Messages in this thread | | | Subject | Re: [patch 3/9] Guest page hinting: volatile page cache. | From | Dave Hansen <> | Date | Fri, 01 Sep 2006 09:37:14 -0700 |
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On Fri, 2006-09-01 at 18:25 +0200, Martin Schwidefsky wrote: > 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 the page is no longer valid?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.
Can you give me the sequence of events that occur so that we need to set, then check PG_discarded? I'm not getting it.
1. there is good data in a page ... 50. ... and PG_discarded gets set ... 99. We check PG_discarded and ...
> > > 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?
No, I'm being tricky. There are struct pages for all memory, including kernel memory. mem_map[] is in kernel memory. So, the memory for the mem_map[] has struct pages, which themselves are in the mem_map[].
void lock_page_for_state_change(struct page *page) { struct pages_backing_page = virt_to_page(page); lock_page(pages_backing_page) }
We did this for a bit with sparsemem, I think. That is, until Andy came up with something even more clever.
-- Dave
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