[PATCH 4.9 057/171] btrfs: fiemap: Cache and merge fiemap extent before submit it to user

Greg Kroah-Hartman
2018-11-08T22:04:25Z

[ Upstream commit 4751832da990a927c37526ae67b9226ea01eb99e ]

[BUG]
Cycle mount btrfs can cause fiemap to return different result.
Like:
# mount /dev/vdb5 /mnt/btrfs
# dd if=/dev/zero bs=16K count=4 oflag=dsync of=/mnt/btrfs/file
# xfs_io -c "fiemap -v" /mnt/btrfs/file
/mnt/test/file:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..127]: 25088..25215 128 0x1
# umount /mnt/btrfs
# mount /dev/vdb5 /mnt/btrfs
# xfs_io -c "fiemap -v" /mnt/btrfs/file
/mnt/test/file:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..31]: 25088..25119 32 0x0
1: [32..63]: 25120..25151 32 0x0
2: [64..95]: 25152..25183 32 0x0
3: [96..127]: 25184..25215 32 0x1
But after above fiemap, we get correct merged result if we call fiemap
again.
# xfs_io -c "fiemap -v" /mnt/btrfs/file
/mnt/test/file:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..127]: 25088..25215 128 0x1

[REASON]
Btrfs will try to merge extent map when inserting new extent map.

btrfs_fiemap(start=0 len=(u64)-1)
|- extent_fiemap(start=0 len=(u64)-1)
|- get_extent_skip_holes(start=0 len=64k)
| |- btrfs_get_extent_fiemap(start=0 len=64k)
| |- btrfs_get_extent(start=0 len=64k)
| | Found on-disk (ino, EXTENT_DATA, 0)
| |- add_extent_mapping()
| |- Return (em->start=0, len=16k)
|
|- fiemap_fill_next_extent(logic=0 phys=X len=16k)
|
|- get_extent_skip_holes(start=0 len=64k)
| |- btrfs_get_extent_fiemap(start=0 len=64k)
| |- btrfs_get_extent(start=16k len=48k)
| | Found on-disk (ino, EXTENT_DATA, 16k)
| |- add_extent_mapping()
| | |- try_merge_map()
| | Merge with previous em start=0 len=16k
| | resulting em start=0 len=32k
| |- Return (em->start=0, len=32K) << Merged result
|- Stripe off the unrelated range (0~16K) of return em
|- fiemap_fill_next_extent(logic=16K phys=X+16K len=16K)
^^^ Causing split fiemap extent.

And since in add_extent_mapping(), em is already merged, in next
fiemap() call, we will get merged result.

[FIX]
Here we introduce a new structure, fiemap_cache, which records previous
fiemap extent.

And will always try to merge current fiemap_cache result before calling
fiemap_fill_next_extent().
Only when we failed to merge current fiemap extent with cached one, we
will call fiemap_fill_next_extent() to submit cached one.

So by this method, we can merge all fiemap extents.

It can also be done in fs/ioctl.c, however the problem is if
fieinfo->fi_extents_max == 0, we have no space to cache previous fiemap
extent.
So I choose to merge it in btrfs.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/extent_io.c | 124 ++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 122 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 2b96ca68dc10..5feaef9bcbda 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4377,6 +4377,123 @@ static struct extent_map *get_extent_skip_holes(struct inode *inode,
return NULL;
}

+/*
+ * To cache previous fiemap extent
+ *
+ * Will be used for merging fiemap extent
+ */
+struct fiemap_cache {
+ u64 offset;
+ u64 phys;
+ u64 len;
+ u32 flags;
+ bool cached;
+};
+
+/*
+ * Helper to submit fiemap extent.
+ *
+ * Will try to merge current fiemap extent specified by @offset, @phys,
+ * @len and @flags with cached one.
+ * And only when we fails to merge, cached one will be submitted as
+ * fiemap extent.
+ *
+ * Return value is the same as fiemap_fill_next_extent().
+ */
+static int emit_fiemap_extent(struct fiemap_extent_info *fieinfo,
+ struct fiemap_cache *cache,
+ u64 offset, u64 phys, u64 len, u32 flags)
+{
+ int ret = 0;
+
+ if (!cache->cached)
+ goto assign;
+
+ /*
+ * Sanity check, extent_fiemap() should have ensured that new
+ * fiemap extent won't overlap with cahced one.
+ * Not recoverable.
+ *
+ * NOTE: Physical address can overlap, due to compression
+ */
+ if (cache->offset + cache->len > offset) {
+ WARN_ON(1);
+ return -EINVAL;
+ }
+
+ /*
+ * Only merges fiemap extents if
+ * 1) Their logical addresses are continuous
+ *
+ * 2) Their physical addresses are continuous
+ * So truly compressed (physical size smaller than logical size)
+ * extents won't get merged with each other
+ *
+ * 3) Share same flags except FIEMAP_EXTENT_LAST
+ * So regular extent won't get merged with prealloc extent
+ */
+ if (cache->offset + cache->len == offset &&
+ cache->phys + cache->len == phys &&
+ (cache->flags & ~FIEMAP_EXTENT_LAST) ==
+ (flags & ~FIEMAP_EXTENT_LAST)) {
+ cache->len += len;
+ cache->flags |= flags;
+ goto try_submit_last;
+ }
+
+ /* Not mergeable, need to submit cached one */
+ ret = fiemap_fill_next_extent(fieinfo, cache->offset, cache->phys,
+ cache->len, cache->flags);
+ cache->cached = false;
+ if (ret)
+ return ret;
+assign:
+ cache->cached = true;
+ cache->offset = offset;
+ cache->phys = phys;
+ cache->len = len;
+ cache->flags = flags;
+try_submit_last:
+ if (cache->flags & FIEMAP_EXTENT_LAST) {
+ ret = fiemap_fill_next_extent(fieinfo, cache->offset,
+ cache->phys, cache->len, cache->flags);
+ cache->cached = false;
+ }
+ return ret;
+}
+
+/*
+ * Sanity check for fiemap cache
+ *
+ * All fiemap cache should be submitted by emit_fiemap_extent()
+ * Iteration should be terminated either by last fiemap extent or
+ * fieinfo->fi_extents_max.
+ * So no cached fiemap should exist.
+ */
+static int check_fiemap_cache(struct btrfs_fs_info *fs_info,
+ struct fiemap_extent_info *fieinfo,
+ struct fiemap_cache *cache)
+{
+ int ret;
+
+ if (!cache->cached)
+ return 0;
+
+ /* Small and recoverbale problem, only to info developer */
+#ifdef CONFIG_BTRFS_DEBUG
+ WARN_ON(1);
+#endif
+ btrfs_warn(fs_info,
+ "unhandled fiemap cache detected: offset=%llu phys=%llu len=%llu flags=0x%x",
+ cache->offset, cache->phys, cache->len, cache->flags);
+ ret = fiemap_fill_next_extent(fieinfo, cache->offset, cache->phys,
+ cache->len, cache->flags);
+ cache->cached = false;
+ if (ret > 0)
+ ret = 0;
+ return ret;
+}
+
int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
__u64 start, __u64 len, get_extent_t *get_extent)
{
@@ -4394,6 +4511,7 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
struct extent_state *cached_state = NULL;
struct btrfs_path *path;
struct btrfs_root *root = BTRFS_I(inode)->root;
+ struct fiemap_cache cache = { 0 };
int end = 0;
u64 em_start = 0;
u64 em_len = 0;
@@ -4573,8 +4691,8 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
flags |= FIEMAP_EXTENT_LAST;
end = 1;
}
- ret = fiemap_fill_next_extent(fieinfo, em_start, disko,
- em_len, flags);
+ ret = emit_fiemap_extent(fieinfo, &cache, em_start, disko,
+ em_len, flags);
if (ret) {
if (ret == 1)
ret = 0;
@@ -4582,6 +4700,8 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
}
}
out_free:
+ if (!ret)
+ ret = check_fiemap_cache(root->fs_info, fieinfo, &cache);
free_extent_map(em);
out:
btrfs_free_path(path);
--
2.17.1