ext4: fix lockdep warning when enabling MMP

When we enable MMP in ext4_multi_mount_protect() during mount or
remount, we end up calling sb_start_write() from write_mmp_block(). This
triggers lockdep warning because freeze protection ranks above s_umount
semaphore we are holding during mount / remount. The problem is harmless
because we are guaranteed the filesystem is not frozen during mount /
remount but still let's fix the warning by not grabbing freeze
protection from ext4_multi_mount_protect().

Cc: stable@kernel.org
Reported-by: syzbot+6b7df7d5506b32467149@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=ab7e5b6f400b7778d46f01841422e5718fb81843
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230411121019.21940-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index 4022bc713421555741ab6b28c4027db6a77efc60..0aaf38ffcb6ec6600ae91b5bd41e2b6fe3fc501a 100644 (file)
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -39,28 +39,36 @@ static void ext4_mmp_csum_set(struct super_block *sb, struct mmp_struct *mmp)
  * Write the MMP block using REQ_SYNC to try to get the block on-disk
  * faster.
  */
-static int write_mmp_block(struct super_block *sb, struct buffer_head *bh)
+static int write_mmp_block_thawed(struct super_block *sb,
+                                 struct buffer_head *bh)
 {
        struct mmp_struct *mmp = (struct mmp_struct *)(bh->b_data);
 
-       /*
-        * We protect against freezing so that we don't create dirty buffers
-        * on frozen filesystem.
-        */
-       sb_start_write(sb);
        ext4_mmp_csum_set(sb, mmp);
        lock_buffer(bh);
        bh->b_end_io = end_buffer_write_sync;
        get_bh(bh);
        submit_bh(REQ_OP_WRITE | REQ_SYNC | REQ_META | REQ_PRIO, bh);
        wait_on_buffer(bh);
-       sb_end_write(sb);
        if (unlikely(!buffer_uptodate(bh)))
                return -EIO;
-
        return 0;
 }
 
+static int write_mmp_block(struct super_block *sb, struct buffer_head *bh)
+{
+       int err;
+
+       /*
+        * We protect against freezing so that we don't create dirty buffers
+        * on frozen filesystem.
+        */
+       sb_start_write(sb);
+       err = write_mmp_block_thawed(sb, bh);
+       sb_end_write(sb);
+       return err;
+}
+
 /*
  * Read the MMP block. It _must_ be read from disk and hence we clear the
  * uptodate flag on the buffer.
@@ -344,7 +352,11 @@ skip:
        seq = mmp_new_seq();
        mmp->mmp_seq = cpu_to_le32(seq);
 
-       retval = write_mmp_block(sb, bh);
+       /*
+        * On mount / remount we are protected against fs freezing (by s_umount
+        * semaphore) and grabbing freeze protection upsets lockdep
+        */
+       retval = write_mmp_block_thawed(sb, bh);
        if (retval)
                goto failed;