From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 15 Jan 2008 10:16:19 -0800 (PST) From: Christoph Lameter Subject: Re: [kvm-devel] mmu notifiers In-Reply-To: <478CF609.3090304@qumranet.com> Message-ID: References: <20080109181908.GS6958@v2.random> <47860512.3040607@qumranet.com> <47891A5C.8060907@qumranet.com> <478C62F8.2070702@qumranet.com> <478CF30F.1010100@qumranet.com> <478CF609.3090304@qumranet.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Avi Kivity Cc: kvm-devel@lists.sourceforge.net, linux-mm@kvack.org, Daniel J Blueman , Andrea Arcangeli List-ID: On Tue, 15 Jan 2008, Avi Kivity wrote: > > But each guest has its own page structs. They cannot share page structs. > > Concurrent access of two independent kernel instances for synchronization > > and status maintenance to a single page struct? > > > > There's a host page struct (that the guest know nothing about and cannot > touch), and optionally a guest page struct for each guest (that the host and > the other guest know nothing about). Ok so if two linux guests want to share memory three page structs are involved: 1. Host page struct 2. Guest #1 page struct 3. Guest #2 page struct I can understand that 1 and 2 point to the same physical page. Even all three could point to the same page if the page is readonly. However, lets say that Guest #1 allocates some anonymous memory and wants to share it with Guest #2. In that case something like PFNMAP is likely going to be used? Or are you remapping the physical page so that #1 and #2 share it? In that case two page struct describe state of the same physical page and we have no effective synchronization for writeback etc. > The host page struct may disappear if the host decides to swap the page into > its backing store and free the page. The guest page structs (if any) would > remain. Page structs never disappear. The pte's may disappear and the page may be unmapped from an address space of a process but the page struct stays. Page struct can only disappear if memory hotplug is activated and memory is taken out of the system. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org