From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f65.google.com ([209.85.210.65]:43046 "EHLO mail-ot1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726676AbeJEIyb (ORCPT ); Fri, 5 Oct 2018 04:54:31 -0400 Received: by mail-ot1-f65.google.com with SMTP id e21-v6so11207578otk.10 for ; Thu, 04 Oct 2018 18:58:04 -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> In-Reply-To: <20181004162748.GI28384@quack2.suse.cz> From: Dan Williams Date: Thu, 4 Oct 2018 18:57:52 -0700 Message-ID: Subject: Re: [PATCH] dax: Fix deadlock in dax_lock_mapping_entry() To: Jan Kara Cc: Matthew Wilcox , 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 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.