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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 4EC6EC433E0 for ; Thu, 21 Jan 2021 08:30:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 020BC23976 for ; Thu, 21 Jan 2021 08:30:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725933AbhAUI3m (ORCPT ); Thu, 21 Jan 2021 03:29:42 -0500 Received: from mx2.suse.de ([195.135.220.15]:54702 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727529AbhAUI2e (ORCPT ); Thu, 21 Jan 2021 03:28:34 -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 2C576AAAE; Thu, 21 Jan 2021 08:27:52 +0000 (UTC) Date: Thu, 21 Jan 2021 09:27:49 +0100 From: Oscar Salvador To: Mike Kravetz Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Michal Hocko , Naoya Horiguchi , Muchun Song , David Hildenbrand , Matthew Wilcox , Andrew Morton Subject: Re: [PATCH v2 2/5] hugetlb: convert page_huge_active() HPageMigratable flag Message-ID: <20210121082749.GB9553@linux> References: <20210120013049.311822-1-mike.kravetz@oracle.com> <20210120013049.311822-3-mike.kravetz@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210120013049.311822-3-mike.kravetz@oracle.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 19, 2021 at 05:30:46PM -0800, Mike Kravetz wrote: > Use the new hugetlb page specific flag HPageMigratable to replace the > page_huge_active interfaces. By it's name, page_huge_active implied > that a huge page was on the active list. However, that is not really > what code checking the flag wanted to know. It really wanted to determine > if the huge page could be migrated. This happens when the page is actually > added the page cache and/or task page table. This is the reasoning behind > the name change. > > The VM_BUG_ON_PAGE() calls in the *_huge_active() interfaces are not > really necessary as we KNOW the page is a hugetlb page. Therefore, they > are removed. > > The routine page_huge_active checked for PageHeadHuge before testing the > active bit. This is unnecessary in the case where we hold a reference or > lock and know it is a hugetlb head page. page_huge_active is also called > without holding a reference or lock (scan_movable_pages), and can race with > code freeing the page. The extra check in page_huge_active shortened the > race window, but did not prevent the race. Offline code calling > scan_movable_pages already deals with these races, so removing the check > is acceptable. Add comment to racy code. > > Signed-off-by: Mike Kravetz Reviewed-by: Oscar Salvador > -/* > - * Test to determine whether the hugepage is "active/in-use" (i.e. being linked > - * to hstate->hugepage_activelist.) > - * > - * This function can be called for tail pages, but never returns true for them. > - */ > -bool page_huge_active(struct page *page) > -{ > - return PageHeadHuge(page) && PagePrivate(&page[1]); This made me think once again. I wonder if we could ever see a scenario where page[0] is a rightful page while page[1] is poisoned/unitialized (poison_pages()). A lot of things would have to happen between the two checks, so I do not see it possible and as you mentioned earlier, the race is already there. Just wanted to speak up my mind. -- Oscar Salvador SUSE L3