Commit 11feeb49 authored Jul 25, 2013 by Andrea Arcangeli Committed by Gleb Natapov Aug 27, 2013

kvm: optimize away THP checks in kvm_is_mmio_pfn()



The checks on PG_reserved in the page structure on head and tail pages
aren't necessary because split_huge_page wouldn't transfer the
PG_reserved bit from head to tail anyway.

This was a forward-thinking check done in the case PageReserved was
set by a driver-owned page mapped in userland with something like
remap_pfn_range in a VM_PFNMAP region, but using hugepmds (not
possible right now). It was meant to be very safe, but it's overkill
as it's unlikely split_huge_page could ever run without the driver
noticing and tearing down the hugepage itself.

And if a driver in the future will really want to map a reserved
hugepage in userland using an huge pmd it should simply take care of
marking all subpages reserved too to keep KVM safe. This of course
would require such a hypothetical driver to tear down the huge pmd
itself and splitting the hugepage itself, instead of relaying on
split_huge_page, but that sounds very reasonable, especially
considering split_huge_page wouldn't currently transfer the reserved
bit anyway.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

parent 0bd50dc9

virt/kvm/kvm_main.c

+2 −22

Original line number	Diff line number	Diff line
		@@ -102,28 +102,8 @@ static bool largepages_enabled = true;

		bool kvm_is_mmio_pfn(pfn_t pfn)
		{
		if (pfn_valid(pfn)) {
		int reserved;
		struct page *tail = pfn_to_page(pfn);
		struct page *head = compound_trans_head(tail);
		reserved = PageReserved(head);
		if (head != tail) {
		/*
		* "head" is not a dangling pointer
		* (compound_trans_head takes care of that)
		* but the hugepage may have been splitted
		* from under us (and we may not hold a
		* reference count on the head page so it can
		* be reused before we run PageReferenced), so
		* we've to check PageTail before returning
		* what we just read.
		*/
		smp_rmb();
		if (PageTail(tail))
		return reserved;
		}
		return PageReserved(tail);
		}
		if (pfn_valid(pfn))
		return PageReserved(pfn_to_page(pfn));

		return true;
		}