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>

1 files changed, 2 insertions, 22 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d2836788561e..0fc25aed79a8 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;
 }