From: Andrew Morton Subject: Re: [PATCH 1/9] readahead: introduce PG_readahead Date: Sat, 19 May 2007 08:25:04 -0700 Message-ID: <20070519082504.8baa8d19.akpm@linux-foundation.org> References: <20070516224752.500812933@mail.ustc.edu.cn> <379355695.83536@ustc.edu.cn> <20070518232824.9917d794.akpm@linux-foundation.org> <379577826.15160@ustc.edu.cn> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, Andi Kleen , Jens Axboe , Oleg Nesterov , Steven Pratt , Ram Pai , "linux-ext4@vger.kernel.org" To: Fengguang Wu Return-path: Received: from smtp2.linux-foundation.org ([207.189.120.14]:57645 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759275AbXESPaW (ORCPT ); Sat, 19 May 2007 11:30:22 -0400 In-Reply-To: <379577826.15160@ustc.edu.cn> Sender: linux-ext4-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Sat, 19 May 2007 20:30:31 +0800 Fengguang Wu wrote: > On Fri, May 18, 2007 at 11:28:24PM -0700, Andrew Morton wrote: > > On Thu, 17 May 2007 06:47:53 +0800 Fengguang Wu wrote: > > > > > Introduce a new page flag: PG_readahead. > > > > Is there any way in which we can avoid adding a new page flag? > > > > We have the advantage that if the kernel very occasionally gets the wrong > > result for PageReadahead(page), nothing particularly bad will happen, so we > > can do racy things. > > > > >From a quick peek, it appears that PG_readahead is only ever set against > > non-uptodate pages. If true we could perhaps exploit that: say, > > PageReadahead(page) == PG_referenced && !PG_uptodate? > > PG_uptodate will flip to 1 before the reader touches the page :( OK. > However, it may be possible to share the same bit with PG_reclaim or PG_booked. > Which one would be preferred? I'd like PG_booked to go away too - I don't think we've put that under the microscope yet. If it remains then its scope will be "defined by the filesystem", so readahead shouldn't use it. PG_reclaim belongs to core VFS so that's much better. Let's not do anything ugly, slow or silly in there, but please do take a look, see if there is an opportunity here.