*
  		 * lend == -1 indicates end-of-file so we have to set 'end'
  		 * to the highest possible pgoff_t and since the type is
  		 * unsigned we're using -1.
  		 */
  		end = -1;
  	else
  		end = (lend + 1) >> PAGE_CACHE_SHIFT;

  	pagevec_init(&pvec, 0);

  	index = start;

  	while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
  			min(end - index, (pgoff_t)PAGEVEC_SIZE),
  			indices)) {

  		mem_cgroup_uncharge_start();

  		for (i = 0; i < pagevec_count(&pvec); i++) {
  			struct page *page = pvec.pages[i];

  			/* We rely upon deletion not changing page->index */

  			index = indices[i];

  			if (index >= end)

  				break;

  			if (radix_tree_exceptional_entry(page)) {
  				clear_exceptional_entry(mapping, index, page);
  				continue;
  			}

  			if (!trylock_page(page))

  				continue;

  			WARN_ON(page->index != index);

  			if (PageWriteback(page)) {
  				unlock_page(page);
  				continue;
  			}

  			truncate_inode_page(mapping, page);

  			unlock_page(page);
  		}

  		pagevec_remove_exceptionals(&pvec);

  		pagevec_release(&pvec);

  		mem_cgroup_uncharge_end();

  		cond_resched();

  		index++;

  	}

  	if (partial_start) {

  		struct page *page = find_lock_page(mapping, start - 1);
  		if (page) {

  			unsigned int top = PAGE_CACHE_SIZE;
  			if (start > end) {
  				/* Truncation within a single page */
  				top = partial_end;
  				partial_end = 0;
  			}

  			wait_on_page_writeback(page);

  			zero_user_segment(page, partial_start, top);
  			cleancache_invalidate_page(mapping, page);
  			if (page_has_private(page))
  				do_invalidatepage(page, partial_start,
  						  top - partial_start);

  			unlock_page(page);
  			page_cache_release(page);
  		}
  	}

  	if (partial_end) {
  		struct page *page = find_lock_page(mapping, end);
  		if (page) {
  			wait_on_page_writeback(page);
  			zero_user_segment(page, 0, partial_end);
  			cleancache_invalidate_page(mapping, page);
  			if (page_has_private(page))
  				do_invalidatepage(page, 0,
  						  partial_end);
  			unlock_page(page);
  			page_cache_release(page);
  		}
  	}
  	/*
  	 * If the truncation happened within a single page no pages
  	 * will be released, just zeroed, so we can bail out now.
  	 */
  	if (start >= end)
  		return;

  	index = start;

  	for ( ; ; ) {
  		cond_resched();

  		if (!pagevec_lookup_entries(&pvec, mapping, index,
  			min(end - index, (pgoff_t)PAGEVEC_SIZE),
  			indices)) {

  			if (index == start)

  				break;

  			index = start;

  			continue;
  		}

  		if (index == start && indices[0] >= end) {
  			pagevec_remove_exceptionals(&pvec);

  			pagevec_release(&pvec);
  			break;
  		}

  		mem_cgroup_uncharge_start();

  		for (i = 0; i < pagevec_count(&pvec); i++) {
  			struct page *page = pvec.pages[i];

  			/* We rely upon deletion not changing page->index */

  			index = indices[i];

  			if (index >= end)

  				break;

  			if (radix_tree_exceptional_entry(page)) {
  				clear_exceptional_entry(mapping, index, page);
  				continue;
  			}

  			lock_page(page);

  			WARN_ON(page->index != index);

  			wait_on_page_writeback(page);

  			truncate_inode_page(mapping, page);

  			unlock_page(page);
  		}

  		pagevec_remove_exceptionals(&pvec);

  		pagevec_release(&pvec);

  		mem_cgroup_uncharge_end();

  		index++;

  	}

  	cleancache_invalidate_inode(mapping);

  }

  EXPORT_SYMBOL(truncate_inode_pages_range);

  /**
   * truncate_inode_pages - truncate *all* the pages from an offset
   * @mapping: mapping to truncate
   * @lstart: offset from which to truncate
   *

   * Called under (and serialised by) inode->i_mutex.

   *
   * Note: When this function returns, there can be a page in the process of
   * deletion (inside __delete_from_page_cache()) in the specified range.  Thus
   * mapping->nrpages can be non-zero when this function returns even after
   * truncation of the whole mapping.

   */
  void truncate_inode_pages(struct address_space *mapping, loff_t lstart)
  {
  	truncate_inode_pages_range(mapping, lstart, (loff_t)-1);
  }

  EXPORT_SYMBOL(truncate_inode_pages);

  /**

   * truncate_inode_pages_final - truncate *all* pages before inode dies
   * @mapping: mapping to truncate
   *
   * Called under (and serialized by) inode->i_mutex.
   *
   * Filesystems have to use this in the .evict_inode path to inform the
   * VM that this is the final truncate and the inode is going away.
   */
  void truncate_inode_pages_final(struct address_space *mapping)
  {
  	unsigned long nrshadows;
  	unsigned long nrpages;
  
  	/*
  	 * Page reclaim can not participate in regular inode lifetime
  	 * management (can't call iput()) and thus can race with the
  	 * inode teardown.  Tell it when the address space is exiting,
  	 * so that it does not install eviction information after the
  	 * final truncate has begun.
  	 */
  	mapping_set_exiting(mapping);
  
  	/*
  	 * When reclaim installs eviction entries, it increases
  	 * nrshadows first, then decreases nrpages.  Make sure we see
  	 * this in the right order or we might miss an entry.
  	 */
  	nrpages = mapping->nrpages;
  	smp_rmb();
  	nrshadows = mapping->nrshadows;
  
  	if (nrpages || nrshadows) {
  		/*
  		 * As truncation uses a lockless tree lookup, cycle
  		 * the tree lock to make sure any ongoing tree
  		 * modification that does not see AS_EXITING is
  		 * completed before starting the final truncate.
  		 */
  		spin_lock_irq(&mapping->tree_lock);
  		spin_unlock_irq(&mapping->tree_lock);
  
  		truncate_inode_pages(mapping, 0);
  	}
  }
  EXPORT_SYMBOL(truncate_inode_pages_final);
  
  /**

   * invalidate_mapping_pages - Invalidate all the unlocked pages of one inode
   * @mapping: the address_space which holds the pages to invalidate
   * @start: the offset 'from' which to invalidate
   * @end: the offset 'to' which to invalidate (inclusive)
   *
   * This function only removes the unlocked pages, if you want to
   * remove all the pages of one inode, you must call truncate_inode_pages.
   *
   * invalidate_mapping_pages() will not block on IO activity. It will not
   * invalidate pages which are dirty, locked, under writeback or mapped into
   * pagetables.
   */
  unsigned long invalidate_mapping_pages(struct address_space *mapping,

  		pgoff_t start, pgoff_t end)

  {

  	pgoff_t indices[PAGEVEC_SIZE];

  	struct pagevec pvec;

  	pgoff_t index = start;

  	unsigned long ret;
  	unsigned long count = 0;

  	int i;

  	/*
  	 * Note: this function may get called on a shmem/tmpfs mapping:
  	 * pagevec_lookup() might then return 0 prematurely (because it
  	 * got a gangful of swap entries); but it's hardly worth worrying
  	 * about - it can rarely have anything to free from such a mapping
  	 * (most pages are dirty), and already skips over any difficulties.
  	 */

  	pagevec_init(&pvec, 0);

  	while (index <= end && pagevec_lookup_entries(&pvec, mapping, index,
  			min(end - index, (pgoff_t)PAGEVEC_SIZE - 1) + 1,
  			indices)) {

  		mem_cgroup_uncharge_start();

  		for (i = 0; i < pagevec_count(&pvec); i++) {
  			struct page *page = pvec.pages[i];

  			/* We rely upon deletion not changing page->index */

  			index = indices[i];

  			if (index > end)
  				break;

  			if (radix_tree_exceptional_entry(page)) {
  				clear_exceptional_entry(mapping, index, page);
  				continue;
  			}

  			if (!trylock_page(page))
  				continue;
  			WARN_ON(page->index != index);

  			ret = invalidate_inode_page(page);

  			unlock_page(page);

  			/*
  			 * Invalidation is a hint that the page is no longer
  			 * of interest and try to speed up its reclaim.
  			 */
  			if (!ret)
  				deactivate_page(page);
  			count += ret;

  		}

  		pagevec_remove_exceptionals(&pvec);

  		pagevec_release(&pvec);

  		mem_cgroup_uncharge_end();

  		cond_resched();

  		index++;

  	}

  	return count;

  }

  EXPORT_SYMBOL(invalidate_mapping_pages);

  /*
   * This is like invalidate_complete_page(), except it ignores the page's
   * refcount.  We do this because invalidate_inode_pages2() needs stronger
   * invalidation guarantees, and cannot afford to leave pages behind because

   * shrink_page_list() has a temp ref on them, or because they're transiently
   * sitting in the lru_cache_add() pagevecs.

   */
  static int
  invalidate_complete_page2(struct address_space *mapping, struct page *page)
  {
  	if (page->mapping != mapping)
  		return 0;

  	if (page_has_private(page) && !try_to_release_page(page, GFP_KERNEL))

  		return 0;

  	spin_lock_irq(&mapping->tree_lock);

  	if (PageDirty(page))
  		goto failed;

  	BUG_ON(page_has_private(page));

  	__delete_from_page_cache(page, NULL);

  	spin_unlock_irq(&mapping->tree_lock);

  	mem_cgroup_uncharge_cache_page(page);

  
  	if (mapping->a_ops->freepage)
  		mapping->a_ops->freepage(page);

  	page_cache_release(page);	/* pagecache ref */
  	return 1;
  failed:

  	spin_unlock_irq(&mapping->tree_lock);

  	return 0;
  }

  static int do_launder_page(struct address_space *mapping, struct page *page)
  {
  	if (!PageDirty(page))
  		return 0;
  	if (page->mapping != mapping || mapping->a_ops->launder_page == NULL)
  		return 0;
  	return mapping->a_ops->launder_page(page);
  }

  /**
   * invalidate_inode_pages2_range - remove range of pages from an address_space

   * @mapping: the address_space

   * @start: the page offset 'from' which to invalidate
   * @end: the page offset 'to' which to invalidate (inclusive)
   *
   * Any pages which are found to be mapped into pagetables are unmapped prior to
   * invalidation.
   *

   * Returns -EBUSY if any pages could not be invalidated.

   */
  int invalidate_inode_pages2_range(struct address_space *mapping,
  				  pgoff_t start, pgoff_t end)
  {

  	pgoff_t indices[PAGEVEC_SIZE];

  	struct pagevec pvec;

  	pgoff_t index;

  	int i;
  	int ret = 0;

  	int ret2 = 0;

  	int did_range_unmap = 0;

  	cleancache_invalidate_inode(mapping);

  	pagevec_init(&pvec, 0);

  	index = start;

  	while (index <= end && pagevec_lookup_entries(&pvec, mapping, index,
  			min(end - index, (pgoff_t)PAGEVEC_SIZE - 1) + 1,
  			indices)) {

  		mem_cgroup_uncharge_start();

  		for (i = 0; i < pagevec_count(&pvec); i++) {

  			struct page *page = pvec.pages[i];

  
  			/* We rely upon deletion not changing page->index */

  			index = indices[i];

  			if (index > end)
  				break;

  			if (radix_tree_exceptional_entry(page)) {
  				clear_exceptional_entry(mapping, index, page);
  				continue;
  			}

  			lock_page(page);

  			WARN_ON(page->index != index);

  			if (page->mapping != mapping) {
  				unlock_page(page);
  				continue;
  			}

  			wait_on_page_writeback(page);

  			if (page_mapped(page)) {

  				if (!did_range_unmap) {
  					/*
  					 * Zap the rest of the file in one hit.
  					 */
  					unmap_mapping_range(mapping,

  					   (loff_t)index << PAGE_CACHE_SHIFT,
  					   (loff_t)(1 + end - index)
  							 << PAGE_CACHE_SHIFT,

  					    0);
  					did_range_unmap = 1;
  				} else {
  					/*
  					 * Just zap this page
  					 */
  					unmap_mapping_range(mapping,

  					   (loff_t)index << PAGE_CACHE_SHIFT,
  					   PAGE_CACHE_SIZE, 0);

  				}
  			}

  			BUG_ON(page_mapped(page));

  			ret2 = do_launder_page(mapping, page);
  			if (ret2 == 0) {
  				if (!invalidate_complete_page2(mapping, page))

  					ret2 = -EBUSY;

  			}
  			if (ret2 < 0)
  				ret = ret2;

  			unlock_page(page);
  		}

  		pagevec_remove_exceptionals(&pvec);

  		pagevec_release(&pvec);

  		mem_cgroup_uncharge_end();

  		cond_resched();

  		index++;

  	}

  	cleancache_invalidate_inode(mapping);

  	return ret;
  }
  EXPORT_SYMBOL_GPL(invalidate_inode_pages2_range);
  
  /**
   * invalidate_inode_pages2 - remove all pages from an address_space

   * @mapping: the address_space

   *
   * Any pages which are found to be mapped into pagetables are unmapped prior to
   * invalidation.
   *

   * Returns -EBUSY if any pages could not be invalidated.

   */
  int invalidate_inode_pages2(struct address_space *mapping)
  {
  	return invalidate_inode_pages2_range(mapping, 0, -1);
  }
  EXPORT_SYMBOL_GPL(invalidate_inode_pages2);

  
  /**
   * truncate_pagecache - unmap and remove pagecache that has been truncated
   * @inode: inode

   * @newsize: new file size

   *
   * inode's new i_size must already be written before truncate_pagecache
   * is called.
   *
   * This function should typically be called before the filesystem
   * releases resources associated with the freed range (eg. deallocates
   * blocks). This way, pagecache will always stay logically coherent
   * with on-disk format, and the filesystem would not have to deal with
   * situations such as writepage being called for a page that has already
   * had its underlying blocks deallocated.
   */

  void truncate_pagecache(struct inode *inode, loff_t newsize)

  {

  	struct address_space *mapping = inode->i_mapping;

  	loff_t holebegin = round_up(newsize, PAGE_SIZE);

  
  	/*
  	 * unmap_mapping_range is called twice, first simply for
  	 * efficiency so that truncate_inode_pages does fewer
  	 * single-page unmaps.  However after this first call, and
  	 * before truncate_inode_pages finishes, it is possible for
  	 * private pages to be COWed, which remain after
  	 * truncate_inode_pages finishes, hence the second
  	 * unmap_mapping_range call must be made for correctness.
  	 */

  	unmap_mapping_range(mapping, holebegin, 0, 1);
  	truncate_inode_pages(mapping, newsize);
  	unmap_mapping_range(mapping, holebegin, 0, 1);

  }
  EXPORT_SYMBOL(truncate_pagecache);
  
  /**

   * truncate_setsize - update inode and pagecache for a new file size
   * @inode: inode
   * @newsize: new file size
   *

   * truncate_setsize updates i_size and performs pagecache truncation (if
   * necessary) to @newsize. It will be typically be called from the filesystem's
   * setattr function when ATTR_SIZE is passed in.

   *

   * Must be called with inode_mutex held and before all filesystem specific
   * block truncation has been performed.

   */
  void truncate_setsize(struct inode *inode, loff_t newsize)
  {

  	i_size_write(inode, newsize);

  	truncate_pagecache(inode, newsize);

  }
  EXPORT_SYMBOL(truncate_setsize);
  
  /**

   * truncate_pagecache_range - unmap and remove pagecache that is hole-punched
   * @inode: inode
   * @lstart: offset of beginning of hole
   * @lend: offset of last byte of hole
   *
   * This function should typically be called before the filesystem
   * releases resources associated with the freed range (eg. deallocates
   * blocks). This way, pagecache will always stay logically coherent
   * with on-disk format, and the filesystem would not have to deal with
   * situations such as writepage being called for a page that has already
   * had its underlying blocks deallocated.
   */
  void truncate_pagecache_range(struct inode *inode, loff_t lstart, loff_t lend)
  {
  	struct address_space *mapping = inode->i_mapping;
  	loff_t unmap_start = round_up(lstart, PAGE_SIZE);
  	loff_t unmap_end = round_down(1 + lend, PAGE_SIZE) - 1;
  	/*
  	 * This rounding is currently just for example: unmap_mapping_range
  	 * expands its hole outwards, whereas we want it to contract the hole
  	 * inwards.  However, existing callers of truncate_pagecache_range are

  	 * doing their own page rounding first.  Note that unmap_mapping_range
  	 * allows holelen 0 for all, and we allow lend -1 for end of file.

  	 */
  
  	/*
  	 * Unlike in truncate_pagecache, unmap_mapping_range is called only
  	 * once (before truncating pagecache), and without "even_cows" flag:
  	 * hole-punching should not remove private COWed pages from the hole.
  	 */
  	if ((u64)unmap_end > (u64)unmap_start)
  		unmap_mapping_range(mapping, unmap_start,
  				    1 + unmap_end - unmap_start, 0);
  	truncate_inode_pages_range(mapping, lstart, lend);
  }
  EXPORT_SYMBOL(truncate_pagecache_range);