Qu Wenruo
2014-Sep-18 08:55 UTC
[PATCH v2] 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: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
---
changelog:
v2:
Liu Bo points out that the if() use 'start + len <=
existing->start'
is not equal to original if(), which may cause problem in no-holes
mode.
---
fs/btrfs/inode.c | 79 ++++++++++++++++++++++++++++++++++++++++----------------
1 file changed, 57 insertions(+), 22 deletions(-)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 016c403..b3864b7 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6191,21 +6191,60 @@ 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;
@@ -6482,25 +6521,21 @@ insert:
ret = 0;
- existing = lookup_extent_mapping(em_tree, start, len);
- if (existing && (existing->start > start ||
- existing->start + existing->len <= start)) {
+ 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 < 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);
- existing = NULL;
- }
- if (!existing) {
- existing = lookup_extent_mapping(em_tree, em->start,
- em->len);
- if (existing) {
- err = merge_extent_mapping(em_tree, existing,
- em, start);
- free_extent_map(existing);
- if (err) {
- free_extent_map(em);
- em = NULL;
- }
- } else {
- err = -EIO;
+ if (err) {
free_extent_map(em);
em = NULL;
}
--
2.1.0
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