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

[Lin Feng]

Hi Andreas,

Thanks for your reply and review.

On 06/08/2016 05:01 AM, Andreas Dilger wrote:
> On Jun 2, 2016, at 6:01 AM, Lin Feng <linf@xxxxxxxxxxxxxxxxxx> 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)?

I found this bug by fiddling the block allocation policy for ext4.

In order to see the effect of this patch, we could do the following:

On a fresh created ext4 fs, make a new topdir called b to hash the test to some 
blockgroup relatively empty, trying to not to be effected by original physical 
fragments. Then write a new file in backward mode.

steps:
1. mkdir b && cd b

2. before this patch:
[root@CentOS6 b]# rm dat -f && sync && sleep 2 && dd if=/dev/zero of=dat bs=4k 
count=256 seek=511 conv=notrunc && sync && sleep 2 && dd if=/dev/zero of=dat 
bs=4k count=254 seek=257 conv=notrunc && sync && filefrag -vv dat
256+0 records in
256+0 records out
1048576 bytes (1.0 MB) copied, 0.000825721 s, 1.3 GB/s
254+0 records in
254+0 records out
1040384 bytes (1.0 MB) copied, 0.000766912 s, 1.4 GB/s
Filesystem type is: ef53
File size of dat is 3141632 (767 blocks, blocksize 4096)
 ext logical physical expected length flags
   0     257     9512             254
   1     511   557056     9766    256 eof
dat: 2 extents found

3. after this patch:
[root@CentOS6 b]# rm dat -f && sync && sleep 2 && dd if=/dev/zero of=dat bs=4k 
count=256 seek=511 conv=notrunc && sync && sleep 2 && dd if=/dev/zero of=dat 
bs=4k count=254 seek=257 conv=notrunc && sync && filefrag -vv dat
256+0 records in
256+0 records out
1048576 bytes (1.0 MB) copied, 0.000856416 s, 1.2 GB/s
254+0 records in
254+0 records out
1040384 bytes (1.0 MB) copied, 0.000669862 s, 1.6 GB/s
Filesystem type is: ef53
File size of dat is 3141632 (767 blocks, blocksize 4096)
 ext logical physical expected length flags
   0     257   556802             510 eof
dat: 1 extent found

thanks,
linfeng
> Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>
>
> > Signed-off-by: Lin Feng <linf@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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