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=-5.5 required=3.0 tests=MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_PASS,USER_AGENT_MUTT 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 5AD2CC43441 for ; Wed, 21 Nov 2018 07:11:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2B05F2148E for ; Wed, 21 Nov 2018 07:11:37 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2B05F2148E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726950AbeKURox (ORCPT ); Wed, 21 Nov 2018 12:44:53 -0500 Received: from mx2.suse.de ([195.135.220.15]:58608 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725999AbeKURox (ORCPT ); Wed, 21 Nov 2018 12:44:53 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id C5845ACB4; Wed, 21 Nov 2018 07:11:33 +0000 (UTC) Date: Wed, 21 Nov 2018 08:11:32 +0100 From: Michal Hocko To: Hugh Dickins Cc: linux-mm@kvack.org, Andrew Morton , Oscar Salvador , Pavel Tatashin , David Hildenbrand , LKML , "Kirill A. Shutemov" Subject: Re: [RFC PATCH 3/3] mm, fault_around: do not take a reference to a locked page Message-ID: <20181121071132.GD12932@dhcp22.suse.cz> References: <20181120134323.13007-1-mhocko@kernel.org> <20181120134323.13007-4-mhocko@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Tue 20-11-18 17:47:21, Hugh Dickins wrote: > On Tue, 20 Nov 2018, Michal Hocko wrote: > > > From: Michal Hocko > > > > filemap_map_pages takes a speculative reference to each page in the > > range before it tries to lock that page. While this is correct it > > also can influence page migration which will bail out when seeing > > an elevated reference count. The faultaround code would bail on > > seeing a locked page so we can pro-actively check the PageLocked > > bit before page_cache_get_speculative and prevent from pointless > > reference count churn. > > > > Cc: "Kirill A. Shutemov" > > Suggested-by: Jan Kara > > Signed-off-by: Michal Hocko > > Acked-by: Hugh Dickins Thanks! > though I think this patch is more useful to the avoid atomic ops, > and unnecessary dirtying of the cacheline, than to avoid the very > transient elevation of refcount, which will not affect page migration > very much. Are you sure it would really be transient? In other words is it possible that the fault around can block migration repeatedly under refault heavy workload? I just couldn't convince myself, to be honest. -- Michal Hocko SUSE Labs