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author | Andrea Arcangeli <aarcange@redhat.com> | 2013-07-25 03:04:38 +0200 |
---|---|---|
committer | Gleb Natapov <gleb@redhat.com> | 2013-08-27 11:01:10 +0300 |
commit | 11feeb498086a3a5907b8148bdf1786a9b18fc55 (patch) | |
tree | 9dcef9a577fd410f84f42b972e2fd4e1ff46f68c | |
parent | 0bd50dc971aad3c29043de4fb7bce45c351d1b67 (diff) | |
download | linux-11feeb498086a3a5907b8148bdf1786a9b18fc55.tar.bz2 |
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>
-rw-r--r-- | virt/kvm/kvm_main.c | 24 |
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; } |