Re: [PATCH 4/7] kvm mmu: implement necessary data structures for second huge page accounting

Avi Kivity
2009-03-29T13:37:57Z

Avi Kivity wrote:
> Joerg Roedel wrote:
>> This patch adds the necessary data structures to take care of write
>> protections in place within a second huge page sized page.
>>
>>
>> +#ifdef KVM_PAGES_PER_LHPAGE
>> + if (npages && !new.hpage_info) {
>> + int hugepages = npages / KVM_PAGES_PER_LHPAGE;
>> + if (npages % KVM_PAGES_PER_LHPAGE)
>> + hugepages++;
>> + if (base_gfn % KVM_PAGES_PER_LHPAGE)
>> + hugepages++;
>>
>
> Consider a slot with base_gfn == 1 and npages == 1. This will have
> hugepages == 2, which is wrong.
>
> I think the right calculation is
>
> ((base_gfn + npages - 1) / N) - (base_gfn / N) + 1
>
> i.e. index of last page, plus one so we can store it.
>
> The small huge page calculation is off as well.
>

I fixed the existing case with

commit 1a967084dbe97a2f4be84139d14e2d958d7ffc46
Author: Avi Kivity <avi@redhat.com>
Date: Sun Mar 29 16:31:25 2009 +0300

KVM: MMU: Fix off-by-one calculating large page count

The large page initialization code concludes there are two large
pages spanned
by a slot covering 1 (small) page starting at gfn 1. This is
incorrect, and
also results in incorrect write_count initialization in some cases
(base = 1,
npages = 513 for example).

Cc: stable@kernel.org
Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 8aa3b95..3d31557 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1076,6 +1076,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
int r;
gfn_t base_gfn;
unsigned long npages;
+ int largepages;
unsigned long i;
struct kvm_memory_slot *memslot;
struct kvm_memory_slot old, new;
@@ -1151,11 +1152,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
new.userspace_addr = 0;
}
if (npages && !new.lpage_info) {
- int largepages = npages / KVM_PAGES_PER_HPAGE;
- if (npages % KVM_PAGES_PER_HPAGE)
- largepages++;
- if (base_gfn % KVM_PAGES_PER_HPAGE)
- largepages++;
+ largepages = 1 + (base_gfn + npages - 1) /
KVM_PAGES_PER_HPAGE;
+ largepages -= base_gfn / npages;

new.lpage_info = vmalloc(largepages *
sizeof(*new.lpage_info));

--
error compiling committee.c: too many arguments to function