Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756458Ab3H3Ouc (ORCPT ); Fri, 30 Aug 2013 10:50:32 -0400 Received: from relay.parallels.com ([195.214.232.42]:59217 "EHLO relay.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756354Ab3H3Oub (ORCPT ); Fri, 30 Aug 2013 10:50:31 -0400 Message-ID: <5220B12A.1060008@parallels.com> Date: Fri, 30 Aug 2013 18:50:18 +0400 From: Maxim Patlasov User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130801 Thunderbird/17.0.8 MIME-Version: 1.0 To: Miklos Szeredi CC: , Kirill Korotaev , Pavel Emelianov , fuse-devel , Brian Foster , Kernel Mailing List , James Bottomley , , Al Viro , Linux-Fsdevel , Andrew Morton , , , Mel Gorman Subject: Re: [PATCH 10/16] fuse: Implement writepages callback 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> <20130830101207.GD19636@tucsk.piliscsaba.szeredi.hu> In-Reply-To: <20130830101207.GD19636@tucsk.piliscsaba.szeredi.hu> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.30.17.2] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3956 Lines: 99 Hi Miklos, 08/30/2013 02:12 PM, Miklos Szeredi пишет: > 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. I thought we're protected from the deadlock by the following chunk (in the very beginning of fuse_writepage): > + if (fuse_page_is_writeback(inode, page->index)) { > + if (wbc->sync_mode != WB_SYNC_ALL) { > + redirty_page_for_writepage(wbc, page); > + return 0; > + } > + fuse_wait_on_page_writeback(inode, page->index); > + } Because reclaimer will never call us with WB_SYNC_ALL. Did I miss something? > > 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. Is it exactly how NFS solves similar problem? > > 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... Yes, I agree. I think there are pretty many reasons not to keep original page under writeback for long. And not to make it dependant on userspace process as well. > > The other offender, balance_dirty_pages() is much easier and needs to be tought > about fuse behavior anyway. BTW, strictlimit feature (including its enable for fuse) is already in linux-next. Thanks, Maxim -- 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/