From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 48A30C433F5 for ; Tue, 2 Nov 2021 09:07:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2ED6060E8C for ; Tue, 2 Nov 2021 09:07:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230333AbhKBJJl (ORCPT ); Tue, 2 Nov 2021 05:09:41 -0400 Received: from szxga02-in.huawei.com ([45.249.212.188]:15342 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229577AbhKBJJb (ORCPT ); Tue, 2 Nov 2021 05:09:31 -0400 Received: from dggemv703-chm.china.huawei.com (unknown [172.30.72.56]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4Hk3tZ16Jdz90lF; Tue, 2 Nov 2021 17:06:46 +0800 (CST) Received: from dggpeml500024.china.huawei.com (7.185.36.10) by dggemv703-chm.china.huawei.com (10.3.19.46) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.15; Tue, 2 Nov 2021 17:06:54 +0800 Received: from [10.174.176.231] (10.174.176.231) by dggpeml500024.china.huawei.com (7.185.36.10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.15; Tue, 2 Nov 2021 17:06:53 +0800 Subject: Re: [PATCH] mm, slub: place the trace before freeing memory in kmem_cache_free() To: John Hubbard , , , , , Andrew Morton , , , CC: , Hewenliang References: <867f6da4-6d38-6435-3fbb-a2a3744029f1@huawei.com> From: Yunfeng Ye Message-ID: <2ea4e792-816c-a734-db1f-388516c74ea9@huawei.com> Date: Tue, 2 Nov 2021 17:06:42 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.176.231] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpeml500024.china.huawei.com (7.185.36.10) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/11/2 15:03, John Hubbard wrote: > On 10/30/21 03:11, Yunfeng Ye wrote: >> After the memory is freed, it may be allocated by other CPUs and has >> been recorded by trace. So the timing sequence of the memory tracing is >> inaccurate. >> >> For example, we expect the following timing sequeuce: >> >>      CPU 0                 CPU 1 >> >>    (1) alloc xxxxxx >>    (2) free  xxxxxx >>                           (3) alloc xxxxxx >>                           (4) free  xxxxxx >> >> However, the following timing sequence may occur: >> >>      CPU 0                 CPU 1 >> >>    (1) alloc xxxxxx >>                           (2) alloc xxxxxx >>    (3) free  xxxxxx >>                           (4) free  xxxxxx >> >> So place the trace before freeing memory in kmem_cache_free(). > > Hi Yunfeng, > > Like Muchun, I had some difficulty with the above description, but > now I think I get it. :) > > In order to make it easier for others, how about this wording and subject > line, instead: > Ok,I will modify the description in the next version patch. Thanks. > > mm, slub: emit the "free" trace report before freeing memory in kmem_cache_free() > > After the memory is freed, it can be immediately allocated by other > CPUs, before the "free" trace report has been emitted. This causes > inaccurate traces. > > For example, if the following sequence of events occurs: > >     CPU 0                 CPU 1 > >   (1) alloc xxxxxx >   (2) free  xxxxxx >                          (3) alloc xxxxxx >                          (4) free  xxxxxx > > ...then they will be inaccurately reported via tracing, so that they > appear to have happened in this order. This makes it look like CPU 1 > somehow managed to allocate mmemory that CPU 0 still had allocated for > itself: > >     CPU 0                 CPU 1 > >   (1) alloc xxxxxx >                          (2) alloc xxxxxx >   (3) free  xxxxxx >                          (4) free  xxxxxx > > In order to avoid this, emit the "free xxxxxx" tracing report just > before the actual call to free the memory, instead of just after it. > > >> >> Signed-off-by: Yunfeng Ye >> --- >>   mm/slub.c | 2 +- >>   1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/slub.c b/mm/slub.c >> index 432145d7b4ec..427e62034c3f 100644 >> --- a/mm/slub.c >> +++ b/mm/slub.c >> @@ -3526,8 +3526,8 @@ void kmem_cache_free(struct kmem_cache *s, void *x) >>       s = cache_from_obj(s, x); >>       if (!s) >>           return; >> -    slab_free(s, virt_to_head_page(x), x, NULL, 1, _RET_IP_); >>       trace_kmem_cache_free(_RET_IP_, x, s->name); >> +    slab_free(s, virt_to_head_page(x), x, NULL, 1, _RET_IP_); >>   } >>   EXPORT_SYMBOL(kmem_cache_free); >> > > ...the diffs seem correct, too, but I'm not exactly a slub reviewer, so > take that for what it's worth. > > > thanks,