Hi Ricardo,
On Mon, Apr 18, 2022 at 8:09 PM Ricardo Koller <ricarkol@xxxxxxxxxx> wrote:
>
> On Mon, Apr 18, 2022 at 07:59:04PM -0700, Ricardo Koller wrote:
> > 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()
>
> sorry, meant to say "number of callbacks"
Good point. I don't have data for this, but generally speaking I do
not believe we need to enqueue a callback for every page. In fact,
since we already make the invalid PTE visible in pre-order traversal
we could theoretically free all tables from a single RCU callback (per
fault).
I think if we used synchronize_rcu() then we would need to drop the
mmu lock since it will block the thread.
--
Thanks,
Oliver
