Re: xfs bug in 2.6.26-rc9

[Dave Chinner <david@fromorbit.com>]

On Mon, Jul 14, 2008 at 05:34:51PM +1000, Lachlan McIlroy wrote:
> Mikael Abrahamsson wrote:
>> On Fri, 11 Jul 2008, Dave Chinner wrote:
>>
>>> That aside, what was the assert failure reported prior to the oops?  
>>> i.e. paste the lines in the log before the ---[ cut here ]--- line?  
>>> One of them will start with 'Assertion failed:', I think....
>>
>> These ones?
>>
>> Jul  8 04:44:56 via kernel: [554197.888008] Assertion failed: whichfork 
>> == XFS_ATTR_FORK || ip->i_delayed_blks == 0, file: fs/xfs/xfs_bmap.c,  
>> line: 5879
>> Jul  9 03:25:21 via kernel: [42940.748007] Assertion failed: whichfork  
>> == XFS_ATTR_FORK || ip->i_delayed_blks == 0, file: fs/xfs/xfs_bmap.c,  
>> line: 5879
>
> 	xfs_ilock(ip, XFS_IOLOCK_SHARED);
>
> 	if (whichfork == XFS_DATA_FORK &&
> 		(ip->i_delayed_blks || ip->i_size > ip->i_d.di_size)) {
> 		/* xfs_fsize_t last_byte = xfs_file_last_byte(ip); */
> 		error = xfs_flush_pages(ip, (xfs_off_t)0,
> 					       -1, 0, FI_REMAPF);
> 		if (error) {
> 			xfs_iunlock(ip, XFS_IOLOCK_SHARED);
> 		return error;
> 		}
> 	}
>
> 	ASSERT(whichfork == XFS_ATTR_FORK || ip->i_delayed_blks == 0);
>
> This is a race between xfs_fsr and a mmap write. xfs_fsr acquires the
> iolock and then flushes the file and because it has the iolock it doesn't
> expect any new delayed allocations to occur.  A mmap write can allocate
> delayed allocations without acquiring the iolock so is able to get in
> after the flush but before the ASSERT.

Christoph and I were contemplating this problem with ->page_mkwrite
reecently. The problem is that we can't, right now, return an
EAGAIN-like error to ->page_mkwrite() and have it retry the
page fault. Other parts of the page faulting code can do this,
so it seems like a solvable problem.

The basic concept is that if we can return a EAGAIN result we can
try-lock the inode and hold the locks necessary to avoid this race
or prevent the page fault from dirtying the page until the
filesystem is unfrozen.

Added linux-mm to the cc list for discussion.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com