ext4: Add delayed allocation support in data=writeback mode
Updated with fixes from Mingming Cao <cmm@us.ibm.com> to unlock and
release the page from page cache if the delalloc write_begin failed, and
properly handle preallocated blocks. Also added a fix to clear
buffer_delay in block_write_full_page() after allocating a delayed
buffer.
Updated with fixes from Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
to update i_disksize properly and to add bmap support for delayed
allocation.
Updated with a fix from Valerie Clement <valerie.clement@bull.net> to
avoid filesystem corruption when the filesystem is mounted with the
delalloc option and blocksize < pagesize.
Signed-off-by: Alex Tomas <alex@clusterfs.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 7b95691..2bef4f8 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -32,6 +32,7 @@
#include <linux/string.h>
#include <linux/buffer_head.h>
#include <linux/writeback.h>
+#include <linux/pagevec.h>
#include <linux/mpage.h>
#include <linux/uio.h>
#include <linux/bio.h>
@@ -46,6 +47,8 @@
new_size);
}
+static void ext4_invalidatepage(struct page *page, unsigned long offset);
+
/*
* Test whether an inode is a fast symlink.
*/
@@ -1408,6 +1411,669 @@
}
/*
+ * Delayed allocation stuff
+ */
+
+struct mpage_da_data {
+ struct inode *inode;
+ struct buffer_head lbh; /* extent of blocks */
+ unsigned long first_page, next_page; /* extent of pages */
+ get_block_t *get_block;
+ struct writeback_control *wbc;
+};
+
+/*
+ * mpage_da_submit_io - walks through extent of pages and try to write
+ * them with __mpage_writepage()
+ *
+ * @mpd->inode: inode
+ * @mpd->first_page: first page of the extent
+ * @mpd->next_page: page after the last page of the extent
+ * @mpd->get_block: the filesystem's block mapper function
+ *
+ * By the time mpage_da_submit_io() is called we expect all blocks
+ * to be allocated. this may be wrong if allocation failed.
+ *
+ * As pages are already locked by write_cache_pages(), we can't use it
+ */
+static int mpage_da_submit_io(struct mpage_da_data *mpd)
+{
+ struct address_space *mapping = mpd->inode->i_mapping;
+ struct mpage_data mpd_pp = {
+ .bio = NULL,
+ .last_block_in_bio = 0,
+ .get_block = mpd->get_block,
+ .use_writepage = 1,
+ };
+ int ret = 0, err, nr_pages, i;
+ unsigned long index, end;
+ struct pagevec pvec;
+
+ BUG_ON(mpd->next_page <= mpd->first_page);
+
+ pagevec_init(&pvec, 0);
+ index = mpd->first_page;
+ end = mpd->next_page - 1;
+
+ while (index <= end) {
+ /* XXX: optimize tail */
+ nr_pages = pagevec_lookup(&pvec, mapping, index, PAGEVEC_SIZE);
+ if (nr_pages == 0)
+ break;
+ for (i = 0; i < nr_pages; i++) {
+ struct page *page = pvec.pages[i];
+
+ index = page->index;
+ if (index > end)
+ break;
+ index++;
+
+ err = __mpage_writepage(page, mpd->wbc, &mpd_pp);
+
+ /*
+ * In error case, we have to continue because
+ * remaining pages are still locked
+ * XXX: unlock and re-dirty them?
+ */
+ if (ret == 0)
+ ret = err;
+ }
+ pagevec_release(&pvec);
+ }
+ if (mpd_pp.bio)
+ mpage_bio_submit(WRITE, mpd_pp.bio);
+
+ return ret;
+}
+
+/*
+ * mpage_put_bnr_to_bhs - walk blocks and assign them actual numbers
+ *
+ * @mpd->inode - inode to walk through
+ * @exbh->b_blocknr - first block on a disk
+ * @exbh->b_size - amount of space in bytes
+ * @logical - first logical block to start assignment with
+ *
+ * the function goes through all passed space and put actual disk
+ * block numbers into buffer heads, dropping BH_Delay
+ */
+static void mpage_put_bnr_to_bhs(struct mpage_da_data *mpd, sector_t logical,
+ struct buffer_head *exbh)
+{
+ struct inode *inode = mpd->inode;
+ struct address_space *mapping = inode->i_mapping;
+ int blocks = exbh->b_size >> inode->i_blkbits;
+ sector_t pblock = exbh->b_blocknr, cur_logical;
+ struct buffer_head *head, *bh;
+ unsigned long index, end;
+ struct pagevec pvec;
+ int nr_pages, i;
+
+ index = logical >> (PAGE_CACHE_SHIFT - inode->i_blkbits);
+ end = (logical + blocks - 1) >> (PAGE_CACHE_SHIFT - inode->i_blkbits);
+ cur_logical = index << (PAGE_CACHE_SHIFT - inode->i_blkbits);
+
+ pagevec_init(&pvec, 0);
+
+ while (index <= end) {
+ /* XXX: optimize tail */
+ nr_pages = pagevec_lookup(&pvec, mapping, index, PAGEVEC_SIZE);
+ if (nr_pages == 0)
+ break;
+ for (i = 0; i < nr_pages; i++) {
+ struct page *page = pvec.pages[i];
+
+ index = page->index;
+ if (index > end)
+ break;
+ index++;
+
+ BUG_ON(!PageLocked(page));
+ BUG_ON(PageWriteback(page));
+ BUG_ON(!page_has_buffers(page));
+
+ bh = page_buffers(page);
+ head = bh;
+
+ /* skip blocks out of the range */
+ do {
+ if (cur_logical >= logical)
+ break;
+ cur_logical++;
+ } while ((bh = bh->b_this_page) != head);
+
+ do {
+ if (cur_logical >= logical + blocks)
+ break;
+
+ if (buffer_delay(bh)) {
+ bh->b_blocknr = pblock;
+ clear_buffer_delay(bh);
+ } else if (buffer_mapped(bh)) {
+ BUG_ON(bh->b_blocknr != pblock);
+ }
+
+ cur_logical++;
+ pblock++;
+ } while ((bh = bh->b_this_page) != head);
+ }
+ pagevec_release(&pvec);
+ }
+}
+
+
+/*
+ * __unmap_underlying_blocks - just a helper function to unmap
+ * set of blocks described by @bh
+ */
+static inline void __unmap_underlying_blocks(struct inode *inode,
+ struct buffer_head *bh)
+{
+ struct block_device *bdev = inode->i_sb->s_bdev;
+ int blocks, i;
+
+ blocks = bh->b_size >> inode->i_blkbits;
+ for (i = 0; i < blocks; i++)
+ unmap_underlying_metadata(bdev, bh->b_blocknr + i);
+}
+
+/*
+ * mpage_da_map_blocks - go through given space
+ *
+ * @mpd->lbh - bh describing space
+ * @mpd->get_block - the filesystem's block mapper function
+ *
+ * The function skips space we know is already mapped to disk blocks.
+ *
+ * The function ignores errors ->get_block() returns, thus real
+ * error handling is postponed to __mpage_writepage()
+ */
+static void mpage_da_map_blocks(struct mpage_da_data *mpd)
+{
+ struct buffer_head *lbh = &mpd->lbh;
+ int err = 0, remain = lbh->b_size;
+ sector_t next = lbh->b_blocknr;
+ struct buffer_head new;
+
+ /*
+ * We consider only non-mapped and non-allocated blocks
+ */
+ if (buffer_mapped(lbh) && !buffer_delay(lbh))
+ return;
+
+ while (remain) {
+ new.b_state = lbh->b_state;
+ new.b_blocknr = 0;
+ new.b_size = remain;
+ err = mpd->get_block(mpd->inode, next, &new, 1);
+ if (err) {
+ /*
+ * Rather than implement own error handling
+ * here, we just leave remaining blocks
+ * unallocated and try again with ->writepage()
+ */
+ break;
+ }
+ BUG_ON(new.b_size == 0);
+
+ if (buffer_new(&new))
+ __unmap_underlying_blocks(mpd->inode, &new);
+
+ /*
+ * If blocks are delayed marked, we need to
+ * put actual blocknr and drop delayed bit
+ */
+ if (buffer_delay(lbh))
+ mpage_put_bnr_to_bhs(mpd, next, &new);
+
+ /* go for the remaining blocks */
+ next += new.b_size >> mpd->inode->i_blkbits;
+ remain -= new.b_size;
+ }
+}
+
+#define BH_FLAGS ((1 << BH_Uptodate) | (1 << BH_Mapped) | (1 << BH_Delay))
+
+/*
+ * mpage_add_bh_to_extent - try to add one more block to extent of blocks
+ *
+ * @mpd->lbh - extent of blocks
+ * @logical - logical number of the block in the file
+ * @bh - bh of the block (used to access block's state)
+ *
+ * the function is used to collect contig. blocks in same state
+ */
+static void mpage_add_bh_to_extent(struct mpage_da_data *mpd,
+ sector_t logical, struct buffer_head *bh)
+{
+ struct buffer_head *lbh = &mpd->lbh;
+ sector_t next;
+
+ next = lbh->b_blocknr + (lbh->b_size >> mpd->inode->i_blkbits);
+
+ /*
+ * First block in the extent
+ */
+ if (lbh->b_size == 0) {
+ lbh->b_blocknr = logical;
+ lbh->b_size = bh->b_size;
+ lbh->b_state = bh->b_state & BH_FLAGS;
+ return;
+ }
+
+ /*
+ * Can we merge the block to our big extent?
+ */
+ if (logical == next && (bh->b_state & BH_FLAGS) == lbh->b_state) {
+ lbh->b_size += bh->b_size;
+ return;
+ }
+
+ /*
+ * We couldn't merge the block to our extent, so we
+ * need to flush current extent and start new one
+ */
+ mpage_da_map_blocks(mpd);
+
+ /*
+ * Now start a new extent
+ */
+ lbh->b_size = bh->b_size;
+ lbh->b_state = bh->b_state & BH_FLAGS;
+ lbh->b_blocknr = logical;
+}
+
+/*
+ * __mpage_da_writepage - finds extent of pages and blocks
+ *
+ * @page: page to consider
+ * @wbc: not used, we just follow rules
+ * @data: context
+ *
+ * The function finds extents of pages and scan them for all blocks.
+ */
+static int __mpage_da_writepage(struct page *page,
+ struct writeback_control *wbc, void *data)
+{
+ struct mpage_da_data *mpd = data;
+ struct inode *inode = mpd->inode;
+ struct buffer_head *bh, *head, fake;
+ sector_t logical;
+
+ /*
+ * Can we merge this page to current extent?
+ */
+ if (mpd->next_page != page->index) {
+ /*
+ * Nope, we can't. So, we map non-allocated blocks
+ * and start IO on them using __mpage_writepage()
+ */
+ if (mpd->next_page != mpd->first_page) {
+ mpage_da_map_blocks(mpd);
+ mpage_da_submit_io(mpd);
+ }
+
+ /*
+ * Start next extent of pages ...
+ */
+ mpd->first_page = page->index;
+
+ /*
+ * ... and blocks
+ */
+ mpd->lbh.b_size = 0;
+ mpd->lbh.b_state = 0;
+ mpd->lbh.b_blocknr = 0;
+ }
+
+ mpd->next_page = page->index + 1;
+ logical = (sector_t) page->index <<
+ (PAGE_CACHE_SHIFT - inode->i_blkbits);
+
+ if (!page_has_buffers(page)) {
+ /*
+ * There is no attached buffer heads yet (mmap?)
+ * we treat the page asfull of dirty blocks
+ */
+ bh = &fake;
+ bh->b_size = PAGE_CACHE_SIZE;
+ bh->b_state = 0;
+ set_buffer_dirty(bh);
+ set_buffer_uptodate(bh);
+ mpage_add_bh_to_extent(mpd, logical, bh);
+ } else {
+ /*
+ * Page with regular buffer heads, just add all dirty ones
+ */
+ head = page_buffers(page);
+ bh = head;
+ do {
+ BUG_ON(buffer_locked(bh));
+ if (buffer_dirty(bh))
+ mpage_add_bh_to_extent(mpd, logical, bh);
+ logical++;
+ } while ((bh = bh->b_this_page) != head);
+ }
+
+ return 0;
+}
+
+/*
+ * mpage_da_writepages - walk the list of dirty pages of the given
+ * address space, allocates non-allocated blocks, maps newly-allocated
+ * blocks to existing bhs and issue IO them
+ *
+ * @mapping: address space structure to write
+ * @wbc: subtract the number of written pages from *@wbc->nr_to_write
+ * @get_block: the filesystem's block mapper function.
+ *
+ * This is a library function, which implements the writepages()
+ * address_space_operation.
+ *
+ * In order to avoid duplication of logic that deals with partial pages,
+ * multiple bio per page, etc, we find non-allocated blocks, allocate
+ * them with minimal calls to ->get_block() and re-use __mpage_writepage()
+ *
+ * It's important that we call __mpage_writepage() only once for each
+ * involved page, otherwise we'd have to implement more complicated logic
+ * to deal with pages w/o PG_lock or w/ PG_writeback and so on.
+ *
+ * See comments to mpage_writepages()
+ */
+static int mpage_da_writepages(struct address_space *mapping,
+ struct writeback_control *wbc,
+ get_block_t get_block)
+{
+ struct mpage_da_data mpd;
+ int ret;
+
+ if (!get_block)
+ return generic_writepages(mapping, wbc);
+
+ mpd.wbc = wbc;
+ mpd.inode = mapping->host;
+ mpd.lbh.b_size = 0;
+ mpd.lbh.b_state = 0;
+ mpd.lbh.b_blocknr = 0;
+ mpd.first_page = 0;
+ mpd.next_page = 0;
+ mpd.get_block = get_block;
+
+ ret = write_cache_pages(mapping, wbc, __mpage_da_writepage, &mpd);
+
+ /*
+ * Handle last extent of pages
+ */
+ if (mpd.next_page != mpd.first_page) {
+ mpage_da_map_blocks(&mpd);
+ mpage_da_submit_io(&mpd);
+ }
+
+ return ret;
+}
+
+/*
+ * this is a special callback for ->write_begin() only
+ * it's intention is to return mapped block or reserve space
+ */
+static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
+ struct buffer_head *bh_result, int create)
+{
+ int ret = 0;
+
+ BUG_ON(create == 0);
+ BUG_ON(bh_result->b_size != inode->i_sb->s_blocksize);
+
+ /*
+ * first, we need to know whether the block is allocated already
+ * preallocated blocks are unmapped but should treated
+ * the same as allocated blocks.
+ */
+ ret = ext4_get_blocks_wrap(NULL, inode, iblock, 1, bh_result, 0, 0);
+ if (ret == 0) {
+ /* the block isn't allocated yet, let's reserve space */
+ /* XXX: call reservation here */
+ /*
+ * XXX: __block_prepare_write() unmaps passed block,
+ * is it OK?
+ */
+ map_bh(bh_result, inode->i_sb, 0);
+ set_buffer_new(bh_result);
+ set_buffer_delay(bh_result);
+ } else if (ret > 0) {
+ bh_result->b_size = (ret << inode->i_blkbits);
+ ret = 0;
+ }
+
+ return ret;
+}
+
+static int ext4_da_get_block_write(struct inode *inode, sector_t iblock,
+ struct buffer_head *bh_result, int create)
+{
+ int ret, needed_blocks = ext4_writepage_trans_blocks(inode);
+ unsigned max_blocks = bh_result->b_size >> inode->i_blkbits;
+ loff_t disksize = EXT4_I(inode)->i_disksize;
+ handle_t *handle = NULL;
+
+ if (create) {
+ handle = ext4_journal_start(inode, needed_blocks);
+ if (IS_ERR(handle)) {
+ ret = PTR_ERR(handle);
+ goto out;
+ }
+ }
+
+ ret = ext4_get_blocks_wrap(handle, inode, iblock, max_blocks,
+ bh_result, create, 0);
+ if (ret > 0) {
+ bh_result->b_size = (ret << inode->i_blkbits);
+
+ /*
+ * Update on-disk size along with block allocation
+ * we don't use 'extend_disksize' as size may change
+ * within already allocated block -bzzz
+ */
+ disksize = ((loff_t) iblock + ret) << inode->i_blkbits;
+ if (disksize > i_size_read(inode))
+ disksize = i_size_read(inode);
+ if (disksize > EXT4_I(inode)->i_disksize) {
+ /*
+ * XXX: replace with spinlock if seen contended -bzzz
+ */
+ down_write(&EXT4_I(inode)->i_data_sem);
+ if (disksize > EXT4_I(inode)->i_disksize)
+ EXT4_I(inode)->i_disksize = disksize;
+ up_write(&EXT4_I(inode)->i_data_sem);
+
+ if (EXT4_I(inode)->i_disksize == disksize) {
+ if (handle == NULL)
+ handle = ext4_journal_start(inode, 1);
+ if (!IS_ERR(handle))
+ ext4_mark_inode_dirty(handle, inode);
+ }
+ }
+
+ ret = 0;
+ }
+
+out:
+ if (handle && !IS_ERR(handle))
+ ext4_journal_stop(handle);
+
+ return ret;
+}
+/* FIXME!! only support data=writeback mode */
+static int ext4_da_writepage(struct page *page,
+ struct writeback_control *wbc)
+{
+ struct inode *inode = page->mapping->host;
+ handle_t *handle = NULL;
+ int ret = 0;
+ int err;
+
+ if (ext4_journal_current_handle())
+ goto out_fail;
+
+ handle = ext4_journal_start(inode, ext4_writepage_trans_blocks(inode));
+ if (IS_ERR(handle)) {
+ ret = PTR_ERR(handle);
+ goto out_fail;
+ }
+
+ if (test_opt(inode->i_sb, NOBH) && ext4_should_writeback_data(inode))
+ ret = nobh_writepage(page, ext4_get_block, wbc);
+ else
+ ret = block_write_full_page(page, ext4_get_block, wbc);
+
+ if (!ret && inode->i_size > EXT4_I(inode)->i_disksize) {
+ EXT4_I(inode)->i_disksize = inode->i_size;
+ ext4_mark_inode_dirty(handle, inode);
+ }
+
+ err = ext4_journal_stop(handle);
+ if (!ret)
+ ret = err;
+ return ret;
+
+out_fail:
+ redirty_page_for_writepage(wbc, page);
+ unlock_page(page);
+ return ret;
+}
+
+static int ext4_da_writepages(struct address_space *mapping,
+ struct writeback_control *wbc)
+{
+ return mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
+}
+
+static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
+ loff_t pos, unsigned len, unsigned flags,
+ struct page **pagep, void **fsdata)
+{
+ int ret;
+ struct page *page;
+ pgoff_t index;
+ unsigned from, to;
+ struct inode *inode = mapping->host;
+ handle_t *handle;
+
+ index = pos >> PAGE_CACHE_SHIFT;
+ from = pos & (PAGE_CACHE_SIZE - 1);
+ to = from + len;
+
+ /*
+ * With delayed allocation, we don't log the i_disksize update
+ * if there is delayed block allocation. But we still need
+ * to journalling the i_disksize update if writes to the end
+ * of file which has an already mapped buffer.
+ */
+ handle = ext4_journal_start(inode, 1);
+ if (IS_ERR(handle)) {
+ ret = PTR_ERR(handle);
+ goto out;
+ }
+
+ page = __grab_cache_page(mapping, index);
+ if (!page)
+ return -ENOMEM;
+ *pagep = page;
+
+ ret = block_write_begin(file, mapping, pos, len, flags, pagep, fsdata,
+ ext4_da_get_block_prep);
+ if (ret < 0) {
+ unlock_page(page);
+ ext4_journal_stop(handle);
+ page_cache_release(page);
+ }
+
+out:
+ return ret;
+}
+
+static int ext4_bh_unmapped_or_delay(handle_t *handle, struct buffer_head *bh)
+{
+ return !buffer_mapped(bh) || buffer_delay(bh);
+}
+
+static int ext4_da_write_end(struct file *file,
+ struct address_space *mapping,
+ loff_t pos, unsigned len, unsigned copied,
+ struct page *page, void *fsdata)
+{
+ struct inode *inode = mapping->host;
+ int ret = 0, ret2;
+ handle_t *handle = ext4_journal_current_handle();
+ loff_t new_i_size;
+
+ /*
+ * generic_write_end() will run mark_inode_dirty() if i_size
+ * changes. So let's piggyback the i_disksize mark_inode_dirty
+ * into that.
+ */
+
+ new_i_size = pos + copied;
+ if (new_i_size > EXT4_I(inode)->i_disksize)
+ if (!walk_page_buffers(NULL, page_buffers(page),
+ 0, len, NULL, ext4_bh_unmapped_or_delay)){
+ /*
+ * Updating i_disksize when extending file without
+ * needing block allocation
+ */
+ if (ext4_should_order_data(inode))
+ ret = ext4_jbd2_file_inode(handle, inode);
+
+ EXT4_I(inode)->i_disksize = new_i_size;
+ }
+ ret2 = generic_write_end(file, mapping, pos, len, copied,
+ page, fsdata);
+ copied = ret2;
+ if (ret2 < 0)
+ ret = ret2;
+ ret2 = ext4_journal_stop(handle);
+ if (!ret)
+ ret = ret2;
+
+ return ret ? ret : copied;
+}
+
+static void ext4_da_invalidatepage(struct page *page, unsigned long offset)
+{
+ struct buffer_head *head, *bh;
+ unsigned int curr_off = 0;
+
+ /*
+ * Drop reserved blocks
+ */
+ BUG_ON(!PageLocked(page));
+ if (!page_has_buffers(page))
+ goto out;
+
+ head = page_buffers(page);
+ bh = head;
+ do {
+ unsigned int next_off = curr_off + bh->b_size;
+
+ /*
+ * is this block fully invalidated?
+ */
+ if (offset <= curr_off && buffer_delay(bh)) {
+ clear_buffer_delay(bh);
+ /* XXX: add real stuff here */
+ }
+ curr_off = next_off;
+ bh = bh->b_this_page;
+ } while (bh != head);
+
+out:
+ ext4_invalidatepage(page, offset);
+
+ return;
+}
+
+
+/*
* bmap() is special. It gets used by applications such as lilo and by
* the swapper to find the on-disk block of a specific piece of data.
*
@@ -1427,6 +2093,16 @@
journal_t *journal;
int err;
+ if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY) &&
+ test_opt(inode->i_sb, DELALLOC)) {
+ /*
+ * With delalloc we want to sync the file
+ * so that we can make sure we allocate
+ * blocks for file
+ */
+ filemap_write_and_wait(mapping);
+ }
+
if (EXT4_I(inode)->i_state & EXT4_STATE_JDATA) {
/*
* This is a REALLY heavyweight approach, but the use of
@@ -1471,11 +2147,6 @@
return 0;
}
-static int ext4_bh_unmapped_or_delay(handle_t *handle, struct buffer_head *bh)
-{
- return !buffer_mapped(bh) || buffer_delay(bh);
-}
-
/*
* Note that we don't need to start a transaction unless we're journaling data
* because we should have holes filled from ext4_page_mkwrite(). We even don't
@@ -1832,10 +2503,28 @@
.releasepage = ext4_releasepage,
};
+static const struct address_space_operations ext4_da_aops = {
+ .readpage = ext4_readpage,
+ .readpages = ext4_readpages,
+ .writepage = ext4_da_writepage,
+ .writepages = ext4_da_writepages,
+ .sync_page = block_sync_page,
+ .write_begin = ext4_da_write_begin,
+ .write_end = ext4_da_write_end,
+ .bmap = ext4_bmap,
+ .invalidatepage = ext4_da_invalidatepage,
+ .releasepage = ext4_releasepage,
+ .direct_IO = ext4_direct_IO,
+ .migratepage = buffer_migrate_page,
+};
+
void ext4_set_aops(struct inode *inode)
{
if (ext4_should_order_data(inode))
inode->i_mapping->a_ops = &ext4_ordered_aops;
+ else if (ext4_should_writeback_data(inode) &&
+ test_opt(inode->i_sb, DELALLOC))
+ inode->i_mapping->a_ops = &ext4_da_aops;
else if (ext4_should_writeback_data(inode))
inode->i_mapping->a_ops = &ext4_writeback_aops;
else