ext4: check superblock mapped prior to committing

This patch attempts to close a hole leading to a BUG seen with hot
removals during writes [1].

A block device (NVME namespace in this test case) is formatted to EXT4
without partitions. It's mounted and write I/O is run to a file, then
the device is hot removed from the slot. The superblock attempts to be
written to the drive which is no longer present.

The typical chain of events leading to the BUG:
ext4_commit_super()
  __sync_dirty_buffer()
    submit_bh()
      submit_bh_wbc()
        BUG_ON(!buffer_mapped(bh));

This fix checks for the superblock's buffer head being mapped prior to
syncing.

[1] https://www.spinics.net/lists/linux-ext4/msg56527.html

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index b37b00befd65c44d7b1ff8c70933449e55f2aa1c..f36c9f774e831b68db426bfa1eca9adf11c0707d 100644 (file)
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4808,6 +4808,14 @@ static int ext4_commit_super(struct super_block *sb, int sync)
 
        if (!sbh || block_device_ejected(sb))
                return error;
+
+       /*
+        * The superblock bh should be mapped, but it might not be if the
+        * device was hot-removed. Not much we can do but fail the I/O.
+        */
+       if (!buffer_mapped(sbh))
+               return error;
+
        /*
         * If the file system is mounted read-only, don't update the
         * superblock write time.  This avoids updating the superblock