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

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

[Lin Feng]

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.

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

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,

Am I missing something or it's not important at all?

Other things:
[linfeng@localhost ext4]$ grep EXT4_MB_HINT_MERGE ./* -rHn
./ext4.h:111:#define EXT4_MB_HINT_MERGE        0x0001
./mballoc.c:1871:    } else if (max > 0 && (ac->ac_flags & EXT4_MB_HINT_MERGE)) {

EXT4_MB_HINT_MERGE is only tested once and nowhere teaches how to use it.
IIUC it also should be folded into EXT4_MB_HINT_TRY_GOAL path or simply skip
EXT4_MB_HINT_MERGE test at -L1871.

thanks,
linfeng


On 06/02/2016 08:01 PM, Lin Feng 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.
>
> 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;
>   	}
>
>

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

[Andreas Dilger]

[-- Attachment #1: Type: text/plain, Size: 2131 bytes --]

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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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@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)?

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@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
>
>
>
>
>

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

[Darrick J. Wong]

On Wed, Jun 08, 2016 at 02:08:21PM +0800, Lin Feng wrote:
> 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@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)?
> 
> 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

Sure would be nice to have an xfstests for this...

--D

> 
> 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@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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