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| author | Chunguang Xu <brookxu@tencent.com> | 2020-12-04 11:05:43 +0800 |
|---|---|---|
| committer | Theodore Ts'o <tytso@mit.edu> | 2020-12-22 13:08:45 -0500 |
| commit | 82ef1370b0c1757ab4ce29f34c52b4e93839b0aa (patch) | |
| tree | 825a0e93809324de1600cb9d0f12225c30f6992d /fs | |
| parent | c92dc856848f32781e37b88c1b7f875e274f5efb (diff) | |
| download | lwn-82ef1370b0c1757ab4ce29f34c52b4e93839b0aa.tar.gz lwn-82ef1370b0c1757ab4ce29f34c52b4e93839b0aa.zip | |
ext4: avoid s_mb_prefetch to be zero in individual scenarios
Commit cfd732377221 ("ext4: add prefetching for block allocation
bitmaps") introduced block bitmap prefetch, and expects to read block
bitmaps of flex_bg through an IO. However, it seems to ignore the
value range of s_log_groups_per_flex. In the scenario where the value
of s_log_groups_per_flex is greater than 27, s_mb_prefetch or
s_mb_prefetch_limit will overflow, cause a divide zero exception.
In addition, the logic of calculating nr is also flawed, because the
size of flexbg is fixed during a single mount, but s_mb_prefetch can
be modified, which causes nr to fail to meet the value condition of
[1, flexbg_size].
To solve this problem, we need to set the upper limit of
s_mb_prefetch. Since we expect to load block bitmaps of a flex_bg
through an IO, we can consider determining a reasonable upper limit
among the IO limit parameters. After consideration, we chose
BLK_MAX_SEGMENT_SIZE. This is a good choice to solve divide zero
problem and avoiding performance degradation.
[ Some minor code simplifications to make the changes easy to follow -- TYT ]
Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Reviewed-by: Samuel Liao <samuelliao@tencent.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/1607051143-24508-1-git-send-email-brookxu@tencent.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Diffstat (limited to 'fs')
| -rw-r--r-- | fs/ext4/mballoc.c | 9 |
1 files changed, 5 insertions, 4 deletions
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index 77815cd110b2..99bf091fee10 100644 --- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -2372,9 +2372,9 @@ repeat: nr = sbi->s_mb_prefetch; if (ext4_has_feature_flex_bg(sb)) { - nr = (group / sbi->s_mb_prefetch) * - sbi->s_mb_prefetch; - nr = nr + sbi->s_mb_prefetch - group; + nr = 1 << sbi->s_log_groups_per_flex; + nr -= group & (nr - 1); + nr = min(nr, sbi->s_mb_prefetch); } prefetch_grp = ext4_mb_prefetch(sb, group, nr, &prefetch_ios); @@ -2710,7 +2710,8 @@ static int ext4_mb_init_backend(struct super_block *sb) if (ext4_has_feature_flex_bg(sb)) { /* a single flex group is supposed to be read by a single IO */ - sbi->s_mb_prefetch = 1 << sbi->s_es->s_log_groups_per_flex; + sbi->s_mb_prefetch = min(1 << sbi->s_es->s_log_groups_per_flex, + BLK_MAX_SEGMENT_SIZE >> (sb->s_blocksize_bits - 9)); sbi->s_mb_prefetch *= 8; /* 8 prefetch IOs in flight at most */ } else { sbi->s_mb_prefetch = 32; |