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 <oupton@xxxxxxxxxx>
> ---
> 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
>
