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authorAndrea Arcangeli <aarcange@redhat.com>2013-07-25 03:04:38 +0200
committerGleb Natapov <gleb@redhat.com>2013-08-27 11:01:10 +0300
commit11feeb498086a3a5907b8148bdf1786a9b18fc55 (patch)
tree9dcef9a577fd410f84f42b972e2fd4e1ff46f68c /virt
parent0bd50dc971aad3c29043de4fb7bce45c351d1b67 (diff)
downloadlinux-11feeb498086a3a5907b8148bdf1786a9b18fc55.tar.gz
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>
Diffstat (limited to 'virt')
-rw-r--r--virt/kvm/kvm_main.c24
1 files changed, 2 insertions, 22 deletions
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d2836788561..0fc25aed79a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -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;
}