On Fri, Apr 07, 2017 at 10:43:15AM +0800, Qu Wenruo wrote:
> [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.
>
If I understand it correctly, what it's missing currently is 'merge', a
straightfoward idea is to make use of the 'merge' ability of btrfs_get_extent.
Since btrfs_get_extent_fiemap is a wrapper of btrfs_get_extent, does it make
sense if we make btrfs_get_extent_fiemap iterate all extent maps within the
range passing to it or just one more extent map to check if btrfs_get_extent
could return a merged extent map before returning?
If no extent map could be merged, which is the worst case, the first
btrfs_get_extent_fiemap will read file metadata into memory from disk and the
following btrfs_get_extent_fiemap will read the rest of file metadata from
the in-memory extent map tree directly.
Thanks,
-liubo
> 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 <quwen...@cn.fujitsu.com>
> ---
> v2:
> Since fiemap_extent_info has a limit for number of fiemap_extent, it's
> possible
> that fiemap_fill_next_extent() return 1 halfway. Remove the WARN_ON() which
> can
> cause kernel warning if we fiemap is called on large compressed file.
> v3:
> Rename finish_fiemap_extent() to check_fiemap_extent(), as in v3 we ensured
> submit_fiemap_extent() to submit fiemap cache, so it just acts as a
> sanity check.
> Remove BTRFS_MAX_EXTENT_SIZE limit in submit_fiemap_extent(), as
> extent map can be larger than BTRFS_MAX_EXTENT_SIZE.
> Don't do backward jump, suggested by David.
> Better sanity check and recoverable fix.
>
> To David:
> What about adding a btrfs_debug_warn(), which will only call WARN_ON(1) if
> BTRFS_CONFIG_DEBUG is specified for recoverable bug?
>
> And modify ASSERT() to always WARN_ON() and exit error code?
> ---
> 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 28e8192..c4cb65d 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4353,6 +4353,123 @@ 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 submit_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 submit_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)
> {
> @@ -4370,6 +4487,7 @@ 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;
> @@ -4549,8 +4667,8 @@ 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 = submit_fiemap_extent(fieinfo, &cache, em_start, disko,
> + em_len, flags);
> if (ret) {
> if (ret == 1)
> ret = 0;
> @@ -4558,6 +4676,8 @@ 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);
> --
> 2.9.3
>
>
>
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