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Message-ID: <1bac514c-e4f0-a609-96ed-f48ef3461da1@arm.com> Date: Thu, 11 Apr 2019 16:16:48 +0100 From: Suzuki K Poulose <suzuki.poulose@....com> To: yuzenghui@...wei.com, linux-arm-kernel@...ts.infradead.org Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu, julien.thierry@....com, christoffer.dall@....com, marc.zyngier@....com, andrew.murray@....com, eric.auger@...hat.com, zhengxiang9@...wei.com, wanghaibin.wang@...wei.com Subject: Re: [PATCH 2/2] kvm: arm: Unify handling THP backed host memory 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@....com> >> Cc: Christoffer Dall <christoffer.dall@....com> >> Cc: Zneghui Yu <yuzenghui@...wei.com> >> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....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. 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)); if (PageTransCompoundMap(page) &&>> + fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) { ... Cheers Suzuki
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