Re: [PATCH v2] vfio/type1: avoid redundant PageReserved checking

From: Ben Luo
Date: Thu Aug 29 2019 - 12:58:29 EST

在 2019/8/28 下午11:55, Alex Williamson 写道:
On Wed, 28 Aug 2019 12:28:04 +0800
Ben Luo <luoben@xxxxxxxxxxxxxxxxx> wrote:

currently, if the page is not a tail of compound page, it will be
checked twice for the same thing.

Signed-off-by: Ben Luo <luoben@xxxxxxxxxxxxxxxxx>
drivers/vfio/vfio_iommu_type1.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 054391f..d0f7346 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -291,11 +291,10 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
static bool is_invalid_reserved_pfn(unsigned long pfn)
if (pfn_valid(pfn)) {
- bool reserved;
struct page *tail = pfn_to_page(pfn);
struct page *head = compound_head(tail);
- reserved = !!(PageReserved(head));
if (head != tail) {
+ bool reserved = PageReserved(head);
* "head" is not a dangling pointer
* (compound_head takes care of that)
Thinking more about this, the code here was originally just a copy of
kvm_is_mmio_pfn() which was simplified in v3.12 with the commit below.
Should we instead do the same thing here? Thanks,

ok, and kvm_is_mmio_pfn() has also been updated since then, I will take a look at that and compose a new patch

commit 11feeb498086a3a5907b8148bdf1786a9b18fc55
Author: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Date: Thu Jul 25 03:04:38 2013 +0200

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@xxxxxxxxxx>
Signed-off-by: Gleb Natapov <gleb@xxxxxxxxxx>

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;