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Re: xfs bug in 2.6.26-rc9

To: Lachlan McIlroy <lachlan@xxxxxxx>
Subject: Re: xfs bug in 2.6.26-rc9
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 14 Jul 2008 22:13:32 +1000
Cc: Mikael Abrahamsson <swmike@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-mm@xxxxxxxxx
In-reply-to: <487B019B.9090401@xxxxxxx>
Mail-followup-to: Lachlan McIlroy <lachlan@xxxxxxx>, Mikael Abrahamsson <swmike@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-mm@xxxxxxxxx
References: <alpine.DEB.1.10.0807110939520.30192@xxxxxxxxxxxxxxxx> <20080711084248.GU29319@disturbed> <alpine.DEB.1.10.0807111215040.30192@xxxxxxxxxxxxxxxx> <487B019B.9090401@xxxxxxx>
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User-agent: Mutt/1.5.17+20080114 (2008-01-14)
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@xxxxxxxxxxxxx


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