From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6039DC4741F for ; Thu, 5 Nov 2020 14:03:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 19E7820759 for ; Thu, 5 Nov 2020 14:03:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730938AbgKEODe (ORCPT ); Thu, 5 Nov 2020 09:03:34 -0500 Received: from mx2.suse.de ([195.135.220.15]:44876 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730822AbgKEODe (ORCPT ); Thu, 5 Nov 2020 09:03:34 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 6DE01AAF1; Thu, 5 Nov 2020 14:03:32 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 0C66A1E130F; Thu, 5 Nov 2020 15:03:32 +0100 (CET) Date: Thu, 5 Nov 2020 15:03:32 +0100 From: Jan Kara To: Chengguang Xu Cc: Jan Kara , miklos , amir73il , linux-unionfs , linux-fsdevel , charliecgxu Subject: Re: [RFC PATCH v2 5/8] ovl: mark overlayfs' inode dirty on shared writable mmap Message-ID: <20201105140332.GG32718@quack2.suse.cz> References: <20201025034117.4918-1-cgxu519@mykernel.net> <20201025034117.4918-6-cgxu519@mykernel.net> <20201102173052.GF23988@quack2.suse.cz> <175931b5387.1349cecf47061.3904278910555065520@mykernel.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <175931b5387.1349cecf47061.3904278910555065520@mykernel.net> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed 04-11-20 19:54:03, Chengguang Xu wrote: > ---- 在 星期二, 2020-11-03 01:30:52 Jan Kara 撰写 ---- > > On Sun 25-10-20 11:41:14, Chengguang Xu wrote: > > > Overlayfs cannot be notified when mmapped area gets dirty, > > > so we need to proactively mark inode dirty in ->mmap operation. > > > > > > Signed-off-by: Chengguang Xu > > > --- > > > fs/overlayfs/file.c | 4 ++++ > > > 1 file changed, 4 insertions(+) > > > > > > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c > > > index efccb7c1f9bc..cd6fcdfd81a9 100644 > > > --- a/fs/overlayfs/file.c > > > +++ b/fs/overlayfs/file.c > > > @@ -486,6 +486,10 @@ static int ovl_mmap(struct file *file, struct vm_area_struct *vma) > > > /* Drop reference count from new vm_file value */ > > > fput(realfile); > > > } else { > > > + if (vma->vm_flags & (VM_SHARED|VM_MAYSHARE) && > > > + vma->vm_flags & (VM_WRITE|VM_MAYWRITE)) > > > + ovl_mark_inode_dirty(file_inode(file)); > > > + > > > > But does this work reliably? I mean once writeback runs, your inode (as > > well as upper inode) is cleaned. Then a page fault comes so file has dirty > > pages again and would need flushing but overlayfs inode stays clean? Am I > > missing something? > > > > Yeah, this is key point of this approach, in order to fix the issue I > explicitly set I_DIRTY_SYNC flag in ovl_mark_inode_dirty(), so what i > mean is during writeback we will call into ->write_inode() by this > flag(I_DIRTY_SYNC) and at that place we get chance to check mapping and > re-dirty overlay's inode. The code logic like below in ovl_write_inode(). > > if (mapping_writably_mapped(upper->i_mapping) || > mapping_tagged(upper->i_mapping, PAGECACHE_TAG_WRITEBACK)) > iflag |= I_DIRTY_PAGES; OK, but suppose the upper mapping is clean at this moment (upper inode has been fully written out for whatever reason, but it is still mapped) so your overlayfs inode becomes clean as well. Then I don't see a mechanism which would make your overlayfs inode dirty again when a write to mmap happens, set_page_dirty() will end up marking upper inode with I_DIRTY_PAGES flag. Note that ovl_mmap() gets called only at mmap(2) syscall time but then pages get faulted in, dirtied, cleaned fully at discretion of the mm / writeback subsystem. Honza -- Jan Kara SUSE Labs, CR