From: Chunguang Xu brookxu@tencent.com
[ Upstream commit 82ef1370b0c1757ab4ce29f34c52b4e93839b0aa ]
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.... Signed-off-by: Theodore Ts'o tytso@mit.edu Signed-off-by: Sasha Levin sashal@kernel.org --- fs/ext4/mballoc.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index 37a619bf1ac7c..e67d5de6f28ca 100644 --- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -2395,9 +2395,9 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
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); @@ -2733,7 +2733,8 @@ 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;