Re: xfs bug in 2.6.26-rc9

[Lachlan McIlroy]

Dave Chinner wrote:
> 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.
Why do we need to try-lock the inode?  Will we have an ABBA deadlock
if we block on the iolock in ->page_mkwrite()?
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