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Commit 2d90c160 authored by Jan Kara's avatar Jan Kara Committed by Theodore Ts'o
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ext4: more efficient SEEK_DATA implementation 



Using SEEK_DATA in a huge sparse file can easily lead to sotflockups as
ext4_seek_data() iterates hole block-by-block. Fix the problem by using
returned hole size from ext4_map_blocks() and thus skip the hole in one
go.

Update also SEEK_HOLE implementation to follow the same pattern as
SEEK_DATA to make future maintenance easier.

Furthermore we add cond_resched() to both ext4_seek_data() and
ext4_seek_hole() to avoid softlockups in case evil user creates huge
fragmented file and we have to go through lots of extents.

Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent e3fb8eb1

fs/ext4/ext4.h

+3 −0
Original line number Diff line number Diff line
@@ -2546,6 +2546,9 @@ extern void ext4_da_update_reserve_space(struct inode *inode, 
					int used, int quota_claim);

extern int ext4_issue_zeroout(struct inode *inode, ext4_lblk_t lblk,

			      ext4_fsblk_t pblk, ext4_lblk_t len);

extern int ext4_get_next_extent(struct inode *inode, ext4_lblk_t lblk,

				unsigned int map_len,

				struct extent_status *result);



/* indirect.c */

extern int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,

fs/ext4/file.c

+36 −61
Original line number Diff line number Diff line
@@ -426,7 +426,7 @@ static int ext4_file_open(struct inode * inode, struct file * filp) 
 */

static int ext4_find_unwritten_pgoff(struct inode *inode,

				     int whence,

				     struct ext4_map_blocks *map,

				     ext4_lblk_t end_blk,

				     loff_t *offset)

{

	struct pagevec pvec;
@@ -441,7 +441,7 @@ static int ext4_find_unwritten_pgoff(struct inode *inode, 
	blkbits = inode->i_sb->s_blocksize_bits;

	startoff = *offset;

	lastoff = startoff;

	endoff = (loff_t)(map->m_lblk + map->m_len) << blkbits;

	endoff = (loff_t)end_blk << blkbits;



	index = startoff >> PAGE_CACHE_SHIFT;

	end = endoff >> PAGE_CACHE_SHIFT;
@@ -559,12 +559,11 @@ static int ext4_find_unwritten_pgoff(struct inode *inode, 
static loff_t ext4_seek_data(struct file *file, loff_t offset, loff_t maxsize)

{

	struct inode *inode = file->f_mapping->host;

	struct ext4_map_blocks map;

	struct extent_status es;

	ext4_lblk_t start, last, end;

	loff_t dataoff, isize;

	int blkbits;

	int ret = 0;

	int ret;



	inode_lock(inode);
@@ -581,41 +580,32 @@ static loff_t ext4_seek_data(struct file *file, loff_t offset, loff_t maxsize) 
	dataoff = offset;



	do {

		map.m_lblk = last;

		map.m_len = end - last + 1;

		ret = ext4_map_blocks(NULL, inode, &map, 0);

		if (ret > 0 && !(map.m_flags & EXT4_MAP_UNWRITTEN)) {

			if (last != start)

				dataoff = (loff_t)last << blkbits;

			break;

		ret = ext4_get_next_extent(inode, last, end - last + 1, &es);

		if (ret <= 0) {

			/* No extent found -> no data */

			if (ret == 0)

				ret = -ENXIO;

			inode_unlock(inode);

			return ret;

		}



		/*

		 * If there is a delay extent at this offset,

		 * it will be as a data.

		 */

		ext4_es_find_delayed_extent_range(inode, last, last, &es);

		if (es.es_len != 0 && in_range(last, es.es_lblk, es.es_len)) {

		last = es.es_lblk;

		if (last != start)

			dataoff = (loff_t)last << blkbits;

		if (!ext4_es_is_unwritten(&es))

			break;

		}



		/*

		 * If there is a unwritten extent at this offset,

		 * it will be as a data or a hole according to page

		 * cache that has data or not.

		 */

		if (map.m_flags & EXT4_MAP_UNWRITTEN) {

			int unwritten;

			unwritten = ext4_find_unwritten_pgoff(inode, SEEK_DATA,

							      &map, &dataoff);

			if (unwritten)

		if (ext4_find_unwritten_pgoff(inode, SEEK_DATA,

					      es.es_lblk + es.es_len, &dataoff))

			break;

		}



		last++;

		last += es.es_len;

		dataoff = (loff_t)last << blkbits;

		cond_resched();

	} while (last <= end);



	inode_unlock(inode);
@@ -632,12 +622,11 @@ static loff_t ext4_seek_data(struct file *file, loff_t offset, loff_t maxsize) 
static loff_t ext4_seek_hole(struct file *file, loff_t offset, loff_t maxsize)

{

	struct inode *inode = file->f_mapping->host;

	struct ext4_map_blocks map;

	struct extent_status es;

	ext4_lblk_t start, last, end;

	loff_t holeoff, isize;

	int blkbits;

	int ret = 0;

	int ret;



	inode_lock(inode);
@@ -654,44 +643,30 @@ static loff_t ext4_seek_hole(struct file *file, loff_t offset, loff_t maxsize) 
	holeoff = offset;



	do {

		map.m_lblk = last;

		map.m_len = end - last + 1;

		ret = ext4_map_blocks(NULL, inode, &map, 0);

		if (ret > 0 && !(map.m_flags & EXT4_MAP_UNWRITTEN)) {

			last += ret;

			holeoff = (loff_t)last << blkbits;

			continue;

		ret = ext4_get_next_extent(inode, last, end - last + 1, &es);

		if (ret < 0) {

			inode_unlock(inode);

			return ret;

		}



		/*

		 * If there is a delay extent at this offset,

		 * we will skip this extent.

		 */

		ext4_es_find_delayed_extent_range(inode, last, last, &es);

		if (es.es_len != 0 && in_range(last, es.es_lblk, es.es_len)) {

			last = es.es_lblk + es.es_len;

		/* Found a hole? */

		if (ret == 0 || es.es_lblk > last) {

			if (last != start)

				holeoff = (loff_t)last << blkbits;

			continue;

			break;

		}



		/*

		 * If there is a unwritten extent at this offset,

		 * it will be as a data or a hole according to page

		 * cache that has data or not.

		 */

		if (map.m_flags & EXT4_MAP_UNWRITTEN) {

			int unwritten;

			unwritten = ext4_find_unwritten_pgoff(inode, SEEK_HOLE,

							      &map, &holeoff);

			if (!unwritten) {

				last += ret;

				holeoff = (loff_t)last << blkbits;

				continue;

			}

		}



		/* find a hole */

		if (ext4_es_is_unwritten(&es) &&

		    ext4_find_unwritten_pgoff(inode, SEEK_HOLE,

					      last + es.es_len, &holeoff))

			break;



		last += es.es_len;

		holeoff = (loff_t)last << blkbits;

		cond_resched();

	} while (last <= end);



	inode_unlock(inode);

fs/ext4/inode.c

+67 −0
Original line number Diff line number Diff line
@@ -5596,3 +5596,70 @@ int ext4_filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf) 


	return err;

}



/*

 * Find the first extent at or after @lblk in an inode that is not a hole.

 * Search for @map_len blocks at most. The extent is returned in @result.

 *

 * The function returns 1 if we found an extent. The function returns 0 in

 * case there is no extent at or after @lblk and in that case also sets

 * @result->es_len to 0. In case of error, the error code is returned.

 */

int ext4_get_next_extent(struct inode *inode, ext4_lblk_t lblk,

			 unsigned int map_len, struct extent_status *result)

{

	struct ext4_map_blocks map;

	struct extent_status es = {};

	int ret;



	map.m_lblk = lblk;

	map.m_len = map_len;



	/*

	 * For non-extent based files this loop may iterate several times since

	 * we do not determine full hole size.

	 */

	while (map.m_len > 0) {

		ret = ext4_map_blocks(NULL, inode, &map, 0);

		if (ret < 0)

			return ret;

		/* There's extent covering m_lblk? Just return it. */

		if (ret > 0) {

			int status;



			ext4_es_store_pblock(result, map.m_pblk);

			result->es_lblk = map.m_lblk;

			result->es_len = map.m_len;

			if (map.m_flags & EXT4_MAP_UNWRITTEN)

				status = EXTENT_STATUS_UNWRITTEN;

			else

				status = EXTENT_STATUS_WRITTEN;

			ext4_es_store_status(result, status);

			return 1;

		}

		ext4_es_find_delayed_extent_range(inode, map.m_lblk,

						  map.m_lblk + map.m_len - 1,

						  &es);

		/* Is delalloc data before next block in extent tree? */

		if (es.es_len && es.es_lblk < map.m_lblk + map.m_len) {

			ext4_lblk_t offset = 0;



			if (es.es_lblk < lblk)

				offset = lblk - es.es_lblk;

			result->es_lblk = es.es_lblk + offset;

			ext4_es_store_pblock(result,

					     ext4_es_pblock(&es) + offset);

			result->es_len = es.es_len - offset;

			ext4_es_store_status(result, ext4_es_status(&es));



			return 1;

		}

		/* There's a hole at m_lblk, advance us after it */

		map.m_lblk += map.m_len;

		map_len -= map.m_len;

		map.m_len = map_len;

		cond_resched();

	}

	result->es_len = 0;

	return 0;

}