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[195.135.220.15]) by mx.google.com with ESMTPS id l26si544117ejs.214.2019.03.11.07.45.28 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Mar 2019 07:45:28 -0700 (PDT) Received-SPF: pass (google.com: domain of jack@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jack@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=jack@suse.cz X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id E1BFAAFBB; Mon, 11 Mar 2019 14:45:27 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 594E51E426A; Mon, 11 Mar 2019 15:45:27 +0100 (CET) Date: Mon, 11 Mar 2019 15:45:27 +0100 From: Jan Kara To: Andrea Arcangeli Cc: Jerome Glisse , "Michael S. Tsirkin" , Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, peterx@redhat.com, linux-mm@kvack.org, Jan Kara Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Message-ID: <20190311144527.GM11553@quack2.suse.cz> References: <1551856692-3384-1-git-send-email-jasowang@redhat.com> <1551856692-3384-6-git-send-email-jasowang@redhat.com> <20190306092837-mutt-send-email-mst@kernel.org> <15105894-4ec1-1ed0-1976-7b68ed9eeeda@redhat.com> <20190307101708-mutt-send-email-mst@kernel.org> <20190307190910.GE3835@redhat.com> <20190307193838.GQ23850@redhat.com> <20190307201722.GG3835@redhat.com> <20190307212717.GS23850@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190307212717.GS23850@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu 07-03-19 16:27:17, Andrea Arcangeli wrote: > > driver that GUP page for hours/days/weeks/months ... obviously the > > race window is big enough here. It affects many fs (ext4, xfs, ...) > > in different ways. I think ext4 is the most obvious because of the > > kernel log trace it leaves behind. > > > > Bottom line is for set_page_dirty to be safe you need the following: > > lock_page() > > page_mkwrite() > > set_pte_with_write() > > unlock_page() > > I also wondered why ext4 writepage doesn't recreate the bh if they got > dropped by the VM and page->private is 0. I mean, page->index and > page->mapping are still there, that's enough info for writepage itself > to take a slow path and calls page_mkwrite to find where to write the > page on disk. There are two problems: 1) What to do with errors that page_mkwrite() can generate (ENOMEM, ENOSPC, EIO). On page fault you just propagate them to userspace, on set_page_dirty() you have no chance so you just silently loose data. 2) We need various locks to protect page_mkwrite(), possibly do some IO. set_page_dirty() is rather uncertain context to acquire locks or do IO... Honza -- Jan Kara SUSE Labs, CR