From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1946317AbXBIKxI (ORCPT ); Fri, 9 Feb 2007 05:53:08 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1946367AbXBIKxI (ORCPT ); Fri, 9 Feb 2007 05:53:08 -0500 Received: from smtp.osdl.org ([65.172.181.24]:43024 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1946317AbXBIKxE (ORCPT ); Fri, 9 Feb 2007 05:53:04 -0500 Date: Fri, 9 Feb 2007 02:52:49 -0800 From: Andrew Morton To: Nick Piggin Cc: Linux Filesystems , Linux Kernel , Linus Torvalds Subject: Re: [rfc][patch 0/3] a faster buffered write deadlock fix? Message-Id: <20070209025249.0a87a435.akpm@linux-foundation.org> In-Reply-To: <20070209103258.GC14398@wotan.suse.de> References: <20070208105437.26443.35653.sendpatchset@linux.site> <20070209004101.3e4a88fc.akpm@linux-foundation.org> <20070209095405.GA14398@wotan.suse.de> <20070209020954.4951256e.akpm@linux-foundation.org> <20070209103258.GC14398@wotan.suse.de> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 9 Feb 2007 11:32:58 +0100 Nick Piggin wrote: > On Fri, Feb 09, 2007 at 02:09:54AM -0800, Andrew Morton wrote: > > On Fri, 9 Feb 2007 10:54:05 +0100 Nick Piggin wrote: > > > > > > > > That's still got a deadlock, > > > > It does? > > Yes, PG_lock vs mm->mmap_sem. Where? It requires that someone hold mmap_sem for writing as well as a page lock (in an order which would require some thought). Do we ever do that? > > > and also it doesn't work if we want to lock > > > the page when performing a minor fault (which I want to fix fault vs > > > invalidate), > > > > It's hard to discuss this without a description of what you want to fix > > there, and a description of how you plan to fix it. > > http://marc.theaimsgroup.com/?l=linux-mm&m=116865911432667&w=2 mutter. Could perhaps fix that by running ClearPageUptodate in invalidate, thus forcing the pagefault code to take the page lock (which we already hold). That does mean that we'll fleetingly have a non-uptodate page in pagetables which is a bit nasty. Or, probably better, we could add a new page flag (heh) telling nopage that it needs to lock the page even if it's uptodate. > > > and also assumes nobody's ->nopage locks the page or > > > requires any resources that are held by prepare_write (something not > > > immediately clear to me with the cluster filesystems, at least). > > > > The nopage handler is filemap_nopage(). Are there any exceptions to that? > > OCFS2 and GFS2. So the rule is that ->nopage handlers against pagecache mustn't lock the page if it's already uptodate. That's OK. But it might conflict with the above invalidate proposal. Gad. ocfs2_nopage() diddles with signals. > > > But that all becomes legacy path, so do we really care? Supposing fs > > > maintainers like perform_write, then after the main ones have implementations > > > we could switch over to the slow-but-correct prepare_write legacy path. > > > Or we could leave it, or we could use Linus's slightly-less-buggy scheme... > > > by that point I expect I'd be sick of arguing about it ;) > > > > It's worth "arguing" about. This is write(). What matters more?? > > That's the legacy path that uses prepare/commit (ie. supposing that all > major filesystems did get converted to perform_write). We'll never, ever, ever update and test all filesytems. What you're calling "legacy" code will be there for all time. I haven't had time to look at the perform_write stuff yet. > Of course I would still want my correct-but-slow version in that case, > but I just wouldn't care to argue if you still wanted to keep it fast. This is write(). We just cannot go and double-copy all the memory or take mmap_sem and do a full pagetable walk in there. It just means that we haven't found a suitable solution yet.