Re: [PATCH] kvm: arm: Fix handling of stage2 huge mappings
From: Suzuki K Poulose
Date: Wed Mar 20 2019 - 05:44:47 EST
Hi Marc,
On 20/03/2019 08:15, Marc Zyngier wrote:
Hi Suzuki,
On Tue, 19 Mar 2019 14:11:08 +0000,
Suzuki K Poulose <suzuki.poulose@xxxxxxx> wrote:
We rely on the mmu_notifier call backs to handle the split/merge
of huge pages and thus we are guaranteed that, while creating a
block mapping, either the entire block is unmapped at stage2 or it
is missing permission.
However, we miss a case where the block mapping is split for dirty
logging case and then could later be made block mapping, if we cancel the
dirty logging. This not only creates inconsistent TLB entries for
the pages in the the block, but also leakes the table pages for
PMD level.
Handle this corner case for the huge mappings at stage2 by
unmapping the non-huge mapping for the block. This could potentially
release the upper level table. So we need to restart the table walk
once we unmap the range.
Fixes : ad361f093c1e31d ("KVM: ARM: Support hugetlbfs backed huge pages")
Reported-by: Zheng Xiang <zhengxiang9@xxxxxxxxxx>
Cc: Zheng Xiang <zhengxiang9@xxxxxxxxxx>
Cc: Zhengui Yu <yuzenghui@xxxxxxxxxx>
Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
Signed-off-by: Suzuki K Poulose
...
+retry:
pmd = stage2_get_pmd(kvm, cache, addr);
VM_BUG_ON(!pmd);
...
if (pmd_present(old_pmd)) {
/*
- * Multiple vcpus faulting on the same PMD entry, can
- * lead to them sequentially updating the PMD with the
- * same value. Following the break-before-make
- * (pmd_clear() followed by tlb_flush()) process can
- * hinder forward progress due to refaults generated
- * on missing translations.
+ * If we already have PTE level mapping for this block,
+ * we must unmap it to avoid inconsistent TLB state and
+ * leaking the table page. We could end up in this situation
+ * if the memory slot was marked for dirty logging and was
+ * reverted, leaving PTE level mappings for the pages accessed
+ * during the period. So, unmap the PTE level mapping for this
+ * block and retry, as we could have released the upper level
+ * table in the process.
*
- * Skip updating the page table if the entry is
- * unchanged.
+ * Normal THP split/merge follows mmu_notifier callbacks and do
+ * get handled accordingly.
*/
- if (pmd_val(old_pmd) == pmd_val(*new_pmd))
- return 0;
-
+ if (!pmd_thp_or_huge(old_pmd)) {
+ unmap_stage2_range(kvm, addr & S2_PMD_MASK, S2_PMD_SIZE);
+ goto retry;
This looks slightly dodgy. Doing this retry results in another call to
stage2_get_pmd(), which may or may not result in allocating a PUD. I
think this is safe as if we managed to get here, it means the whole
hierarchy was already present and nothing was allocated in the first
round.
Somehow, I would feel more comfortable with just not even trying.
Unmap, don't fix the fault, let the vcpu come again for additional
punishment. But this is probably more invasive, as none of the
stage2_set_p*() return value is ever evaluated. Oh well.
Yes. The other option was to unmap_stage2_ptes() and get the page refcount
on the new pmd. But that kind of makes it a bit difficult to follow the
code.
if (stage2_pud_present(kvm, old_pud)) {
- stage2_pud_clear(kvm, pudp);
- kvm_tlb_flush_vmid_ipa(kvm, addr);
+ /*
+ * If we already have table level mapping for this block, unmap
+ * the range for this block and retry.
+ */
+ if (!stage2_pud_huge(kvm, old_pud)) {
+ unmap_stage2_range(kvm, addr & S2_PUD_MASK, S2_PUD_SIZE);
This broke 32bit. I've added the following hunk to fix it:
Grrr! Sorry about that.
diff --git a/arch/arm/include/asm/stage2_pgtable.h b/arch/arm/include/asm/stage2_pgtable.h
index de2089501b8b..b8f21088a744 100644
--- a/arch/arm/include/asm/stage2_pgtable.h
+++ b/arch/arm/include/asm/stage2_pgtable.h
@@ -68,6 +68,9 @@ stage2_pmd_addr_end(struct kvm *kvm, phys_addr_t addr, phys_addr_t end)
#define stage2_pmd_table_empty(kvm, pmdp) kvm_page_empty(pmdp)
#define stage2_pud_table_empty(kvm, pudp) false
+#define S2_PUD_MASK PGDIR_MASK
+#define S2_PUD_SIZE PGDIR_SIZE
+
We should really get rid of the S2_P{U/M}D_* definitions, as they are
always the same as the host. The only thing that changes is the PGD size
which varies according to the IPA and the concatenation.
static inline bool kvm_stage2_has_pud(struct kvm *kvm)
{
return false;
+ goto retry;
+ } else {
+ WARN_ON_ONCE(pud_pfn(old_pud) != pud_pfn(*new_pudp));
+ stage2_pud_clear(kvm, pudp);
+ kvm_tlb_flush_vmid_ipa(kvm, addr);
+ }
The 'else' line could go, and would make the code similar to the PMD path.
Yep. I think the pud_pfn() may not be defined for some configs, if the hugetlbfs
is not selected on arm32. So, we should move them to kvm_pud_pfn() instead.
} else {
get_page(virt_to_page(pudp));
}
--
2.7.4
If you're OK with the above nits, I'll squash them into the patch.
With the kvm_pud_pfn() changes, yes. Alternately, I could resend the updated
patch, fixing the typo in Zenghui's name. Let me know.
Cheers
Suzuki