From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751851Ab0KQXbm (ORCPT ); Wed, 17 Nov 2010 18:31:42 -0500 Received: from smtp-out.google.com ([74.125.121.35]:59925 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751238Ab0KQXbl convert rfc822-to-8bit (ORCPT ); Wed, 17 Nov 2010 18:31:41 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=e65S8XIaqFAHF9CmO/t472G5lIBjuFQRPamsP86EtBbgysaveht/kovj/h8lZ/0onQ JL0N6XgD89BhurtHrqcQ== MIME-Version: 1.0 In-Reply-To: <20101117231143.GQ22876@dastard> References: <1289996638-21439-1-git-send-email-walken@google.com> <1289996638-21439-4-git-send-email-walken@google.com> <20101117125756.GA5576@amd> <1290007734.2109.941.camel@laptop> <20101117231143.GQ22876@dastard> Date: Wed, 17 Nov 2010 15:31:37 -0800 Message-ID: Subject: Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback From: Michel Lespinasse To: Dave Chinner Cc: Peter Zijlstra , Nick Piggin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Hugh Dickins , Rik van Riel , Kosaki Motohiro , Theodore Tso , Michael Rubin , Suleiman Souhlal Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 17, 2010 at 3:11 PM, Dave Chinner wrote: >> Really, my understanding is that not pre-allocating filesystem blocks >> is just fine. This is, after all, what happens with ext3 and it's >> never been reported as a bug (that I know of). > > It's not ext3 you have to worry about - it's the filesystems that > need special state set up on their pages/buffers for ->writepage to > work correctly that are the problem. You need to call > ->write_begin/->write_end to get the state set up properly. > > If this state is not set up properly, silent data loss will occur > during mmap writes either by ENOSPC or failing to set up writes into > unwritten extents correctly (i.e. we'll be back to where we were in > 2.6.15). > > I don't think ->page_mkwrite can be worked around - we need that to > be called on the first write fault of any mmap()d page to ensure it > is set up correctly for writeback.  If we don't get write faults > after the page is mlock()d, then we need the ->page_mkwrite() call > during the mlock() call. Just to be clear - I'm proposing to skip the entire do_wp_page() call by doing a read fault rather than a write fault. If the page wasn't dirty already, it will stay clean and with a non-writable PTE until it gets actually written to, at which point we'll get a write fault and do_wp_page will be invoked as usual. I am not proposing to skip the page_mkwrite() while upgrading the PTE permissions, which I think is what you were arguing against ? -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies.