Commit e6c4efd8 authored by Qu Wenruo's avatar Qu Wenruo Committed by Chris Mason
Browse files

btrfs: Fix and enhance merge_extent_mapping() to insert best fitted extent map 



The following commit enhanced the merge_extent_mapping() to reduce
fragment in extent map tree, but it can't handle case which existing
lies before map_start:
51f39 btrfs: Use right extent length when inserting overlap extent map.

[BUG]
When existing extent map's start is before map_start,
the em->len will be minus, which will corrupt the extent map and fail to
insert the new extent map.
This will happen when someone get a large extent map, but when it is
going to insert it into extent map tree, some one has already commit
some write and split the huge extent into small parts.

[REPRODUCER]
It is very easy to tiger using filebench with randomrw personality.
It is about 100% to reproduce when using 8G preallocated file in 60s
randonrw test.

[FIX]
This patch can now handle any existing extent position.
Since it does not directly use existing->start, now it will find the
previous and next extent around map_start.
So the old existing->start < map_start bug will never happen again.

[ENHANCE]
This patch will insert the best fitted extent map into extent map tree,
other than the oldest [map_start, map_start + sectorsize) or the
relatively newer but not perfect [map_start, existing->start).

The patch will first search existing extent that does not intersects with
the desired map range [map_start, map_start + len).
The existing extent will be either before or behind map_start, and based
on the existing extent, we can find out the previous and next extent
around map_start.

So the best fitted extent would be [prev->end, next->start).
For prev or next is not found, em->start would be prev->end and em->end
wold be next->start.

With this patch, the fragment in extent map tree should be reduced much
more than the 51f39 commit and reduce an unneeded extent map tree search.

Reported-by: default avatarTsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: default avatarQu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: default avatarChris Mason <clm@fb.com>
parent 4d1a40c6

fs/btrfs/inode.c

+57 −22
Original line number Diff line number Diff line
@@ -6249,21 +6249,60 @@ out_fail_inode: 
	goto out_fail;

}



/* Find next extent map of a given extent map, caller needs to ensure locks */

static struct extent_map *next_extent_map(struct extent_map *em)

{

	struct rb_node *next;



	next = rb_next(&em->rb_node);

	if (!next)

		return NULL;

	return container_of(next, struct extent_map, rb_node);

}



static struct extent_map *prev_extent_map(struct extent_map *em)

{

	struct rb_node *prev;



	prev = rb_prev(&em->rb_node);

	if (!prev)

		return NULL;

	return container_of(prev, struct extent_map, rb_node);

}



/* helper for btfs_get_extent.  Given an existing extent in the tree,

 * the existing extent is the nearest extent to map_start,

 * and an extent that you want to insert, deal with overlap and insert

 * the new extent into the tree.

 * the best fitted new extent into the tree.

 */

static int merge_extent_mapping(struct extent_map_tree *em_tree,

				struct extent_map *existing,

				struct extent_map *em,

				u64 map_start)

{

	struct extent_map *prev;

	struct extent_map *next;

	u64 start;

	u64 end;

	u64 start_diff;



	BUG_ON(map_start < em->start || map_start >= extent_map_end(em));

	start_diff = map_start - em->start;

	em->start = map_start;

	em->len = existing->start - em->start;



	if (existing->start > map_start) {

		next = existing;

		prev = prev_extent_map(next);

	} else {

		prev = existing;

		next = next_extent_map(prev);

	}



	start = prev ? extent_map_end(prev) : em->start;

	start = max_t(u64, start, em->start);

	end = next ? next->start : extent_map_end(em);

	end = min_t(u64, end, extent_map_end(em));

	start_diff = start - em->start;

	em->start = start;

	em->len = end - start;

	if (em->block_start < EXTENT_MAP_LAST_BYTE &&

	    !test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {

		em->block_start += start_diff;
@@ -6540,16 +6579,17 @@ insert: 


		ret = 0;



		existing = lookup_extent_mapping(em_tree, start, len);

		if (existing && (existing->start > start ||

		    existing->start + existing->len <= start)) {

			free_extent_map(existing);

			existing = NULL;

		}

		if (!existing) {

			existing = lookup_extent_mapping(em_tree, em->start,

							 em->len);

			if (existing) {

		existing = search_extent_mapping(em_tree, start, len);

		/*

		 * existing will always be non-NULL, since there must be

		 * extent causing the -EEXIST.

		 */

		if (start >= extent_map_end(existing) ||

		    start + len <= existing->start) {

			/*

			 * The existing extent map is the one nearest to

			 * the [start, start + len) range which overlaps

			 */

			err = merge_extent_mapping(em_tree, existing,

						   em, start);

			free_extent_map(existing);
@@ -6557,11 +6597,6 @@ insert: 
				free_extent_map(em);

				em = NULL;

			}

			} else {

				err = -EIO;

				free_extent_map(em);

				em = NULL;

			}

		} else {

			free_extent_map(em);

			em = existing;

CRA Git | Maintained and supported by SUSTech CRA and CCSE