FAILED: patch "[PATCH] ext4: fix memory leak in ext4_fill_super" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From afd09b617db3786b6ef3dc43e28fe728cfea84df Mon Sep 17 00:00:00 2001
From: Alexey Makhalov <[email protected]>
Date: Fri, 21 May 2021 07:55:33 +0000
Subject: [PATCH] ext4: fix memory leak in ext4_fill_super

Buffer head references must be released before calling kill_bdev(); otherwise the buffer head (and its page referenced by b_data) will not be freed by kill_bdev, and subsequently that bh will be leaked.

If blocksizes differ, sb_set_blocksize() will kill current buffers and page cache by using kill_bdev(). And then super block will be reread again but using correct blocksize this time. sb_set_blocksize() didn't fully free superblock page and buffer head, and being busy, they were not freed and instead leaked.

This can easily be reproduced by calling an infinite loop of:

systemctl start <ext4_on_lvm>.mount, and
systemctl stop <ext4_on_lvm>.mount

... since systemd creates a cgroup for each slice which it mounts, and the bh leak get amplified by a dying memory cgroup that also never gets freed, and memory consumption is much more easily noticed.

index 886e0d518668..f66c7301b53a 100644
@@ -4462,14 +4462,20 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	if (sb->s_blocksize != blocksize) {
+		/*
+		 * bh must be released before kill_bdev(), otherwise
+		 * it won't be freed and its page also. kill_bdev()
+		 * is called by sb_set_blocksize().
+		 */
+		brelse(bh);
 		/* Validate the filesystem blocksize */
 		if (!sb_set_blocksize(sb, blocksize)) {
 			ext4_msg(sb, KERN_ERR, "bad block size %d",
 					blocksize);
+			bh = NULL;
 			goto failed_mount;
 		}
 
-		brelse(bh);
 		logical_sb_block = sb_block * EXT4_MIN_BLOCK_SIZE;
 		offset = do_div(logical_sb_block, blocksize);
 		bh = ext4_sb_bread_unmovable(sb, logical_sb_block);

@@ -5202,8 +5208,9 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		kfree(get_qf_name(sb, sbi, i));
 #endif
 	fscrypt_free_dummy_policy(&sbi->s_dummy_enc_policy);
-	ext4_blkdev_remove(sbi);
+	/* ext4_blkdev_remove() calls kill_bdev(), release bh before it. */
 	brelse(bh);
+	ext4_blkdev_remove(sbi);
 out_fail:
 	sb->s_fs_info = NULL;
 	kfree(sbi->s_blockgroup_lock);