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Date: Thu, 17 Dec 2020 01:14:01 -0500 From: "Theodore Y. Ts'o" <tytso@....edu> To: Chunguang Xu <brookxu.cn@...il.com> Cc: adilger.kernel@...ger.ca, bzzz@...mcloud.com, linux-ext4@...r.kernel.org Subject: Re: [PATCH v2] ext4: avoid s_mb_prefetch to be zero in individual scenarios 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@...cent.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@...cent.com> Signed-off-by: Chunguang Xu <brookxu@...cent.com> Reviewed-by: Samuel Liao <samuelliao@...cent.com> Reviewed-by: Andreas Dilger <adilger@...ger.ca> Link: https://lore.kernel.org/r/1607051143-24508-1-git-send-email-brookxu@tencent.com Signed-off-by: Theodore Ts'o <tytso@....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;
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