linux-kernel - [PATCH] ext4: fix wrong calculation of minlen in ext4_trim

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Message-Id: <20230406150621.3854298-1-chao@kernel.org>
Date:   Thu,  6 Apr 2023 23:06:21 +0800
From:   Chao Yu <chao@...nel.org>
To:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Cc:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chao Yu <chao@...nel.org>
Subject: [PATCH] ext4: fix wrong calculation of minlen in ext4_trim_fs()

As Ted pointed out as below:

"
However the minlen variable in ext4_trim_fs is in units of *clusters*.
And so it gets rounded up two places.  The first time is when it is
converted into units of a cluster:

	minlen = EXT4_NUM_B2C(EXT4_SB(sb),
			      range->minlen >> sb->s_blocksize_bits);

Oh, and by the way, that first conversion is not correct as currently
written in ext4_fs_trim().   It should be

	minlen = (range->minlen + EXT4_CLUSTER_SIZE(sb) - 1) >>
		(sb->s_blocksize_bits + EXT4_CLUSTER_BITS(sb));
"

The reason is if range->minlen is smaller than block size of ext4,
original calculation "range->minlen >> sb->s_blocksize_bits" may
return zero, but since EXT4_NUM_B2C() expects a non-zero in-parameter,
so it needs to round up range->minlen to cluster size directly as above.

Link: https://lore.kernel.org/lkml/20230311031843.GF860405@mit.edu/
Suggested-by: Theodore Ts'o <tytso@....edu>
Signed-off-by: Chao Yu <chao@...nel.org>
---
 fs/ext4/mballoc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 5b2ae37a8b80..d8b9d6a83d1e 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -6478,8 +6478,8 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
 
 	start = range->start >> sb->s_blocksize_bits;
 	end = start + (range->len >> sb->s_blocksize_bits) - 1;
-	minlen = EXT4_NUM_B2C(EXT4_SB(sb),
-			      range->minlen >> sb->s_blocksize_bits);
+	minlen = (range->minlen + EXT4_CLUSTER_SIZE(sb) - 1) >>
+			(sb->s_blocksize_bits + EXT4_CLUSTER_BITS(sb));
 
 	if (minlen > EXT4_CLUSTERS_PER_GROUP(sb) ||
 	    start >= max_blks ||
-- 
2.25.1