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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 8AC70C43381 for ; Thu, 7 Jan 2021 11:19:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6D7C823125 for ; Thu, 7 Jan 2021 11:19:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727978AbhAGLTQ (ORCPT ); Thu, 7 Jan 2021 06:19:16 -0500 Received: from mx2.suse.de ([195.135.220.15]:50232 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726326AbhAGLTP (ORCPT ); Thu, 7 Jan 2021 06:19:15 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1610018308; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=RQpGpOyPrE4bOtyYyPbs1TfHW7kVTnWw1fBHDIw2fno=; b=PYrkANYpWF+ZDJpX+msYjR159brUpvCPsnc5tplfYlWtO2QGfSMmXxjU+E0dIVwZrhKwOo pfxPlm973ZJSrdwtvKvw2LcE4/HWBd0X/j13V4kEbHldGjhWzOweWXObbWm6NPQY+ll532 VmK0CAc+Q0iWvlU0qJTFUp3sOcm2HXU= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 34BEFAD12; Thu, 7 Jan 2021 11:18:28 +0000 (UTC) Date: Thu, 7 Jan 2021 12:18:27 +0100 From: Michal Hocko To: Muchun Song Cc: Mike Kravetz , Andrew Morton , Naoya Horiguchi , Andi Kleen , Linux Memory Management List , LKML Subject: Re: [External] Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page Message-ID: <20210107111827.GG13207@dhcp22.suse.cz> References: <20210106084739.63318-1-songmuchun@bytedance.com> <20210106084739.63318-4-songmuchun@bytedance.com> <20210106165632.GT13207@dhcp22.suse.cz> <20210107084146.GD13207@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 07-01-21 16:53:13, Muchun Song wrote: > On Thu, Jan 7, 2021 at 4:41 PM Michal Hocko wrote: > > > > On Thu 07-01-21 13:39:38, Muchun Song wrote: > > > On Thu, Jan 7, 2021 at 12:56 AM Michal Hocko wrote: > > > > > > > > On Wed 06-01-21 16:47:36, Muchun Song wrote: > > > > > There is a race condition between __free_huge_page() > > > > > and dissolve_free_huge_page(). > > > > > > > > > > CPU0: CPU1: > > > > > > > > > > // page_count(page) == 1 > > > > > put_page(page) > > > > > __free_huge_page(page) > > > > > dissolve_free_huge_page(page) > > > > > spin_lock(&hugetlb_lock) > > > > > // PageHuge(page) && !page_count(page) > > > > > update_and_free_page(page) > > > > > // page is freed to the buddy > > > > > spin_unlock(&hugetlb_lock) > > > > > spin_lock(&hugetlb_lock) > > > > > clear_page_huge_active(page) > > > > > enqueue_huge_page(page) > > > > > // It is wrong, the page is already freed > > > > > spin_unlock(&hugetlb_lock) > > > > > > > > > > The race windows is between put_page() and spin_lock() which > > > > > is in the __free_huge_page(). > > > > > > > > The race window reall is between put_page and dissolve_free_huge_page. > > > > And the result is that the put_page path would clobber an unrelated page > > > > (either free or already reused page) which is quite serious. > > > > Fortunatelly pages are dissolved very rarely. I believe that user would > > > > require to be privileged to hit this by intention. > > > > > > > > > We should make sure that the page is already on the free list > > > > > when it is dissolved. > > > > > > > > Another option would be to check for PageHuge in __free_huge_page. Have > > > > you considered that rather than add yet another state? The scope of the > > > > spinlock would have to be extended. If that sounds more tricky then can > > > > we check the page->lru in the dissolve path? If the page is still > > > > PageHuge and reference count 0 then there shouldn't be many options > > > > where it can be queued, right? > > > > > > Did you mean that we iterate over the free list to check whether > > > the page is on the free list? > > > > No I meant to check that the page is enqueued which along with ref count > > = 0 should mean it has been released to the pool unless I am missing > > something. > > The page can be on the free list or active list or empty when it > is freed to the pool. How to check whether it is on the free list? As I've said, I might be missing something here. But if the page is freed why does it matter whether it is on a active list or free list from the dissolve operation POV? -- Michal Hocko SUSE Labs