From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f66.google.com ([209.85.210.66]:32826 "EHLO mail-ot1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727133AbeJEK6W (ORCPT ); Fri, 5 Oct 2018 06:58:22 -0400 Received: by mail-ot1-f66.google.com with SMTP id q50so7259371otd.0 for ; Thu, 04 Oct 2018 21:01:34 -0700 (PDT) MIME-Version: 1.0 References: <20180927112332.3649-1-jack@suse.cz> <20180927132843.GA19006@bombadil.infradead.org> <20180927134129.GB8800@quack2.suse.cz> <20181004162748.GI28384@quack2.suse.cz> <20181005025212.GA12446@bombadil.infradead.org> In-Reply-To: <20181005025212.GA12446@bombadil.infradead.org> From: Dan Williams Date: Thu, 4 Oct 2018 21:01:20 -0700 Message-ID: Subject: Re: [PATCH] dax: Fix deadlock in dax_lock_mapping_entry() To: Matthew Wilcox Cc: Jan Kara , linux-fsdevel , linux-nvdimm , Barret Rhoden Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, Oct 4, 2018 at 7:52 PM Matthew Wilcox wrote: > > On Thu, Oct 04, 2018 at 06:57:52PM -0700, Dan Williams wrote: > > On Thu, Oct 4, 2018 at 9:27 AM Jan Kara wrote: > > > > > > On Thu 27-09-18 11:22:22, Dan Williams wrote: > > > > On Thu, Sep 27, 2018 at 6:41 AM Jan Kara 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? > > > > KVM is enough... however, I have found a hack that makes the test pass: > > > > diff --git a/mm/filemap.c b/mm/filemap.c > > index 52517f28e6f4..d7f035b1846e 100644 > > --- a/mm/filemap.c > > +++ b/mm/filemap.c > > @@ -1668,6 +1668,9 @@ unsigned find_get_entries(struct address_space *mapping, > > goto repeat; > > } > > export: > > + if (iter.index < start) > > + continue; > > + > > indices[ret] = iter.index; > > entries[ret] = page; > > if (++ret == nr_entries) > > > > Is this a radix bug? I would never expect: > > > > radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) > > > > ...to return entries with an index < start. Without that change above > > we loop forever because dax_layout_busy_page() can't make forward > > progress. I'll dig into the radix code tomorrow, but maybe someone > > else will be me to it overnight. > > If 'start' is within a 2MB entry, iter.index can absolutely be less > than start. I forget exactly what the radix tree code does, but I think > it returns index set to the canonical/base index of the entry. Ok, that makes sense. Then the bug is in dax_layout_busy_page() which needs to increment 'index' by the entry size. This might also explain why not every run sees it because you may get lucky and have a 4K entry.