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Re: [PATCH v2 1/3] xfs: add scan owner field to xfs_eofblocks

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH v2 1/3] xfs: add scan owner field to xfs_eofblocks
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Tue, 27 May 2014 22:30:19 -0700
Cc: Brian Foster <bfoster@xxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140527212653.GC6677@dastard>
References: <1400845950-41435-1-git-send-email-bfoster@xxxxxxxxxx> <1400845950-41435-2-git-send-email-bfoster@xxxxxxxxxx> <20140527104428.GC1440@xxxxxxxxxxxxx> <20140527121810.GB63281@xxxxxxxxxxxxxxx> <20140527212653.GC6677@dastard>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, May 28, 2014 at 07:26:53AM +1000, Dave Chinner wrote:
> > Right... maybe I'm not parsing your point. The purpose here is to avoid
> > the trylock entirely. E.g., Indicate that we have already acquired the
> > lock and can proceed with xfs_free_eofblocks(), rather than fail a
> > trylock and skip (which appears to be a potential infinite loop scenario
> > here due to how the AG walking code handles EAGAIN).
> 
> I think Christoph's concern here is that we are calling a function
> that can take the iolock while we already hold the iolock. i.e. the
> reason we have to add the anti-deadlock code in the first place.

Indeed.

> To
> address that, can we restructure xfs_file_buffered_aio_write() such
> that the ENOSPC/EDQUOT flush is done outside the iolock?
> 
> >From a quick check, I don't think there is any problem with dropping
> the iolock, doing the flushes and then going all the way back to the
> start of the function again, but closer examination and testing is
> warranted...

I think we'd need some form of early space reservation, otherwise we'd
get non-atomic writes.  Time to get those batches write patches out
again..

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