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Date: Wed, 08 Jun 2016 14:08:21 +0800 From: Lin Feng <linf@...nanetcenter.com> To: Andreas Dilger <adilger@...ger.ca> CC: Theodore Ts'o <tytso@....edu>, linux-ext4 <linux-ext4@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] ext4: mballoc.c: fix ac_g_ex and ac_f_ex misuse bug in EXT4_MB_HINT_TRY_GOAL path 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@...nanetcenter.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@...tOS6 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@...tOS6 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@...ger.ca> > >> Signed-off-by: Lin Feng <linf@...nanetcenter.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 > > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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