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List:       linux-xfs
Subject:    Re: [PATCH v2] xfs: avoid AGI/AGF deadlock scenario for inode chunk allocation
From:       Dave Chinner <david () fromorbit ! com>
Date:       2014-02-28 22:20:48
Message-ID: 20140228222048.GD13647 () dastard
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On Fri, Feb 28, 2014 at 02:24:18PM -0500, Brian Foster wrote:
> On Tue, Feb 11, 2014 at 01:07:46PM -0500, Brian Foster wrote:
> > The inode chunk allocation path can lead to deadlock conditions if
> > a transaction is dirtied with an AGF (to fix up the freelist) for
> > an AG that cannot satisfy the actual allocation request. This code
> > path is written to try and avoid this scenario, but it can be
> > reproduced by running xfstests generic/270 in a loop on a 512b fs.
> > 
> > An example situation is:
> > - process A attempts an inode allocation on AG 3, modifies
> >   the freelist, fails the allocation and ultimately moves on to
> >   AG 0 with the AG 3 AGF held
> > - process B is doing a free space operation (i.e., truncate) and
> >   acquires the AG 0 AGF, waits on the AG 3 AGF
> > - process A acquires the AG 0 AGI, waits on the AG 0 AGF (deadlock)
> > 
> > The problem here is that process A acquired the AG 3 AGF while
> > moving on to AG 0 (and releasing the AG 3 AGI with the AG 3 AGF
> > held). xfs_dialloc() makes one pass through each of the AGs when
> > attempting to allocate an inode chunk. The expectation is a clean
> > transaction if a particular AG cannot satisfy the allocation
> > request. xfs_ialloc_ag_alloc() is written to support this through
> > use of the minalignslop allocation args field.
> > 
> > When using the agi->agi_newino optimization, we attempt an exact
> > bno allocation request based on the location of the previously
> > allocated chunk. minalignslop is set to inform the allocator that
> > we will require alignment on this chunk, and thus to not allow the
> > request for this AG if the extra space is not available. Suppose
> > that the AG in question has just enough space for this request, but
> > not at the requested bno. xfs_alloc_fix_freelist() will proceed as
> > normal as it determines the request should succeed, and thus it is
> > allowed to modify the agf. xfs_alloc_ag_vextent() ultimately fails
> > because the requested bno is not available. In response, the caller
> > moves on to a NEAR_BNO allocation request for the same AG. The
> > alignment is set, but the minalignslop field is never reset. This
> > increases the overall requirement of the request from the first
> > attempt. If this delta is the difference between allocation success
> > and failure for the AG, xfs_alloc_fix_freelist() rejects this
> > request outright the second time around and causes the allocation
> > request to unnecessarily fail for this AG.
> > 
> > To address this situation, reset the minalignslop field immediately
> > after use and prevent it from leaking into subsequent requests.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> > 
> > v2:
> > - Reset minalignslop immediately after use rather than prior to the
> >   subsequent request and add a comment. [dchinner]
> > 
> 
> ping? Any chance to get this committed?

I'm sorry, Brian, I thought I had committed it - I'll get it in the
next round.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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