Messages in this thread | | | Subject | Re: [PATCH 2/2] kvm: arm: Unify handling THP backed host memory | From | Zenghui Yu <> | Date | Fri, 12 Apr 2019 17:34:53 +0800 |
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On 2019/4/11 23:16, Suzuki K Poulose wrote: > Hi Zhengui, > > On 11/04/2019 02:59, Zenghui Yu wrote: >> Hi Suzuki, >> >> On 2019/4/10 23:23, Suzuki K Poulose wrote: >>> We support mapping host memory backed by PMD transparent hugepages >>> at stage2 as huge pages. However the checks are now spread across >>> two different places. Let us unify the handling of the THPs to >>> keep the code cleaner (and future proof for PUD THP support). >>> This patch moves transparent_hugepage_adjust() closer to the caller >>> to avoid a forward declaration for >>> fault_supports_stage2_huge_mappings(). >>> >>> Also, since we already handle the case where the host VA and the guest >>> PA may not be aligned, the explicit VM_BUG_ON() is not required. >>> >>> Cc: Marc Zyngier <marc.zyngier@arm.com> >>> Cc: Christoffer Dall <christoffer.dall@arm.com> >>> Cc: Zneghui Yu <yuzenghui@huawei.com> >>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com> >>> --- >>> virt/kvm/arm/mmu.c | 123 >>> +++++++++++++++++++++++++++-------------------------- >>> 1 file changed, 62 insertions(+), 61 deletions(-) >>> >>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c >>> index 6d73322..714eec2 100644 >>> --- a/virt/kvm/arm/mmu.c >>> +++ b/virt/kvm/arm/mmu.c >>> @@ -1380,53 +1380,6 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, >>> phys_addr_t guest_ipa, >>> return ret; >>> } >>> -static bool transparent_hugepage_adjust(kvm_pfn_t *pfnp, phys_addr_t >>> *ipap) >>> -{ >>> - kvm_pfn_t pfn = *pfnp; >>> - gfn_t gfn = *ipap >> PAGE_SHIFT; >>> - struct page *page = pfn_to_page(pfn); >>> - >>> - /* >>> - * PageTransCompoundMap() returns true for THP and >>> - * hugetlbfs. Make sure the adjustment is done only for THP >>> - * pages. >>> - */ >>> - if (!PageHuge(page) && PageTransCompoundMap(page)) { >>> - unsigned long mask; >>> - /* >>> - * The address we faulted on is backed by a transparent huge >>> - * page. However, because we map the compound huge page and >>> - * not the individual tail page, we need to transfer the >>> - * refcount to the head page. We have to be careful that the >>> - * THP doesn't start to split while we are adjusting the >>> - * refcounts. >>> - * >>> - * We are sure this doesn't happen, because mmu_notifier_retry >>> - * was successful and we are holding the mmu_lock, so if this >>> - * THP is trying to split, it will be blocked in the mmu >>> - * notifier before touching any of the pages, specifically >>> - * before being able to call __split_huge_page_refcount(). >>> - * >>> - * We can therefore safely transfer the refcount from PG_tail >>> - * to PG_head and switch the pfn from a tail page to the head >>> - * page accordingly. >>> - */ >>> - mask = PTRS_PER_PMD - 1; >>> - VM_BUG_ON((gfn & mask) != (pfn & mask)); >>> - if (pfn & mask) { >>> - *ipap &= PMD_MASK; >>> - kvm_release_pfn_clean(pfn); >>> - pfn &= ~mask; >>> - kvm_get_pfn(pfn); >>> - *pfnp = pfn; >>> - } >>> - >>> - return true; >>> - } >>> - >>> - return false; >>> -} >>> - >>> /** >>> * stage2_wp_ptes - write protect PMD range >>> * @pmd: pointer to pmd entry >>> @@ -1677,6 +1630,61 @@ static bool >>> fault_supports_stage2_huge_mapping(struct kvm_memory_slot *memslot, >>> (hva & ~(map_size - 1)) + map_size <= uaddr_end; >>> } >>> +/* >>> + * Check if the given hva is backed by a transparent huge page (THP) >>> + * and whether it can be mapped using block mapping in stage2. If >>> so, adjust >>> + * the stage2 PFN and IPA accordingly. Only PMD_SIZE THPs are currently >>> + * supported. This will need to be updated to support other THP sizes. >>> + * >>> + * Returns the size of the mapping. >>> + */ >>> +static unsigned long >>> +transparent_hugepage_adjust(struct kvm_memory_slot *memslot, >>> + unsigned long hva, kvm_pfn_t *pfnp, >>> + phys_addr_t *ipap) >>> +{ >>> + kvm_pfn_t pfn = *pfnp; >>> + struct page *page = pfn_to_page(pfn); >>> + >>> + /* >>> + * PageTransCompoundMap() returns true for THP and >>> + * hugetlbfs. Make sure the adjustment is done only for THP >>> + * pages. Also make sure that the HVA and IPA are sufficiently >>> + * aligned and that the block map is contained within the memslot. >>> + */ >>> + if (!PageHuge(page) && PageTransCompoundMap(page) && >> >> We managed to get here, ensure that we only play with normal size pages >> and no hugetlbfs pages will be involved. "!PageHuge(page)" will always >> return true and we can let it go. > > I think that is a bit tricky. If someone ever modifies the user_mem_abort() > and we end up in getting called with a HugeTLB backed page things could go > wrong.
That will be bad. I'm not sure if it's possible in the future.
> I could do remove the check, but would like to add a WARN_ON_ONCE() to make > sure our assumption is held. > > i.e, > WARN_ON_ONCE(PageHuge(page));
But this is a careful approach. I think this will be valuable both for developers and the code itself. Thanks!
zenghui
> > if (PageTransCompoundMap(page) &&>> + > fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) { > > ... >
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