From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f199.google.com (mail-pg1-f199.google.com [209.85.215.199]) by kanga.kvack.org (Postfix) with ESMTP id F13AF6B0006 for ; Mon, 16 Jul 2018 19:12:53 -0400 (EDT) Received: by mail-pg1-f199.google.com with SMTP id a3-v6so4086028pgv.10 for ; Mon, 16 Jul 2018 16:12:53 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id q13-v6si22493961pgc.670.2018.07.16.16.12.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 16 Jul 2018 16:12:51 -0700 (PDT) Date: Mon, 16 Jul 2018 16:12:49 -0700 From: Andrew Morton Subject: Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers Message-Id: <20180716161249.c76240cd487c070fb271d529@linux-foundation.org> In-Reply-To: <20180716115058.5559-1-mhocko@kernel.org> References: <20180716115058.5559-1-mhocko@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: LKML , linux-mm@kvack.org, Michal Hocko , "David (ChunMing) Zhou" , Paolo Bonzini , Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= , Alex Deucher , David Airlie , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Doug Ledford , Jason Gunthorpe , Mike Marciniszyn , Dennis Dalessandro , Sudeep Dutt , Ashutosh Dixit , Dimitri Sivanich , Boris Ostrovsky , Juergen Gross , =?ISO-8859-1?Q?J=E9r=F4me?= Glisse , Andrea Arcangeli , Felix Kuehling , kvm@vger.kernel.org, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, linux-rdma@vger.kernel.org, xen-devel@lists.xenproject.org, Christian =?ISO-8859-1?Q?K=F6nig?= , David Rientjes , Leon Romanovsky On Mon, 16 Jul 2018 13:50:58 +0200 Michal Hocko wrote: > From: Michal Hocko > > There are several blockable mmu notifiers which might sleep in > mmu_notifier_invalidate_range_start and that is a problem for the > oom_reaper because it needs to guarantee a forward progress so it cannot > depend on any sleepable locks. > > Currently we simply back off and mark an oom victim with blockable mmu > notifiers as done after a short sleep. That can result in selecting a > new oom victim prematurely because the previous one still hasn't torn > its memory down yet. > > We can do much better though. Even if mmu notifiers use sleepable locks > there is no reason to automatically assume those locks are held. > Moreover majority of notifiers only care about a portion of the address > space and there is absolutely zero reason to fail when we are unmapping an > unrelated range. Many notifiers do really block and wait for HW which is > harder to handle and we have to bail out though. > > This patch handles the low hanging fruid. __mmu_notifier_invalidate_range_start > gets a blockable flag and callbacks are not allowed to sleep if the > flag is set to false. This is achieved by using trylock instead of the > sleepable lock for most callbacks and continue as long as we do not > block down the call chain. I assume device driver developers are wondering "what does this mean for me". As I understand it, the only time they will see blockable==false is when their driver is being called in response to an out-of-memory condition, yes? So it is a very rare thing. Any suggestions regarding how the driver developers can test this code path? I don't think we presently have a way to fake an oom-killing event? Perhaps we should add such a thing, given the problems we're having with that feature. > I think we can improve that even further because there is a common > pattern to do a range lookup first and then do something about that. > The first part can be done without a sleeping lock in most cases AFAICS. > > The oom_reaper end then simply retries if there is at least one notifier > which couldn't make any progress in !blockable mode. A retry loop is > already implemented to wait for the mmap_sem and this is basically the > same thing. > > ... > > +static inline int mmu_notifier_invalidate_range_start_nonblock(struct mm_struct *mm, > + unsigned long start, unsigned long end) > +{ > + int ret = 0; > + if (mm_has_notifiers(mm)) > + ret = __mmu_notifier_invalidate_range_start(mm, start, end, false); > + > + return ret; > } nit, { if (mm_has_notifiers(mm)) return __mmu_notifier_invalidate_range_start(mm, start, end, false); return 0; } would suffice. > > ... > > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -3074,7 +3074,7 @@ void exit_mmap(struct mm_struct *mm) > * reliably test it. > */ > mutex_lock(&oom_lock); > - __oom_reap_task_mm(mm); > + (void)__oom_reap_task_mm(mm); > mutex_unlock(&oom_lock); What does this do? > set_bit(MMF_OOM_SKIP, &mm->flags); > > ... >