From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f170.google.com ([209.85.216.170]:35295 "EHLO mail-qt0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751195AbcH2Rws (ORCPT ); Mon, 29 Aug 2016 13:52:48 -0400 Received: by mail-qt0-f170.google.com with SMTP id 93so72360173qtg.2 for ; Mon, 29 Aug 2016 10:52:48 -0700 (PDT) Message-ID: <1472493162.16070.10.camel@poochiereds.net> Subject: Re: OOM detection regressions since 4.7 From: Jeff Layton To: Linus Torvalds , Olaf Hering , Bruce Fields Cc: Michal Hocko , Andrew Morton , Markus Trippelsdorf , Arkadiusz Miskiewicz , Ralf-Peter Rohbeck , Jiri Slaby , Greg KH , Vlastimil Babka , Joonsoo Kim , linux-mm , LKML , Linux NFS Mailing List Date: Mon, 29 Aug 2016 13:52:42 -0400 In-Reply-To: References: <20160822093249.GA14916@dhcp22.suse.cz> <20160822093707.GG13596@dhcp22.suse.cz> <20160822100528.GB11890@kroah.com> <20160822105441.GH13596@dhcp22.suse.cz> <20160822133114.GA15302@kroah.com> <20160822134227.GM13596@dhcp22.suse.cz> <20160822150517.62dc7cce74f1af6c1f204549@linux-foundation.org> <20160823074339.GB23577@dhcp22.suse.cz> <20160825071103.GC4230@dhcp22.suse.cz> <20160825071728.GA3169@aepfle.de> <20160829145203.GA30660@aepfle.de> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Sender: linux-nfs-owner@vger.kernel.org List-ID: On Mon, 2016-08-29 at 10:28 -0700, Linus Torvalds wrote: > > On Mon, Aug 29, 2016 at 7:52 AM, Olaf Hering wrote: > > > > > > Today I noticed the nfsserver was disabled, probably since months already. > > Starting it gives a OOM, not sure if this is new with 4.7+. > > That's not an oom, that's just an allocation failure. > > And with order-4, that's actually pretty normal. Nobody should use > order-4 (that's 16 contiguous pages, fragmentation can easily make > that hard - *much* harder than the small order-2 or order-2 cases that > we should largely be able to rely on). > > In fact, people who do multi-order allocations should always have a > fallback, and use __GFP_NOWARN. > > > > > [93348.306406] Call Trace: > > [93348.306490]  [] __alloc_pages_slowpath+0x1af/0xa10 > > [93348.306501]  [] __alloc_pages_nodemask+0x250/0x290 > > [93348.306511]  [] cache_grow_begin+0x8d/0x540 > > [93348.306520]  [] fallback_alloc+0x161/0x200 > > [93348.306530]  [] __kmalloc+0x1d2/0x570 > > [93348.306589]  [] nfsd_reply_cache_init+0xaa/0x110 [nfsd] > > Hmm. That's kmalloc itself falling back after already failing to grow > the slab cache earlier (the earlier allocations *were* done with > NOWARN afaik). > > It does look like nfsdstarts out by allocating the hash table with one > single fairly big allocation, and has no fallback position. > > I suspect the code expects to be started at boot time, when this just > isn't an issue. The fact that you loaded the nfsd kernel module with > memory already fragmented after heavy use is likely why nobody else > has seen this. > > Adding the nfsd people to the cc, because just from a robustness > standpoint I suspect it would be better if the code did something like > >  (a) shrink the hash table if the allocation fails (we've got some > examples of that elsewhere) > > or > >  (b) fall back on a vmalloc allocation (that's certainly the simpler model) > > We do have a "kvfree()" helper function for the "free either a kmalloc > or vmalloc allocation" but we don't actually have a good helper > pattern for the allocation side. People just do it by hand, at least > partly because we have so many different ways to allocate things - > zeroing, non-zeroing, node-specific or not, atomic or not (atomic > cannot fall back to vmalloc, obviously) etc etc. > > Bruce, Jeff, comments? > >              Linus Yeah, that makes total sense. Hmm...we _do_ already auto-size the hash at init time already, so shrinking it downward and retrying if the allocation fails wouldn't be hard to do. Maybe I can just cut it in half and throw a pr_warn to tell the admin in that case. In any case...I'll take a look at how we can improve it. Thanks for the heads-up! --  Jeff Layton