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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 785A3C35254 for ; Mon, 17 Feb 2020 09:31:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5662C20679 for ; Mon, 17 Feb 2020 09:31:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728911AbgBQJbe (ORCPT ); Mon, 17 Feb 2020 04:31:34 -0500 Received: from mx2.suse.de ([195.135.220.15]:49106 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728773AbgBQJbd (ORCPT ); Mon, 17 Feb 2020 04:31:33 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id CD095C05B; Mon, 17 Feb 2020 09:31:31 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id C981E1E0D47; Mon, 17 Feb 2020 10:31:28 +0100 (CET) Date: Mon, 17 Feb 2020 10:31:28 +0100 From: Jan Kara To: Minchan Kim Cc: Andrew Morton , linux-mm , LKML , Jan Kara , Matthew Wilcox , Josef Bacik , Johannes Weiner , Robert Stupp Subject: Re: [PATCH 3/3] mm: make PageReadahead more strict Message-ID: <20200217093128.GB12032@quack2.suse.cz> References: <20200212221614.215302-1-minchan@kernel.org> <20200212221614.215302-3-minchan@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200212221614.215302-3-minchan@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 12-02-20 14:16:14, Minchan Kim wrote: > PG_readahead flag is shared with PG_reclaim but PG_reclaim is only > used in write context while PG_readahead is used for read context. > > To make it clear, let's introduce PageReadahead wrapper with > !PageWriteback so it could make code clear and we could drop > PageWriteback check in page_cache_async_readahead, which removes > pointless dropping mmap_sem. > > Signed-off-by: Minchan Kim ... > +/* Clear PG_readahead only if it's PG_readahead, not PG_reclaim */ > +static inline int TestClearPageReadahead(struct page *page) > +{ > + VM_BUG_ON_PGFLAGS(PageCompound(page), page); > + > + return !PageWriteback(page) || > + test_and_clear_bit(PG_reclaim, &page->flags); > +} I think this is still wrong - if PageWriteback is not set, it will never clear PG_reclaim bit so effectively the page will stay in PageReadahead state! The logic you really want to implement is: if (PageReadahead(page)) { <- this is your new PageReadahead implementation clear_bit(PG_reclaim, &page->flags); return 1; } return 0; Now this has the problem that it is not atomic. The only way I see to make this fully atomic is using cmpxchg(). If we wanted to make this kinda-sorta OK, the proper condition would look like: return !PageWriteback(page) **&&** test_and_clear_bit(PG_reclaim, &page->flags); Which is similar to what you originally had but different because in C '&&' operator is not commutative due to side-effects committed at sequence points. BTW: I share Andrew's view that we are piling hacks to fix problems caused by older hacks. But I don't see any good option how to unalias PG_readahead and PG_reclaim. Honza -- Jan Kara SUSE Labs, CR