On Fri, Aug 8, 2014 at 6:06 AM, Qu Wenruo <quwenruo@xxxxxxxxxxxxxx> wrote:
> When current btrfs finds that a new extent map is going to be insereted
> but failed with -EEXIST, it will try again to insert the extent map
> but with the length of sectorsize.
> This is OK if we don't enable 'no-holes' feature since all extent space
> is continuous, we will not go into the not found->insert routine.
>
> But if we enable 'no-holes' feature, it will make things out of control.
> e.g. in 4K sectorsize, we pass the following args to btrfs_get_extent():
> btrfs_get_extent() args: start: 27874 len 4100
> 28672 27874 28672 27874+4100 32768
> |-----------------------|
> |---------hole--------------------|---------data----------|
>
> 1) not found and insert
> Since no extent map containing the range, btrfs_get_extent() will go
> into the not_found and insert routine, which will try to insert the
> extent map (27874, 27847 + 4100).
>
> 2) first overlap
> But it overlaps with (28672, 32768) extent, so -EEXIST will be returned
> by add_extent_mapping().
>
> 3) retry but still overlap
> After catching the -EEXIST, then btrfs_get_extent() will try insert it
> again but with 4K length, which still overlaps, so -EEXIST will be
> returned.
>
> This makes the following patch fail to punch hole.
> d77815461f047e561f77a07754ae923ade597d4e btrfs: Avoid trucating page or punching hole in a already existed hole.
>
> This patch will use the right length, which is the (exsisting->start -
> em->start) to insert, making the above patch works in 'no-holes' mode.
> Also, some small code style problems in above patch is fixed too.
>
> Reported-by: Filipe David Manana <fdmanana@xxxxxxxxx>
> Signed-off-by: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
Reviewed-by: Filipe David Manana <fdmanana@xxxxxxxx>
Tested-by: Filipe David Manana <fdmanana@xxxxxxxx>
Verified it makes all xfstests pass with and without no-holes.
Also with change, the large sparse files created by generic/299 now
have about ~2k extent maps when before they had ~11M (no-holes
enabled) and ~2M (without no-holes).
Thanks Qu
> ---
> fs/btrfs/file.c | 4 ++--
> fs/btrfs/inode.c | 7 +++----
> 2 files changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 1f2b99c..6eb71e6 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -2240,7 +2240,7 @@ static int btrfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> goto out_only_mutex;
> }
>
> - lockstart = round_up(offset , BTRFS_I(inode)->root->sectorsize);
> + lockstart = round_up(offset, BTRFS_I(inode)->root->sectorsize);
> lockend = round_down(offset + len,
> BTRFS_I(inode)->root->sectorsize) - 1;
> same_page = ((offset >> PAGE_CACHE_SHIFT) ==
> @@ -2301,7 +2301,7 @@ static int btrfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> tail_start + tail_len, 0, 1);
> if (ret)
> goto out_only_mutex;
> - }
> + }
> }
> }
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 3668048..391dcd3 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -6085,14 +6085,14 @@ out_fail:
> static int merge_extent_mapping(struct extent_map_tree *em_tree,
> struct extent_map *existing,
> struct extent_map *em,
> - u64 map_start, u64 map_len)
> + u64 map_start)
> {
> 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 = map_len;
> + em->len = existing->start - em->start;
> if (em->block_start < EXTENT_MAP_LAST_BYTE &&
> !test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
> em->block_start += start_diff;
> @@ -6378,8 +6378,7 @@ insert:
> em->len);
> if (existing) {
> err = merge_extent_mapping(em_tree, existing,
> - em, start,
> - root->sectorsize);
> + em, start);
> free_extent_map(existing);
> if (err) {
> free_extent_map(em);
> --
> 2.0.4
>
--
Filipe David Manana,
"Reasonable men adapt themselves to the world.
Unreasonable men adapt the world to themselves.
That's why all progress depends on unreasonable men."
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