Re: [PATCH] dax: Fix deadlock in dax_lock_mapping_entry()

From: Jan Kara <jack@suse.cz>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	brho@google.com
Subject: Re: [PATCH] dax: Fix deadlock in dax_lock_mapping_entry()
Date: Thu, 4 Oct 2018 18:27:48 +0200	[thread overview]
Message-ID: <20181004162748.GI28384@quack2.suse.cz> (raw)
In-Reply-To: <CAPcyv4hut3P5fiNHgjpf4Qg_nBo9JKkovCdrUnzXyji=ey8zJw@mail.gmail.com>

On Thu 27-09-18 11:22:22, Dan Williams wrote:
> On Thu, Sep 27, 2018 at 6:41 AM Jan Kara <jack@suse.cz> wrote:
> >
> > On Thu 27-09-18 06:28:43, Matthew Wilcox wrote:
> > > On Thu, Sep 27, 2018 at 01:23:32PM +0200, Jan Kara wrote:
> > > > When dax_lock_mapping_entry() has to sleep to obtain entry lock, it will
> > > > fail to unlock mapping->i_pages spinlock and thus immediately deadlock
> > > > against itself when retrying to grab the entry lock again. Fix the
> > > > problem by unlocking mapping->i_pages before retrying.
> > >
> > > It seems weird that xfstests doesn't provoke this ...
> >
> > The function currently gets called only from mm/memory-failure.c. And yes,
> > we are lacking DAX hwpoison error tests in fstests...
> 
> I have an item on my backlog to port the ndctl unit test that does
> memory_failure() injection vs ext4 over to fstests. That said I've
> been investigating a deadlock on ext4 caused by this test. When I saw
> this patch I hoped it was root cause, but the test is still failing
> for me. Vishal is able to pass the test on his system, so the failure
> mode is timing dependent. I'm running this patch on top of -rc5 and
> still seeing the following deadlock.

I went through the code but I don't see where the problem could be. How can
I run that test? Is KVM enough or do I need hardware with AEP dimms?

								Honza

> 
>     EXT4-fs (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
>     EXT4-fs (pmem0): mounted filesystem with ordered data mode. Opts: dax
>     Injecting memory failure for pfn 0x208900 at process virtual
> address 0x7f5872900000
>     Memory failure: 0x208900: Killing dax-pmd:7095 due to hardware
> memory corruption
>     Memory failure: 0x208900: recovery action for dax page: Recovered
>     watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [dax-pmd:7095]
>     [..]
>     irq event stamp: 121911146
>     hardirqs last  enabled at (121911145): [<ffffffff81aa1bd9>]
> _raw_spin_unlock_irq+0x29/0x40    hardirqs last disabled at
> (121911146): [<ffffffff810037a3>] trace_hardirqs_off_thunk+0x1a/0x1c
>     softirqs last  enabled at (78238674): [<ffffffff81e0032e>]
> __do_softirq+0x32e/0x428
>     softirqs last disabled at (78238627): [<ffffffff810bc6f6>]
> irq_exit+0xf6/0x100
>     CPU: 35 PID: 7095 Comm: dax-pmd Tainted: G           OE
> 4.19.0-rc5+ #2394
>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> rel-1.11.1-0-g0551a4be2c-prebuilt.qemu-project.org 04/01/2014
>     RIP: 0010:lock_release+0x134/0x2a0
>     [..]
>     Call Trace:
>      find_get_entries+0x299/0x3c0
>      pagevec_lookup_entries+0x1a/0x30
>      dax_layout_busy_page+0x9c/0x280
>      ? __lock_acquire+0x12fa/0x1310
>      ext4_break_layouts+0x48/0x100
>      ? ext4_punch_hole+0x108/0x5a0
>      ext4_punch_hole+0x110/0x5a0
>      ext4_fallocate+0x189/0xa40
>      ? rcu_read_lock_sched_held+0x6b/0x80
>      ? rcu_sync_lockdep_assert+0x2e/0x60
>      vfs_fallocate+0x13f/0x270
> 
> The same test against xfs is not failing for me. I have been seeking
> some focus time to dig in on this.
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR