Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755455Ab3H3KML (ORCPT ); Fri, 30 Aug 2013 06:12:11 -0400 Received: from mail-we0-f177.google.com ([74.125.82.177]:58758 "EHLO mail-we0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754238Ab3H3KMI (ORCPT ); Fri, 30 Aug 2013 06:12:08 -0400 Date: Fri, 30 Aug 2013 12:12:07 +0200 From: Miklos Szeredi To: Maxim Patlasov Cc: riel@redhat.com, Kirill Korotaev , Pavel Emelianov , fuse-devel , Brian Foster , Kernel Mailing List , James Bottomley , linux-mm@kvack.org, Al Viro , Linux-Fsdevel , Andrew Morton , fengguang.wu@intel.com, devel@openvz.org, Mel Gorman Subject: Re: [PATCH 10/16] fuse: Implement writepages callback Message-ID: <20130830101207.GD19636@tucsk.piliscsaba.szeredi.hu> References: <20130629172211.20175.70154.stgit@maximpc.sw.ru> <20130629174525.20175.18987.stgit@maximpc.sw.ru> <20130719165037.GA18358@tucsk.piliscsaba.szeredi.hu> <51FBD2DF.50506@parallels.com> <52050474.8040608@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <52050474.8040608@parallels.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3116 Lines: 76 On Fri, Aug 09, 2013 at 07:02:12PM +0400, Maxim Patlasov wrote: > 08/06/2013 08:25 PM, Miklos Szeredi пишет: > >Hmm. Direct IO on an mmaped file will do get_user_pages() which will > >do the necessary page fault magic and ->page_mkwrite() will be called. > >At least AFAICS. > > Yes, I agree. > > > > >The page cannot become dirty through a memory mapping without first > >switching the pte from read-only to read-write first. Page accounting > >logic relies on this too. The other way the page can become dirty is > >through write(2) on the fs. But we do get notified about that too. > > Yes, that's correct, but I don't understand why you disregard two > other cases of marking page dirty (both related to direct AIO read > from a file to a memory region mmap-ed to a fuse file): > > 1. dio_bio_submit() --> > bio_set_pages_dirty() --> > set_page_dirty_lock() > > 2. dio_bio_complete() --> > bio_check_pages_dirty() --> > bio_dirty_fn() --> > bio_set_pages_dirty() --> > set_page_dirty_lock() > > As soon as a page became dirty through a memory mapping (exactly as > you explained), nothing would prevent it to be written-back. And > fuse will call end_page_writeback almost immediately after copying > the real page to a temporary one. Then dio_bio_submit may re-dirty > page speculatively w/o notifying fuse. And again, since then nothing > would prevent it to be written-back once more. Hence we can end up > in more then one temporary page in fuse write-back. And similar > concern for dio_bio_complete() re-dirty. > > This make me think that we do need fuse_page_is_writeback() in > fuse_writepages_fill(). But it shouldn't be harmful because it will > no-op practically always due to waiting for fuse writeback in > ->page_mkwrite() and in course of handling write(2). The problem is: if we need it in ->writepages, we need it in ->writepage too. And that's where we can't have it because it would deadlock in reclaim. There's a way to work around this: - if the request is still in queue, just update it with the contents of the new page - if the request already in userspace, create a new reqest, but only let userspace have it once the previous request for the same page completes, so the ordering is not messed up But that's a lot of hairy code. Any other ideas? The best would be if we could get rid of the ugly temporary page requirement for fuse writeback. The big blocker has always been direct reclaim: allocation must not wait on fuse writebacks. AFAICS there's still a wait_on_page_writeback() in relation to memcg. And it interacts with page migration which also has them. Those are the really difficult ones... The other offender, balance_dirty_pages() is much easier and needs to be tought about fuse behavior anyway. Thoughts? Thanks, Miklos -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/