Re: Infinite loop with DAX PMD faults

From: "Kani, Toshimitsu" <toshi.kani@hpe.com>
To: "dan.j.williams@intel.com" <dan.j.williams@intel.com>,
	"ross.zwisler@linux.intel.com" <ross.zwisler@linux.intel.com>
Cc: "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"jack@suse.cz" <jack@suse.cz>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>
Subject: Re: Infinite loop with DAX PMD faults
Date: Thu, 27 Oct 2016 21:48:41 +0000	[thread overview]
Message-ID: <1477604810.20881.104.camel@hpe.com> (raw)
In-Reply-To: <20161027210343.GA12217@linux.intel.com>

On Thu, 2016-10-27 at 15:03 -0600, Ross Zwisler wrote:
> On Thu, Oct 27, 2016 at 12:46:32PM -0700, Dan Williams wrote:
> > 
> > On Thu, Oct 27, 2016 at 12:07 PM, Jan Kara <jack@suse.cz> 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?
> > 
> > Can you bisect it with CONFIG_BROKEN removed from older kernels?
> > 
> > I remember tracking down something like this when initially doing
> > the pmd support.  It ended up being a missed pmd_devmap() check in
> > the fault path, so it may not be the same issue.  It would at least
> > be interesting to see if 4.6 fails in a similar manner with this
> > test and FS_DAX_PMD enabled.
> 
> I've been able to reproduce this with my v4.9-rc2 branch, but it
> doesn't reproduce with the old v4.6 kernel.

Not sure if it's relevant, but as FYI I fixed a similar issue before.

commit 59bf4fb9d386601cbaa70a9b00159abb846dedaa
dax: Split pmd map when fallback on COW

-Toshi
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