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
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