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kvm: optimize away THP checks in kvm_is_mmio_pfn()

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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>
This commit is contained in:
Andrea Arcangeli 2013-07-25 03:04:38 +02:00 committed by Gleb Natapov
parent 0bd50dc971
commit 11feeb4980
1 changed files with 2 additions and 22 deletions
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24
virt/kvm/kvm_main.c
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@ -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;
}