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 X-Spam-Level: X-Spam-Status: No, score=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1BAD7C2B9F4 for ; Fri, 25 Jun 2021 09:32:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0149161427 for ; Fri, 25 Jun 2021 09:32:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231437AbhFYJeu (ORCPT ); Fri, 25 Jun 2021 05:34:50 -0400 Received: from szxga02-in.huawei.com ([45.249.212.188]:5427 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231430AbhFYJer (ORCPT ); Fri, 25 Jun 2021 05:34:47 -0400 Received: from dggeme703-chm.china.huawei.com (unknown [172.30.72.54]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4GBBXL66bxz74H0; Fri, 25 Jun 2021 17:29:06 +0800 (CST) Received: from [10.174.177.120] (10.174.177.120) by dggeme703-chm.china.huawei.com (10.1.199.99) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.2176.2; Fri, 25 Jun 2021 17:32:23 +0800 Subject: Re: [Phishing Risk] [External] [PATCH 2/3] mm/zsmalloc.c: combine two atomic ops in zs_pool_dec_isolated() From: Miaohe Lin To: Muchun Song CC: Andrew Morton , Minchan Kim , , , LKML , Linux Memory Management List References: <20210624123930.1769093-1-linmiaohe@huawei.com> <20210624123930.1769093-3-linmiaohe@huawei.com> <1b38b33f-316e-1816-216f-9923f612ceb6@huawei.com> <01117bc0-53b1-d81a-a4d8-2a1dbe5dcd94@huawei.com> Message-ID: <97fdc2f3-6757-7ca1-6323-02b618b85894@huawei.com> Date: Fri, 25 Jun 2021 17:32:23 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <01117bc0-53b1-d81a-a4d8-2a1dbe5dcd94@huawei.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.120] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) To dggeme703-chm.china.huawei.com (10.1.199.99) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/6/25 16:46, Miaohe Lin wrote: > On 2021/6/25 15:29, Muchun Song wrote: >> On Fri, Jun 25, 2021 at 2:32 PM Miaohe Lin wrote: >>> >>> On 2021/6/25 13:01, Muchun Song wrote: >>>> On Thu, Jun 24, 2021 at 8:40 PM Miaohe Lin wrote: >>>>> >>>>> atomic_long_dec_and_test() is equivalent to atomic_long_dec() and >>>>> atomic_long_read() == 0. Use it to make code more succinct. >>>> >>>> Actually, they are not equal. atomic_long_dec_and_test implies a >>>> full memory barrier around it but atomic_long_dec and atomic_long_read >>>> don't. >>>> >>> >>> Many thanks for comment. They are indeed not completely equal as you said. >>> What I mean is they can do the same things we want in this specified context. >>> Thanks again. >> >> I don't think so. Using individual operations can eliminate memory barriers. >> We will pay for the barrier if we use atomic_long_dec_and_test here. > > The combination of atomic_long_dec and atomic_long_read usecase is rare and looks somehow > weird. I think it's worth to do this with the cost of barrier. > It seems there is race between zs_pool_dec_isolated and zs_unregister_migration if pool->destroying is reordered before the atomic_long_dec and atomic_long_read ops. So this memory barrier is necessary: zs_pool_dec_isolated zs_unregister_migration pool->destroying != true pool->destroying = true; smp_mb(); wait_for_isolated_drain wait_event with atomic_long_read(&pool->isolated_pages) != 0 atomic_long_dec(&pool->isolated_pages); atomic_long_read(&pool->isolated_pages) == 0 Thus wake_up_all is missed. And the comment in zs_pool_dec_isolated() said: /* * There's no possibility of racing, since wait_for_isolated_drain() * checks the isolated count under &class->lock after enqueuing * on migration_wait. */ But I found &class->lock is indeed not acquired for wait_for_isolated_drain(). So I think the above race is possible. Does this make senses for you ? Thanks. >> >>> >>>> That RMW operations that have a return value is equal to the following. >>>> >>>> smp_mb__before_atomic() >>>> non-RMW operations or RMW operations that have no return value >>>> smp_mb__after_atomic() >>>> >>>> Thanks. >>>> >>>>> >>>>> Signed-off-by: Miaohe Lin >>>>> --- >>>>> mm/zsmalloc.c | 3 +-- >>>>> 1 file changed, 1 insertion(+), 2 deletions(-) >>>>> >>>>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c >>>>> index 1476289b619f..0b4b23740d78 100644 >>>>> --- a/mm/zsmalloc.c >>>>> +++ b/mm/zsmalloc.c >>>>> @@ -1828,13 +1828,12 @@ static void putback_zspage_deferred(struct zs_pool *pool, >>>>> static inline void zs_pool_dec_isolated(struct zs_pool *pool) >>>>> { >>>>> VM_BUG_ON(atomic_long_read(&pool->isolated_pages) <= 0); >>>>> - atomic_long_dec(&pool->isolated_pages); >>>>> /* >>>>> * There's no possibility of racing, since wait_for_isolated_drain() >>>>> * checks the isolated count under &class->lock after enqueuing >>>>> * on migration_wait. >>>>> */ >>>>> - if (atomic_long_read(&pool->isolated_pages) == 0 && pool->destroying) >>>>> + if (atomic_long_dec_and_test(&pool->isolated_pages) && pool->destroying) >>>>> wake_up_all(&pool->migration_wait); >>>>> } >>>>> >>>>> -- >>>>> 2.23.0 >>>>> >>>> . >>>> >>> >> . >> >