[PATCH 5.10 62/63] ext4: avoid s_mb_prefetch to be zero in individual scenarios

Greg Kroah-Hartman
2021-01-04T16:04:15Z

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.com
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;
--
2.27.0