ext4: check superblock mapped prior to committing

commit a17712c8e4be4fa5404d20e9cd3b2b21eae7bc56 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index fc8686afd6ecda490ee744abfba15272e1eeb190..49af3c50b263a18738688275c7dd42b1e90f3daf 100644 (file)
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4442,6 +4442,14 @@ 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 (buffer_write_io_error(sbh)) {
                /*
                 * Oh, dear.  A previous attempt to write the