Commit a17712c8 authored Jul 02, 2018 by Jon Derrick Committed by Theodore Ts'o Jul 02, 2018

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

parent bfe0a5f4

fs/ext4/super.c

+8 −0

Original line number	Original line	Diff line number	Diff line
	@@ -4808,6 +4808,14 @@ static int ext4_commit_super(struct super_block *sb, int sync)

	if (!sbh \|\| block_device_ejected(sb))		if (!sbh \|\| block_device_ejected(sb))
	return error;		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		* If the file system is mounted read-only, don't update the
	* superblock write time. This avoids updating the superblock		* superblock write time. This avoids updating the superblock