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authorChunguang Xu <brookxu@tencent.com>2020-12-04 11:05:43 +0800
committerTheodore Ts'o <tytso@mit.edu>2020-12-22 13:08:45 -0500
commit82ef1370b0c1757ab4ce29f34c52b4e93839b0aa (patch)
tree825a0e93809324de1600cb9d0f12225c30f6992d /fs
parentc92dc856848f32781e37b88c1b7f875e274f5efb (diff)
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.c9
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;