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=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 B5699C433DB for ; Thu, 21 Jan 2021 08:27:55 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 32ADE23977 for ; Thu, 21 Jan 2021 08:27:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 32ADE23977 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.de Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 7B54A6B0012; Thu, 21 Jan 2021 03:27:54 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 765836B0022; Thu, 21 Jan 2021 03:27:54 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 67B0A6B0023; Thu, 21 Jan 2021 03:27:54 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0077.hostedemail.com [216.40.44.77]) by kanga.kvack.org (Postfix) with ESMTP id 533626B0012 for ; Thu, 21 Jan 2021 03:27:54 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 16747181AEF39 for ; Thu, 21 Jan 2021 08:27:54 +0000 (UTC) X-FDA: 77729104068.30.trade41_3a0198b27561 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin30.hostedemail.com (Postfix) with ESMTP id E87AE180B3AB8 for ; Thu, 21 Jan 2021 08:27:53 +0000 (UTC) X-HE-Tag: trade41_3a0198b27561 X-Filterd-Recvd-Size: 3364 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf08.hostedemail.com (Postfix) with ESMTP for ; Thu, 21 Jan 2021 08:27:53 +0000 (UTC) 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) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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