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[34.83.12.150]) by smtp.gmail.com with ESMTPSA id q4-20020a17090a7a8400b001cd4a0c3270sm13901593pjf.7.2022.04.18.19.59.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 18 Apr 2022 19:59:07 -0700 (PDT) Date: Mon, 18 Apr 2022 19:59:04 -0700 From: Ricardo Koller To: Oliver Upton Subject: Re: [RFC PATCH 14/17] KVM: arm64: Punt last page reference to rcu callback for parallel walk Message-ID: References: <20220415215901.1737897-1-oupton@google.com> <20220415215901.1737897-15-oupton@google.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20220415215901.1737897-15-oupton@google.com> Cc: kvm@vger.kernel.org, Marc Zyngier , Ben Gardon , Peter Shier , David Matlack , Paolo Bonzini , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Fri, Apr 15, 2022 at 09:58:58PM +0000, Oliver Upton wrote: > It is possible that a table page remains visible to another thread until > the next rcu synchronization event. To that end, we cannot drop the last > page reference synchronous with post-order traversal for a parallel > table walk. > > Schedule an rcu callback to clean up the child table page for parallel > walks. > > Signed-off-by: Oliver Upton > --- > arch/arm64/include/asm/kvm_pgtable.h | 3 ++ > arch/arm64/kvm/hyp/pgtable.c | 24 +++++++++++++-- > arch/arm64/kvm/mmu.c | 44 +++++++++++++++++++++++++++- > 3 files changed, 67 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h > index 74955aba5918..52e55e00f0ca 100644 > --- a/arch/arm64/include/asm/kvm_pgtable.h > +++ b/arch/arm64/include/asm/kvm_pgtable.h > @@ -81,6 +81,8 @@ static inline bool kvm_level_supports_block_mapping(u32 level) > * @put_page: Decrement the refcount on a page. When the > * refcount reaches 0 the page is automatically > * freed. > + * @free_table: Drop the last page reference, possibly in the > + * next RCU sync if doing a shared walk. > * @page_count: Return the refcount of a page. > * @phys_to_virt: Convert a physical address into a virtual > * address mapped in the current context. > @@ -98,6 +100,7 @@ struct kvm_pgtable_mm_ops { > void (*get_page)(void *addr); > void (*put_page)(void *addr); > int (*page_count)(void *addr); > + void (*free_table)(void *addr, bool shared); > void* (*phys_to_virt)(phys_addr_t phys); > phys_addr_t (*virt_to_phys)(void *addr); > void (*dcache_clean_inval_poc)(void *addr, size_t size); > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index 121818d4c33e..a9a48edba63b 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -147,12 +147,19 @@ static inline void kvm_pgtable_walk_end(void) > {} > > #define kvm_dereference_ptep rcu_dereference_raw > + > +static inline void kvm_pgtable_destroy_barrier(void) > +{} > + > #else > #define kvm_pgtable_walk_begin rcu_read_lock > > #define kvm_pgtable_walk_end rcu_read_unlock > > #define kvm_dereference_ptep rcu_dereference > + > +#define kvm_pgtable_destroy_barrier rcu_barrier > + > #endif > > static kvm_pte_t *kvm_pte_follow(kvm_pte_t pte, struct kvm_pgtable_mm_ops *mm_ops) > @@ -1063,7 +1070,12 @@ static int stage2_map_walk_table_post(u64 addr, u64 end, u32 level, > childp = kvm_pte_follow(*old, mm_ops); > } > > - mm_ops->put_page(childp); > + /* > + * If we do not have exclusive access to the page tables it is possible > + * the unlinked table remains visible to another thread until the next > + * rcu synchronization. > + */ > + mm_ops->free_table(childp, shared); > mm_ops->put_page(ptep); > > return ret; > @@ -1203,7 +1215,7 @@ static int stage2_unmap_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep, > kvm_granule_size(level)); > > if (childp) > - mm_ops->put_page(childp); > + mm_ops->free_table(childp, shared); > > return 0; > } > @@ -1433,7 +1445,7 @@ static int stage2_free_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep, > mm_ops->put_page(ptep); > > if (kvm_pte_table(*old, level)) > - mm_ops->put_page(kvm_pte_follow(*old, mm_ops)); > + mm_ops->free_table(kvm_pte_follow(*old, mm_ops), shared); > > return 0; > } > @@ -1452,4 +1464,10 @@ void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt) > pgd_sz = kvm_pgd_pages(pgt->ia_bits, pgt->start_level) * PAGE_SIZE; > pgt->mm_ops->free_pages_exact(pgt->pgd, pgd_sz); > pgt->pgd = NULL; > + > + /* > + * Guarantee that all unlinked subtrees associated with the stage2 page > + * table have also been freed before returning. > + */ > + kvm_pgtable_destroy_barrier(); > } > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > index cc6ed6b06ec2..6ecf37009c21 100644 > --- a/arch/arm64/kvm/mmu.c > +++ b/arch/arm64/kvm/mmu.c > @@ -98,9 +98,50 @@ static bool kvm_is_device_pfn(unsigned long pfn) > static void *stage2_memcache_zalloc_page(void *arg) > { > struct kvm_mmu_caches *mmu_caches = arg; > + struct stage2_page_header *hdr; > + void *addr; > > /* Allocated with __GFP_ZERO, so no need to zero */ > - return kvm_mmu_memory_cache_alloc(&mmu_caches->page_cache); > + addr = kvm_mmu_memory_cache_alloc(&mmu_caches->page_cache); > + if (!addr) > + return NULL; > + > + hdr = kvm_mmu_memory_cache_alloc(&mmu_caches->header_cache); > + if (!hdr) { > + free_page((unsigned long)addr); > + return NULL; > + } > + > + hdr->page = virt_to_page(addr); > + set_page_private(hdr->page, (unsigned long)hdr); > + return addr; > +} > + > +static void stage2_free_page_now(struct stage2_page_header *hdr) > +{ > + WARN_ON(page_ref_count(hdr->page) != 1); > + > + __free_page(hdr->page); > + kmem_cache_free(stage2_page_header_cache, hdr); > +} > + > +static void stage2_free_page_rcu_cb(struct rcu_head *head) > +{ > + struct stage2_page_header *hdr = container_of(head, struct stage2_page_header, > + rcu_head); > + > + stage2_free_page_now(hdr); > +} > + > +static void stage2_free_table(void *addr, bool shared) > +{ > + struct page *page = virt_to_page(addr); > + struct stage2_page_header *hdr = (struct stage2_page_header *)page_private(page); > + > + if (shared) > + call_rcu(&hdr->rcu_head, stage2_free_page_rcu_cb); Can the number of callbacks grow to "dangerous" numbers? can it be bounded with something like the following? if number of readers is really high: synchronize_rcu() else call_rcu() maybe the rcu API has an option for that. > + else > + stage2_free_page_now(hdr); > } > > static void *kvm_host_zalloc_pages_exact(size_t size) > @@ -613,6 +654,7 @@ static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = { > .free_pages_exact = free_pages_exact, > .get_page = kvm_host_get_page, > .put_page = kvm_host_put_page, > + .free_table = stage2_free_table, > .page_count = kvm_host_page_count, > .phys_to_virt = kvm_host_va, > .virt_to_phys = kvm_host_pa, > -- > 2.36.0.rc0.470.gd361397f0d-goog > _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm