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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 5EEE7C433E0 for ; Tue, 23 Feb 2021 15:42:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2CD3760295 for ; Tue, 23 Feb 2021 15:42:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233279AbhBWPmU (ORCPT ); Tue, 23 Feb 2021 10:42:20 -0500 Received: from mx2.suse.de ([195.135.220.15]:35408 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233101AbhBWPmQ (ORCPT ); Tue, 23 Feb 2021 10:42:16 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A7C06AF5A; Tue, 23 Feb 2021 15:41:32 +0000 (UTC) Date: Tue, 23 Feb 2021 16:41:28 +0100 From: Oscar Salvador To: Muchun Song Cc: Mike Kravetz , Jonathan Corbet , Thomas Gleixner , mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org, Peter Zijlstra , viro@zeniv.linux.org.uk, Andrew Morton , paulmck@kernel.org, mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com, Randy Dunlap , oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de, Mina Almasry , David Rientjes , Matthew Wilcox , Michal Hocko , "Song Bao Hua (Barry Song)" , David Hildenbrand , HORIGUCHI =?utf-8?B?TkFPWUEo5aCA5Y+jIOebtOS5nyk=?= , Joao Martins , Xiongchun duan , linux-doc@vger.kernel.org, LKML , Linux Memory Management List , linux-fsdevel Subject: Re: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page Message-ID: <20210223154128.GA21082@localhost.localdomain> References: <20210219104954.67390-1-songmuchun@bytedance.com> <20210219104954.67390-5-songmuchun@bytedance.com> <13a5363c-6af4-1e1f-9a18-972ca18278b5@oracle.com> <20210223092740.GA1998@linux> <20210223104957.GA3844@linux> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210223104957.GA3844@linux> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 23, 2021 at 11:50:05AM +0100, Oscar Salvador wrote: > > CPU0: CPU1: > > set_compound_page_dtor(HUGETLB_PAGE_DTOR); > > memory_failure_hugetlb > > get_hwpoison_page > > __get_hwpoison_page > > get_page_unless_zero > > put_page_testzero() > > > > Maybe this can happen. But it is a very corner case. If we want to > > deal with this. We can put_page_testzero() first and then > > set_compound_page_dtor(HUGETLB_PAGE_DTOR). > > I have to check further, but it looks like this could actually happen. > Handling this with VM_BUG_ON is wrong, because memory_failure/soft_offline are > entitled to increase the refcount of the page. > > AFAICS, > > CPU0: CPU1: > set_compound_page_dtor(HUGETLB_PAGE_DTOR); > memory_failure_hugetlb > get_hwpoison_page > __get_hwpoison_page > get_page_unless_zero > put_page_testzero() > identify_page_state > me_huge_page > > I think we can reach me_huge_page with either refcount = 1 or refcount =2, > depending whether put_page_testzero has been issued. > > For now, I would not re-enqueue the page if put_page_testzero == false. > I have to see how this can be handled gracefully. I took a brief look. It is not really your patch fault. Hugetlb <-> memory-failure synchronization is a bit odd, it definitely needs improvment. The thing is, we can have different scenarios here. E.g: by the time we return from put_page_testzero, we might have refcount == 0 and PageHWPoison, or refcount == 1 PageHWPoison. The former will let a user get a page from the pool and get a sigbus when it faults in the page, and the latter will be even more odd as we will have a self-refcounted page in the free pool (and hwpoisoned). As I said, it is not this patchset fault. I just made me realize this problem. I have to think some more about this. -- Oscar Salvador SUSE L3