On Tue, Aug 06, 2013 at 01:05:48PM +0100, Marc Zyngier wrote:
> When using 64kB pages, we only have two levels of page tables,
> meaning that PGD, PUD and PMD are fused. In this case, trying
> to refcount PUDs and PMDs independantly is a a complete disaster,
independently
> as they are the same.
>
> We manage to get it right for the allocation (stage2_set_pte uses
> {pmd,pud}_none), but the unmapping path clears both pud and pmd
> refcounts, which fails spectacularly with 2-level page tables.
>
> The fix is to avoid calling clear_pud_entry when both the pmd and
> pud pages are empty. For this, and instead of introducing another
> pud_empty function, consolidate both pte_empty and pmd_empty into
> page_empty (the code is actually identical) and use that to also
> test the validity of the pud.
>
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
> arch/arm/kvm/mmu.c | 22 ++++++++--------------
> 1 file changed, 8 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index ca6bea4..7e1d899 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -85,6 +85,12 @@ static void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc)
> return p;
> }
>
> +static bool page_empty(void *ptr)
> +{
> + struct page *ptr_page = virt_to_page(ptr);
> + return page_count(ptr_page) == 1;
> +}
> +
> static void clear_pud_entry(struct kvm *kvm, pud_t *pud, phys_addr_t addr)
> {
> pmd_t *pmd_table = pmd_offset(pud, 0);
> @@ -103,12 +109,6 @@ static void clear_pmd_entry(struct kvm *kvm, pmd_t *pmd, phys_addr_t addr)
> put_page(virt_to_page(pmd));
> }
>
> -static bool pmd_empty(pmd_t *pmd)
> -{
> - struct page *pmd_page = virt_to_page(pmd);
> - return page_count(pmd_page) == 1;
> -}
> -
> static void clear_pte_entry(struct kvm *kvm, pte_t *pte, phys_addr_t addr)
> {
> if (pte_present(*pte)) {
> @@ -118,12 +118,6 @@ static void clear_pte_entry(struct kvm *kvm, pte_t *pte, phys_addr_t addr)
> }
> }
>
> -static bool pte_empty(pte_t *pte)
> -{
> - struct page *pte_page = virt_to_page(pte);
> - return page_count(pte_page) == 1;
> -}
> -
> static void unmap_range(struct kvm *kvm, pgd_t *pgdp,
> unsigned long long start, u64 size)
> {
> @@ -153,10 +147,10 @@ static void unmap_range(struct kvm *kvm, pgd_t *pgdp,
> range = PAGE_SIZE;
>
> /* If we emptied the pte, walk back up the ladder */
> - if (pte_empty(pte)) {
> + if (page_empty(pte)) {
> clear_pmd_entry(kvm, pmd, addr);
> range = PMD_SIZE;
> - if (pmd_empty(pmd)) {
> + if (page_empty(pmd) && !page_empty(pud)) {
> clear_pud_entry(kvm, pud, addr);
> range = PUD_SIZE;
> }
looks right, an alternative would be to check in clear_pud_entry if the
entry actually had a value, but I don't think it's really clearer.
However, this got me thinking a bit. What happens if we pass a non-pmd
aligned address to unmap_range, and let's assume the size of the range
is more than 2MB, won't we be leaking memory by incrementing with
PMD_SIZE? (same argument goes for PUD_SIZE). See the patch below:
diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index ca6bea4..80a83ec 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -132,37 +132,37 @@ static void unmap_range(struct kvm *kvm, pgd_t *pgdp,
pmd_t *pmd;
pte_t *pte;
unsigned long long addr = start, end = start + size;
- u64 range;
+ u64 next;
while (addr < end) {
pgd = pgdp + pgd_index(addr);
pud = pud_offset(pgd, addr);
if (pud_none(*pud)) {
- addr += PUD_SIZE;
+ addr = pud_addr_end(addr, end);
continue;
}
pmd = pmd_offset(pud, addr);
if (pmd_none(*pmd)) {
- addr += PMD_SIZE;
+ addr = pmd_addr_end(addr, end);
continue;
}
pte = pte_offset_kernel(pmd, addr);
clear_pte_entry(kvm, pte, addr);
- range = PAGE_SIZE;
+ next = addr + PAGE_SIZE;
/* If we emptied the pte, walk back up the ladder */
if (pte_empty(pte)) {
clear_pmd_entry(kvm, pmd, addr);
- range = PMD_SIZE;
+ next = pmd_addr_end(addr, end);
if (pmd_empty(pmd)) {
clear_pud_entry(kvm, pud, addr);
- range = PUD_SIZE;
+ next = pud_addr_end(addr, end);
}
}
- addr += range;
+ addr = next;
}
}
--
Christoffer
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