linux-kernel - Re: [PATCH] direct-io: use GFP

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Date:   Thu, 8 Aug 2019 11:13:50 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Mike Snitzer <msnitzer@...hat.com>, junxiao.bi@...cle.com,
        dm-devel@...hat.com, Alasdair Kergon <agk@...hat.com>,
        honglei.wang@...cle.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH] direct-io: use GFP_NOIO to avoid deadlock



On Thu, 8 Aug 2019, Matthew Wilcox wrote:

> On Thu, Aug 08, 2019 at 05:50:10AM -0400, Mikulas Patocka wrote:
> > A deadlock with this stacktrace was observed.
> > 
> > The obvious problem here is that in the call chain 
> > xfs_vm_direct_IO->__blockdev_direct_IO->do_blockdev_direct_IO->kmem_cache_alloc 
> > we do a GFP_KERNEL allocation while we are in a filesystem driver and in a 
> > block device driver.
> 
> But that's not the problem.  The problem is the loop driver calls into the
> filesystem without calling memalloc_noio_save() / memalloc_noio_restore().
> There are dozens of places in XFS which use GFP_KERNEL allocations and
> all can trigger this same problem if called from the loop driver.

OK. I'll send a new patch that sets PF_MEMALLOC_NOIO in the loop driver.

Mikulas