mremap: don't do uneccesary checks if new_len == old_len

[linux/fpc-iii.git] / mm / truncate.c

/*
 * 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 <linux/rmap.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);
}

/*
 * 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);

        /*
         * Some filesystems seem to re-dirty the page even after
         * the VM has canceled the dirty bit (eg ext3 journaling).
         * Hence dirty accounting check is placed after invalidation.
         */
        cancel_dirty_page(page);
        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)) {
                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);
                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 all gone from start onwards, we're done */
                        if (index == start)
                                break;
                        /* Otherwise restart to make sure all gone */
                        index = start;
                        continue;
                }
                if (index == start && indices[0] >= end) {
                        /* All gone out of hole to be punched, we're done */
                        pagevec_remove_exceptionals(&pvec);
                        pagevec_release(&pvec);
                        break;
                }
                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) {
                                /* Restart punch to make sure all gone */
                                index = start - 1;
                                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);
                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;

        pagevec_init(&pvec, 0);
        while (index <= end && pagevec_lookup_entries(&pvec, mapping, index,
                        min(end - index, (pgoff_t)PAGEVEC_SIZE - 1) + 1,
                        indices)) {
                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_file_page(page);
                        count += ret;
                }
                pagevec_remove_exceptionals(&pvec);
                pagevec_release(&pvec);
                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)
{
        struct mem_cgroup *memcg;
        unsigned long flags;

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

        if (page_has_private(page) && !try_to_release_page(page, GFP_KERNEL))
                return 0;

        memcg = mem_cgroup_begin_page_stat(page);
        spin_lock_irqsave(&mapping->tree_lock, flags);
        if (PageDirty(page))
                goto failed;

        BUG_ON(page_has_private(page));
        __delete_from_page_cache(page, NULL, memcg);
        spin_unlock_irqrestore(&mapping->tree_lock, flags);
        mem_cgroup_end_page_stat(memcg);

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

        page_cache_release(page);       /* pagecache ref */
        return 1;
failed:
        spin_unlock_irqrestore(&mapping->tree_lock, flags);
        mem_cgroup_end_page_stat(memcg);
        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)) {
                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);
                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 a lock serializing truncates and writes (generally
 * i_mutex but e.g. xfs uses a different lock) and before all filesystem
 * specific block truncation has been performed.
 */
void truncate_setsize(struct inode *inode, loff_t newsize)
{
        loff_t oldsize = inode->i_size;

        i_size_write(inode, newsize);
        if (newsize > oldsize)
                pagecache_isize_extended(inode, oldsize, newsize);
        truncate_pagecache(inode, newsize);
}
EXPORT_SYMBOL(truncate_setsize);

/**
 * pagecache_isize_extended - update pagecache after extension of i_size
 * @inode:      inode for which i_size was extended
 * @from:       original inode size
 * @to:         new inode size
 *
 * Handle extension of inode size either caused by extending truncate or by
 * write starting after current i_size. We mark the page straddling current
 * i_size RO so that page_mkwrite() is called on the nearest write access to
 * the page.  This way filesystem can be sure that page_mkwrite() is called on
 * the page before user writes to the page via mmap after the i_size has been
 * changed.
 *
 * The function must be called after i_size is updated so that page fault
 * coming after we unlock the page will already see the new i_size.
 * The function must be called while we still hold i_mutex - this not only
 * makes sure i_size is stable but also that userspace cannot observe new
 * i_size value before we are prepared to store mmap writes at new inode size.
 */
void pagecache_isize_extended(struct inode *inode, loff_t from, loff_t to)
{
        int bsize = 1 << inode->i_blkbits;
        loff_t rounded_from;
        struct page *page;
        pgoff_t index;

        WARN_ON(to > inode->i_size);

        if (from >= to || bsize == PAGE_CACHE_SIZE)
                return;
        /* Page straddling @from will not have any hole block created? */
        rounded_from = round_up(from, bsize);
        if (to <= rounded_from || !(rounded_from & (PAGE_CACHE_SIZE - 1)))
                return;

        index = from >> PAGE_CACHE_SHIFT;
        page = find_lock_page(inode->i_mapping, index);
        /* Page not cached? Nothing to do */
        if (!page)
                return;
        /*
         * See clear_page_dirty_for_io() for details why set_page_dirty()
         * is needed.
         */
        if (page_mkclean(page))
                set_page_dirty(page);
        unlock_page(page);
        page_cache_release(page);
}
EXPORT_SYMBOL(pagecache_isize_extended);

/**
 * 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);