Re: [PATCH v2] ext4: avoid s_mb_prefetch to be zero in individual scenarios

["Theodore Y. Ts'o" <tytso@mit.edu>]

I cleaned up the commit description and the code slightly; it doesn't
change the generated object but makes the code a bit more concise and
easier to read:

commit 8333bd298d915a2d1c01cbed9287d284aaa04bb1
Author: Chunguang Xu <brookxu@tencent.com>
Date:   Fri Dec 4 11:05:43 2020 +0800

    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>

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 @@ 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);
@@ -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;