xfs: clean up unwritten buffers on write failure

The xfs_vm_write_failed() handler is currently responsible for cleaning
up any delalloc blocks over the range of a failed write beyond EOF.
Failure to do so results in warning messages and other inconsistencies
between buffer and extent state. The ->releasepage() handler currently
warns in the event of a page being released with either unwritten or
delalloc buffers, as neither is ever expected by the time a page is
released.

As has been reproduced by generic/083 on a -bsize=1k fs, it is currently
possible to trigger the ->releasepage() warning for a page with
unwritten buffers when a filesystem is near ENOSPC. This is reproduced
by the following sequence:

  $ mkfs.xfs -f -b size=1k -d size=100m <dev>
  $ mount <dev> /mnt/
  $
  $ xfs_io -fc "falloc -k 0 1k" /mnt/file
  $ dd if=/dev/zero of=/mnt/enospc conv=notrunc oflag=append
  $
  $ xfs_io -c "pwrite 512 1k" /mnt/file
  $ xfs_io -d -c "pwrite 16k 1k" /mnt/file

The first pwrite command attempts a block unaligned write across an
unwritten block and a hole. The delalloc for the hole fails with ENOSPC
and the subsequent error handling does not clean up the unwritten buffer
that was instantiated during the first ->get_block() call.

The second pwrite triggers a warning as part of the inode mapping
invalidation that occurs prior to direct I/O. The releasepage() handler
detects the unwritten buffer at this time, warns and prevents the
release of the page.

To deal with this problem, update xfs_vm_write_failed() to clean up
unwritten as well as delalloc buffers that are beyond EOF and within the
range of the failed write.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_aops.c

@@ -1783,14 +1783,22 @@ xfs_vm_write_failed(
 		if (block_start >= to)
 			break;
 
-		if (!buffer_delay(bh))
+		/*
+		 * Process delalloc and unwritten buffers beyond EOF. We can
+		 * encounter unwritten buffers in the event that a file has
+		 * post-EOF unwritten extents and an extending write happens to
+		 * fail (e.g., an unaligned write that also involves a delalloc
+		 * to the same page).
+		 */
+		if (!buffer_delay(bh) && !buffer_unwritten(bh))
 			continue;
 
 		if (!buffer_new(bh) && block_offset < i_size_read(inode))
 			continue;
 
-		xfs_vm_kill_delalloc_range(inode, block_offset,
-					   block_offset + bh->b_size);
+		if (buffer_delay(bh))
+			xfs_vm_kill_delalloc_range(inode, block_offset,
+						   block_offset + bh->b_size);
 
 		/*
 		 * This buffer does not contain data anymore. make sure anyone
@@ -1801,6 +1809,7 @@ xfs_vm_write_failed(
 		clear_buffer_mapped(bh);
 		clear_buffer_new(bh);
 		clear_buffer_dirty(bh);
+		clear_buffer_unwritten(bh);
 	}
 
 }