From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f200.google.com (mail-pl1-f200.google.com [209.85.214.200]) by kanga.kvack.org (Postfix) with ESMTP id 3A4A76B2382 for ; Tue, 20 Nov 2018 22:18:31 -0500 (EST) Received: by mail-pl1-f200.google.com with SMTP id y2so5505434plr.8 for ; Tue, 20 Nov 2018 19:18:31 -0800 (PST) Received: from userp2130.oracle.com (userp2130.oracle.com. [156.151.31.86]) by mx.google.com with ESMTPS id d10si30204698pls.170.2018.11.20.19.18.29 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 20 Nov 2018 19:18:29 -0800 (PST) Subject: Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial References: <20181117013335.32220-1-wen.gang.wang@oracle.com> <20181118010229.esa32zk5hpob67y7@master> <20181121030241.h7rgyjtlfcnm3hki@master> From: Wengang Wang Message-ID: <9e238df6-d018-68b8-1c79-0c248abf0804@oracle.com> Date: Tue, 20 Nov 2018 19:18:13 -0800 MIME-Version: 1.0 In-Reply-To: <20181121030241.h7rgyjtlfcnm3hki@master> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Wei Yang Cc: cl@linux.com, penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Hi Wei, I think you will receive my reply to Zhong, But I am copying my comments for that patch here (again): Copy starts ==> I am not sure if the patch you mentioned intended to fix the problem here. With that patch the negative page->pobjects would become a large positive value, it will win the compare with s->cpu_partial and go ahead to unfreeze the partial slabs. Though it may be not a perfect fix for this issue, it really fixes (or workarounds) the issue here. I'd like to skip my patch.. <=== Copy ends thanks, wengang On 2018/11/20 19:02, Wei Yang wrote: > On Tue, Nov 20, 2018 at 09:58:58AM -0800, Wengang Wang wrote: >> Hi Wei, >> >> >> On 2018/11/17 17:02, Wei Yang wrote: >>> On Fri, Nov 16, 2018 at 05:33:35PM -0800, Wengang Wang wrote: >>>> The this_cpu_cmpxchg makes the do-while loop pass as long as the >>>> s->cpu_slab->partial as the same value. It doesn't care what happened to >>>> that slab. Interrupt is not disabled, and new alloc/free can happen in the >>> Well, I seems to understand your description. >>> >>> There are two slabs >>> >>> * one which put_cpu_partial() trying to free an object >>> * one which is the first slab in cpu_partial list >>> >>> There is some tricky case, the first slab in cpu_partial list we >>> reference to will change since interrupt is not disabled. >> Yes, two slabs involved here just as you said above. >> And yes, the case is really tricky, but it's there. >> >>>> interrupt handlers. Theoretically, after we have a reference to the it, >>> ^^^ >>> one more word? >> sorry, "the" should not be there. >> >>>> stored in _oldpage_, the first slab on the partial list on this CPU can be >>> ^^^ >>> One little suggestion here, mayby use cpu_partial would be more easy to >>> understand. I confused this with the partial list in kmem_cache_node at >>> the first time. :-) >> Right, making others understanding easily is very important. I just meant >> cpu_partial. >> >>>> moved to kmem_cache_node and then moved to different kmem_cache_cpu and >>>> then somehow can be added back as head to partial list of current >>>> kmem_cache_cpu, though that is a very rare case. If that rare case really >>> Actually, no matter what happens after the removal of the first slab in >>> cpu_partial, it would leads to problem. >> Maybe you are right, what I see is the problem on the page->pobjects. >> >>>> happened, the reading of oldpage->pobjects may get a 0xdead0000 >>>> unexpectedly, stored in _pobjects_, if the reading happens just after >>>> another CPU removed the slab from kmem_cache_node, setting lru.prev to >>>> LIST_POISON2 (0xdead000000000200). The wrong _pobjects_(negative) then >>>> prevents slabs from being moved to kmem_cache_node and being finally freed. >>>> >>>> We see in a vmcore, there are 375210 slabs kept in the partial list of one >>>> kmem_cache_cpu, but only 305 in-use objects in the same list for >>>> kmalloc-2048 cache. We see negative values for page.pobjects, the last page >>>> with negative _pobjects_ has the value of 0xdead0004, the next page looks >>>> good (_pobjects is 1). >>>> >>>> For the fix, I wanted to call this_cpu_cmpxchg_double with >>>> oldpage->pobjects, but failed due to size difference between >>>> oldpage->pobjects and cpu_slab->partial. So I changed to call >>>> this_cpu_cmpxchg_double with _tid_. I don't really want no alloc/free >>>> happen in between, but just want to make sure the first slab did expereince >>>> a remove and re-add. This patch is more to call for ideas. >>> Maybe not an exact solution. >>> >>> I took a look into the code and change log. >>> >>> _tid_ is introduced by commit 8a5ec0ba42c4 ('Lockless (and preemptless) >>> fastpaths for slub'), which is used to guard cpu_freelist. While we don't >>> modify _tid_ when cpu_partial changes. >>> >>> May need another _tid_ for cpu_partial? >> Right, _tid_ changes later than cpu_partial changes. >> >> As pointed out by Zhong Jiang, the pobjects issue is fixed by commit > Where you discussed this issue? Any reference I could get a look? > >> e5d9998f3e09 (not sure if by side effect, see my replay there), > I took a look at this commit e5d9998f3e09 ('slub: make ->cpu_partial > unsigned int'), but not see some relationship between them. > > Would you mind show me a link or cc me in case you have further > discussion? > > Thanks. > >> I'd skip this patch.?? If we found other problems regarding the change of >> cpu_partial, let's fix them. What do you think? >> >> thanks, >> wengang