From mboxrd@z Thu Jan 1 00:00:00 1970 From: Martin Schwidefsky Subject: Re: [patch 3/9] Guest page hinting: volatile page cache. Date: Fri, 01 Sep 2006 18:25:43 +0200 Message-ID: <1157127943.21733.52.camel@localhost> References: <20060901110948.GD15684@skybase> <1157122667.28577.69.camel@localhost.localdomain> <1157124674.21733.13.camel@localhost> <44F8563B.3050505@shadowen.org> <1157126640.21733.43.camel@localhost> <1157127483.28577.117.camel@localhost.localdomain> Reply-To: schwidefsky@de.ibm.com Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1157127483.28577.117.camel@localhost.localdomain> Sender: linux-kernel-owner@vger.kernel.org To: Dave Hansen Cc: Andy Whitcroft , linux-kernel@vger.kernel.org, virtualization@lists.osdl.org, akpm@osdl.org, nickpiggin@yahoo.com.au, frankeh@watson.ibm.com List-Id: virtualization@lists.linuxfoundation.org On Fri, 2006-09-01 at 09:18 -0700, Dave Hansen wrote: > > 1) The page-is-discarded (PG_discarded) bit is set for pages that have > > been recognized as removed by the host. The page needs to be removed > > from the page cache while there are still page references floating > > around. To prevent multiple removals from the page cache the discarded > > bit is needed. > > OK, so the page has data in it, and is in the page cache. The > hypervisor kills the page, gives the notification to the kernel that the > page has gone away, and the kernel marks PG_discarded. There still > might be active references to the page. No, the hypervisor does not give the notification immediatly. A discard fault is delivered to the guest if it tries to access a page that has been removed by the host. That is the fundamental difference between a memory ballooner and the guest page hinting. > So, is the problem trying to communicate with the reference holders that > the page is no longer valid? How is this fundamentally different from > page truncating? Truncating is similar but the reaction is different. A truncated page is gone and will not be recreated. A discarded page can be reloaded. > > 2) The page-state-change (PG_state_change) bit is required to prevent > > that an make_stable "overtakes" a make_volatile. In order to make a page > > volatile a number of conditions are check. After this is done the state > > change will be done. The critical section is the code that performs the > > checks up to the instruction that does the state change. No make_stable > > may be done in between. The granularity is per page, to use a global > > lock like a spinlock would severly limit the scalability for large smp > > systems. > > How about doing it in the NUMA node? Or the mem_section? Or, even a > bit in the mem_map[] for the area guarding the 'struct page' itself? > Even a hashed table of locks based on the page address. You just need > something that allows _some_ level of concurrency. You certainly never > have a number of CPUs which is anywhere close to the number of 'struct > page's in the system. NUMA node is not granular enough, mem_section is probably doable. I do not understand the part about the bit in the mem_map[] area, a bit in the page->flags is exactly that, isn't it? -- blue skies, Martin. Martin Schwidefsky Linux for zSeries Development & Services IBM Deutschland Entwicklung GmbH "Reality continues to ruin my life." - Calvin.