On Thu, Apr 06, 2017 at 05:05:16PM +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.
>
> 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@xxxxxxxxxxxxxx>
> ---
> 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.
> ---
> fs/btrfs/extent_io.c | 116 ++++++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 110 insertions(+), 6 deletions(-)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 28e81922a21c..84f4090dfaff 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4353,6 +4353,107 @@ 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 {
> + bool cached;
> + u64 offset;
> + u64 phys;
> + u64 len;
> + u32 flags;
Please move bool cached after flags, for better packing.
> +};
> +
> +/*
> + * 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 0 if merged or submitted.
> + * Return <0 for error.
> + */
> +static int submit_fiemap_extent(struct fiemap_extent_info *fieinfo,
> + struct fiemap_cache *cache,
> + u64 offset, u64 phys, u64 len, u32 flags)
> +{
> + int ret;
> +
> + if (!cache->cached) {
> +assign:
> + cache->cached = true;
> + cache->offset = offset;
> + cache->phys = phys;
> + cache->len = len;
> + cache->flags = flags;
> + return 0;
> + }
> +
> + /*
> + * Sanity check, extent_fiemap() should have ensured that new
> + * fiemap extent won't overlap with cahced one.
> + * NOTE: Physical address can overlap, due to compression
> + */
> + WARN_ON(cache->offset + cache->len > offset);
> +
> + /*
> + * Only merge 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
> + *
> + * 4) Merged result is no larger than BTRFS_MAX_EXTENT_SIZE
> + */
> + if (cache->offset + cache->len == offset &&
> + cache->phys + cache->len == phys &&
> + cache->len + len <= BTRFS_MAX_EXTENT_SIZE &&
> + (cache->flags & ~FIEMAP_EXTENT_LAST) ==
> + (flags & ~FIEMAP_EXTENT_LAST)) {
> + cache->len += len;
> + cache->flags |= flags;
> +
> + /* Last extent, submit it anyway */
> + if (cache->flags & FIEMAP_EXTENT_LAST) {
> + cache->cached = false;
> + return fiemap_fill_next_extent(fieinfo, cache->offset,
> + cache->phys, cache->len, cache->flags);
> + }
> + return 0;
> + }
> +
> + /* Not mergeable, need to submit cached one */
> + ret = fiemap_fill_next_extent(fieinfo, cache->offset, cache->phys,
> + cache->len, cache->flags);
> + if (ret)
> + return ret;
> + goto assign;
This pattern is really ugly, jumping from the end of the function to the
beginning just to get to do a return. Please do the opposite, ie, move
the contents of the if statement block here and replace it by a goto.
Other than that, looks good to me so far.
> +}
> +
> +/*
> + * Submit the last cached fiemap extent.
> + */
> +static int finish_fiemap_extent(struct fiemap_extent_info *fieinfo,
> + struct fiemap_cache *cache)
> +{
> + int ret;
> +
> + if (!cache->cached)
> + return 0;
> + ret = fiemap_fill_next_extent(fieinfo, cache->offset, cache->phys,
> + cache->len, cache->flags);
> + cache->cached = false;
> + return ret;
> +}
> +
> int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> __u64 start, __u64 len, get_extent_t *get_extent)
> {
> @@ -4370,6 +4471,7 @@ 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;
> @@ -4549,15 +4651,17 @@ 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);
> - if (ret) {
> - if (ret == 1)
> - ret = 0;
> + ret = submit_fiemap_extent(fieinfo, &cache, em_start, disko,
> + em_len, flags);
> + if (ret)
> goto out_free;
> - }
> }
> out_free:
> + /* Submit any cached one */
> + if (!ret)
> + ret = finish_fiemap_extent(fieinfo, &cache);
> + if (ret > 0)
> + ret = 0;
> free_extent_map(em);
> out:
> btrfs_free_path(path);
--
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