Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:11849 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751210Ab0HTL2H (ORCPT ); Fri, 20 Aug 2010 07:28:07 -0400 Date: Fri, 20 Aug 2010 07:27:57 -0400 From: Jeff Layton To: Christoph Hellwig Cc: Wu Fengguang , linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Chris Mason , Jens Axboe Subject: Re: why are WB_SYNC_NONE COMMITs being done with FLUSH_SYNC set ? Message-ID: <20100820072757.6ae9741a@tlielax.poochiereds.net> In-Reply-To: <20100820091904.GB20138@infradead.org> References: <20100819101525.076831ad@barsoom.rdu.redhat.com> <20100819143710.GA4752@infradead.org> <20100819235553.GB22747@localhost> <20100820091904.GB20138@infradead.org> Content-Type: text/plain; charset=US-ASCII Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Fri, 20 Aug 2010 05:19:04 -0400 Christoph Hellwig wrote: > On Fri, Aug 20, 2010 at 07:55:53AM +0800, Wu Fengguang wrote: > > Since migration and pageout still set nonblocking for ->writepage, we > > may keep them in the near future, until VM does not start IO on itself. > > Why does pageout() and memory migration need to be even more > non-blocking than the already non-blockig WB_SYNC_NONE writeout? > Just an idle thought on this... I think a lot of the confusion here comes from the fact that we have sync_mode and a bunch of flags, and it's not at all clear how filesystems are supposed to treat the union of them. There are also possible unions of flags/sync_modes that never happen in practice. It's not always obvious though and as filesystem implementors we have to consider the possibility that they might occur (consider WB_SYNC_ALL + for_background). Perhaps a lot of this confusion could be lifted by getting rid of the extra flags and adding new sync_mode's. Maybe something like: WB_SYNC_ALL /* wait on everything to complete */ WB_SYNC_NONE /* don't wait on anything */ WB_SYNC_FOR_RECLAIM /* sync for reclaim */ WB_SYNC_FOR_KUPDATED /* sync by kupdate */ ...etc... That does mean that all of the filesystem specific code may need to be touched when new modes are added and removed. I think it would be clearer though about what you're supposed to do in ->writepages. -- Jeff Layton