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Date: Thu, 27 Oct 2016 13:54:49 -0600
From: Ross Zwisler <ross.zwisler@linux.intel.com>
Subject: Re: Infinite loop with DAX PMD faults
Message-ID: <20161027195449.GA29643@linux.intel.com>
References: <20161027190750.GA28888@quack2.suse.cz>
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To: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.kernel.org, linux-nvdimm@lists.01.org
List-ID: <linux-nvdimm@lists.01.org>

On Thu, Oct 27, 2016 at 09:07:50PM +0200, Jan Kara wrote:
> Hello,
> 
> When testing my DAX patches rebased on top of Ross' DAX PMD series, I've
> come across the following issue with generic/344 test from xfstests. The
> test ends in an infinite fault loop when we fault index 0 over and over
> again never finishing the fault. The problem is that we do a write fault
> for index 0 when there is PMD for that index. So we enter wp_huge_pmd().
> For whatever reason that returns VM_FAULT_FALLBACK so we continue to
> handle_pte_fault(). There we do
> 
> 	if (pmd_trans_unstable(vmf->pmd) || pmd_devmap(*vmf->pmd))
> 
> check which is true - the PMD we have is pmd_trans_huge() - so we 'return
> 0' and that results in retrying the fault and all happens from the
> beginning again.
> 
> It isn't quite obvious how to break that cycle to me. The comment before
> pmd_none_or_trans_huge_or_clear_bad() goes to great lengths explaining
> possible races when PMD is pmd_trans_huge() so it needs careful evaluation
> what needs to be done for DAX. Ross, any idea?

I'll try & reproduce this, and I'll get back to you.
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