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 the page is no longer valid?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.
