lkml.org 
[lkml]   [2019]   [Mar]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC] Question about TLB flush while set Stage-2 huge pages
From
Date
Hi Suzuki,

On 2019/3/19 1:34, Suzuki K Poulose wrote:
> Hi !
> On Sun, Mar 17, 2019 at 09:34:11PM +0800, Zenghui Yu wrote:
>> Hi Suzuki,
>>
>> ---8<---
>>
>> test: kvm: arm: Maybe two more fixes
>>
>> Applied based on Suzuki's patch.
>>
>> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
>> ---
>> virt/kvm/arm/mmu.c | 8 ++++++--
>> 1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> index 05765df..ccd5d5d 100644
>> --- a/virt/kvm/arm/mmu.c
>> +++ b/virt/kvm/arm/mmu.c
>> @@ -1089,7 +1089,9 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct
>> kvm_mmu_memory_cache
>> * Normal THP split/merge follows mmu_notifier
>> * callbacks and do get handled accordingly.
>> */
>> - unmap_stage2_range(kvm, (addr & S2_PMD_MASK), S2_PMD_SIZE);
>> + addr &= S2_PMD_MASK;
>> + unmap_stage2_ptes(kvm, pmd, addr, addr + S2_PMD_SIZE);
>> + get_page(virt_to_page(pmd));
>> } else {
>>
>> /*
>> @@ -1138,7 +1140,9 @@ static int stage2_set_pud_huge(struct kvm *kvm, struct
>> kvm_mmu_memory_cache *cac
>> if (stage2_pud_present(kvm, old_pud)) {
>> /* If we have PTE level mapping, unmap the entire range */
>> if (WARN_ON_ONCE(!stage2_pud_huge(kvm, old_pud))) {
>> - unmap_stage2_range(kvm, addr & S2_PUD_MASK, S2_PUD_SIZE);
>> + addr &= S2_PUD_MASK;
>> + unmap_stage2_pmds(kvm, pudp, addr, addr + S2_PUD_SIZE);
>> + get_page(virt_to_page(pudp));
>> } else {
>> stage2_pud_clear(kvm, pudp);
>> kvm_tlb_flush_vmid_ipa(kvm, addr);
>
> This makes it a bit tricky to follow the code. The other option is to
> do something like :

Yes.

>
>
> ---8>---
>
> kvm: arm: Fix handling of stage2 huge mappings
>
> We rely on the mmu_notifier call backs to handle the split/merging
> of huge pages and thus we are guaranteed that while creating a
> block mapping, the entire block is unmapped at stage2. 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 these corner cases for the huge mappings at stage2 by
> unmapping the PTE level mapping. This could potentially release
> the upper level table. So we need to restart the table walk
> once we unmap the range.
>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> virt/kvm/arm/mmu.c | 57 +++++++++++++++++++++++++++++++++++++++---------------
> 1 file changed, 41 insertions(+), 16 deletions(-)
>
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index fce0983..a38a3f1 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1060,25 +1060,41 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
> {
> pmd_t *pmd, old_pmd;
>
> +retry:
> pmd = stage2_get_pmd(kvm, cache, addr);
> VM_BUG_ON(!pmd);
>
> old_pmd = *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.
> + *
> + * Skip updating the page table if the entry is
> + * unchanged.
> + */
> + if (pmd_val(old_pmd) == pmd_val(*new_pmd))
> + return 0;
> +
> 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.
> - *
> - * Skip updating the page table if the entry is
> - * unchanged.
> + * If we already have PTE level mapping for this block,
> + * we must unmap it to avoid inconsistent TLB
> + * state. 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.
> + * Normal THP split/merge follows mmu_notifier
> + * callbacks and do get handled accordingly.
> + * Unmap the PTE level mapping and retry.
> */
> - 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);
Nit: we can get rid of the parentheses around "addr & S2_PMD_MASK" to
make it looks the same as PUD level (but it is not necessary).
> + goto retry;
> + }
> /*
> * Mapping in huge pages should only happen through a
> * fault. If a page is merged into a transparent huge
> @@ -1090,8 +1106,7 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
> * should become splitting first, unmapped, merged,
> * and mapped back in on-demand.
> */
> - VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd));
> -
> + WARN_ON_ONCE(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd));
> pmd_clear(pmd);
> kvm_tlb_flush_vmid_ipa(kvm, addr);
> } else {
> @@ -1107,6 +1122,7 @@ static int stage2_set_pud_huge(struct kvm *kvm, struct kvm_mmu_memory_cache *cac
> {
> pud_t *pudp, old_pud;
>
> +retry:
> pudp = stage2_get_pud(kvm, cache, addr);
> VM_BUG_ON(!pudp);
>
> @@ -1122,8 +1138,17 @@ static int stage2_set_pud_huge(struct kvm *kvm, struct kvm_mmu_memory_cache *cac
> return 0;
>
> if (stage2_pud_present(kvm, old_pud)) {
> - stage2_pud_clear(kvm, pudp);
> - kvm_tlb_flush_vmid_ipa(kvm, addr);
> + /*
> + * If we already have PTE level mapping, unmap the entire
> + * range and retry.
> + */
> + if (!stage2_pud_huge(kvm, old_pud)) {
> + unmap_stage2_range(kvm, addr & S2_PUD_MASK, S2_PUD_SIZE);
> + goto retry;
> + } else {
> + stage2_pud_clear(kvm, pudp);
> + kvm_tlb_flush_vmid_ipa(kvm, addr);
> + }
> } else {
> get_page(virt_to_page(pudp));
> }
>

It look much better, and works fine now!


thanks,

zenghui


\
 
 \ /
  Last update: 2019-03-19 10:09    [W:0.108 / U:71.024 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site