Re: [PATCH] ext4: mballoc.c: fix ac_g_ex and ac_f_ex misuse bug in EXT4_MB_HINT_TRY_GOAL path

[Andreas Dilger <adilger@dilger.ca>]

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On Jun 2, 2016, at 6:01 AM, Lin Feng <linf@chinanetcenter.com> wrote:
> 
> Descriptions:
> ext4 block allocation core stack:
> ext4_mb_new_blocks
>  ext4_mb_normalize_request
>  ext4_mb_regular_allocator
>    ext4_mb_find_by_goal
>      mb_find_extent(e4b, ac->ac_g_ex.fe_start, ac->ac_g_ex.fe_len, &ex);
> 
> The start block searching hint for merging(use EXT4_MB_HINT_TRY_GOAL flag)
> set in ext4_mb_normalize_request is stored in ac_f_ex, while in
> EXT4_MB_HINT_TRY_GOAL path which falls in ext4_mb_find_by_goal always use
> ac_g_ex as a hint and the hint set in ext4_mb_normalize_request is never
> use.
> 
> We could hit this bug by writing a sparse file from backward mode and the
> file may get fragments even if the physical blocks in the hole is free,
> which is expected to be merged into a single extent.

This looks reasonable.  Do you have any kind of test case that shows the
effect of the change (e.g. fragmentation counts per file before/after)?

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

> Signed-off-by: Lin Feng <linf@chinanetcenter.com>
> ---
> fs/ext4/mballoc.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index c1ab3ec..e31fc63 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3198,15 +3198,15 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
> 	if (ar->pright && (ar->lright == (start + size))) {
> 		/* merge to the right */
> 		ext4_get_group_no_and_offset(ac->ac_sb, ar->pright - size,
> -						&ac->ac_f_ex.fe_group,
> -						&ac->ac_f_ex.fe_start);
> +						&ac->ac_g_ex.fe_group,
> +						&ac->ac_g_ex.fe_start);
> 		ac->ac_flags |= EXT4_MB_HINT_TRY_GOAL;
> 	}
> 	if (ar->pleft && (ar->lleft + 1 == start)) {
> 		/* merge to the left */
> 		ext4_get_group_no_and_offset(ac->ac_sb, ar->pleft + 1,
> -						&ac->ac_f_ex.fe_group,
> -						&ac->ac_f_ex.fe_start);
> +						&ac->ac_g_ex.fe_group,
> +						&ac->ac_g_ex.fe_start);
> 		ac->ac_flags |= EXT4_MB_HINT_TRY_GOAL;
> 	}
> 
> --
> 1.9.3
> 
> 


Cheers, Andreas






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