From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751410AbeDEQPF (ORCPT ); Thu, 5 Apr 2018 12:15:05 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:36200 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750835AbeDEQPE (ORCPT ); Thu, 5 Apr 2018 12:15:04 -0400 Date: Thu, 5 Apr 2018 09:15:01 -0700 From: Matthew Wilcox To: Michal Hocko Cc: Joel Fernandes , Steven Rostedt , Zhaoyang Huang , Ingo Molnar , LKML , kernel-patch-test@lists.linaro.org, Andrew Morton , "open list:MEMORY MANAGEMENT" , Vlastimil Babka Subject: Re: [PATCH v1] kernel/trace:check the val against the available mem Message-ID: <20180405161501.GD28128@bombadil.infradead.org> References: <20180404062340.GD6312@dhcp22.suse.cz> <20180404101149.08f6f881@gandalf.local.home> <20180404142329.GI6312@dhcp22.suse.cz> <20180404114730.65118279@gandalf.local.home> <20180405025841.GA9301@bombadil.infradead.org> <20180405142258.GA28128@bombadil.infradead.org> <20180405142749.GL6312@dhcp22.suse.cz> <20180405151359.GB28128@bombadil.infradead.org> <20180405153240.GO6312@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180405153240.GO6312@dhcp22.suse.cz> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 05, 2018 at 05:32:40PM +0200, Michal Hocko wrote: > On Thu 05-04-18 08:13:59, Matthew Wilcox wrote: > > Argh. The comment confused me. OK, now I've read the source and > > understand that GFP_KERNEL | __GFP_RETRY_MAYFAIL tries exactly as hard > > as GFP_KERNEL *except* that it won't cause OOM itself. But any other > > simultaneous GFP_KERNEL allocation without __GFP_RETRY_MAYFAIL will > > cause an OOM. (And that's why we're having a conversation) > > Well, I can udnerstand how this can be confusing. The all confusion > boils down to the small-never-fails semantic we have. So all reclaim > modificators (__GFP_NOFAIL, __GFP_RETRY_MAYFAIL, __GFP_NORETRY) only > modify the _default_ behavior. Now that I understand the flag, I'll try to write a more clear explanation. > > That's a problem because we have places in the kernel that call > > kv[zm]alloc(very_large_size, GFP_KERNEL), and that will turn into vmalloc, > > which will do the exact same thing, only it will trigger OOM all by itself > > (assuming the largest free chunk of address space in the vmalloc area > > is larger than the amount of free memory). > > well, hardcoded GFP_KERNEL from vmalloc guts is yet another, ehm, > herritage that you are not so proud of. Certainly not, but that's not what I'm concerned about; I'm concerned about the allocation of the pages, not the allocation of the array containing the page pointers. > > We could also have a GFP flag that says to only succeed if we're further > > above the existing watermark than normal. __GFP_LOW (==ALLOC_LOW), > > if you like. That would give us the desired behaviour of trying all of > > the reclaim methods that GFP_KERNEL would, but not being able to exhaust > > all the memory that GFP_KERNEL allocations would take. > > Well, I would be really careful with yet another gfp mask. They are so > incredibly hard to define properly and then people kinda tend to screw > your best intentions with their usecases ;) > Failing on low wmark is very close to __GFP_NORETRY or even > __GFP_NOWAIT, btw. So let's try to not overthink this... Oh, indeed. We must be able to clearly communicate to users when they should use this flag. I have in mind something like this: * __GFP_HIGH indicates that the caller is high-priority and that granting * the request is necessary before the system can make forward progress. * For example, creating an IO context to clean pages. * * __GFP_LOW indicates that the caller is low-priority and that it should * not be allocated pages that would cause the system to get into an * out-of-memory situation. For example, allocating multiple individual * pages in order to satisfy a larger request. I think this should actually replace __GFP_RETRY_MAYFAIL. It makes sense to a user: "This is a low priority GFP_KERNEL allocation". I doubt there's one kernel hacker in a hundred who could explain what GFP_RETRY_MAYFAIL does, exactly, and I'm not just saying that because I got it wrong ;-)