Messages in this thread | | | Date | Fri, 1 Feb 2008 11:23:57 -0800 (PST) | From | Christoph Lameter <> | Subject | Re: [PATCH] mmu notifiers #v5 |
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On Fri, 1 Feb 2008, Andrea Arcangeli wrote:
> Note that my #v5 doesn't require to increase the page count all the > time, so GRU will work fine with #v5.
But that comes with the cost of firing invalidate_page for every page being evicted. In order to make your single invalidate_range work without it you need to hold a refcount on the page.
> invalidate_page[s] is always called before the page is freed. This > will require modifications to the tlb flushing code logic to take > advantage of _pages in certain places. For now it's just safe.
Yes so your invalidate_range is still some sort of dysfunctional optimization? Gazillions of invalidate_page's will have to be executed when tearing down large memory areas.
> > How does KVM insure the consistency of the shadow page tables? Atomic ops? > > A per-VM mmu_lock spinlock is taken to serialize the access, plus > atomic ops for the cpu.
And that would not be enough to hold of new references? With small tweaks this should work with a common scheme. We could also redefine the role of _start and _end slightly to just require that the refs are removed when _end completes. That would allow the KVM page count ref to work as is now and would avoid the individual invalidate_page() callouts.
> > The GRU has no page table on its own. It populates TLB entries on demand > > using the linux page table. There is no way it can figure out when to > > drop page counts again. The invalidate calls are turned directly into tlb > > flushes. > > Yes, this is why it can't serialize follow_page with only the PT lock > with your patch. KVM may do it once you add start,end to range_end > only thanks to the additional pin on the page.
Right but that pin requires taking a refcount which we cannot do.
Frankly this looks as if this is a solution that would work only for KVM.
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