thp: kvm mmu transparent hugepage support

This should work for both hugetlbfs and transparent hugepages.

[akpm@linux-foundation.org: bring forward PageTransCompound() addition for bisectability]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 125 insertions, 19 deletions

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 9cafbb499813..47b2c3288b6b 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -554,14 +554,18 @@ static int host_mapping_level(struct kvm *kvm, gfn_t gfn)
         return ret;
 }
 
-static int mapping_level(struct kvm_vcpu *vcpu, gfn_t large_gfn)
+static bool mapping_level_dirty_bitmap(struct kvm_vcpu *vcpu, gfn_t large_gfn)
 {
         struct kvm_memory_slot *slot;
-        int host_level, level, max_level;
-
         slot = gfn_to_memslot(vcpu->kvm, large_gfn);
         if (slot && slot->dirty_bitmap)
-                return PT_PAGE_TABLE_LEVEL;
+                return true;
+        return false;
+}
+
+static int mapping_level(struct kvm_vcpu *vcpu, gfn_t large_gfn)
+{
+        int host_level, level, max_level;
 
         host_level = host_mapping_level(vcpu->kvm, large_gfn);
 
@@ -2281,6 +2285,48 @@ static int kvm_handle_bad_page(struct kvm *kvm, gfn_t gfn, pfn_t pfn)
         return 1;
 }
 
+static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
+                                        gfn_t *gfnp, pfn_t *pfnp, int *levelp)
+{
+        pfn_t pfn = *pfnp;
+        gfn_t gfn = *gfnp;
+        int level = *levelp;
+
+        /*
+         * Check if it's a transparent hugepage. If this would be an
+         * hugetlbfs page, level wouldn't be set to
+         * PT_PAGE_TABLE_LEVEL and there would be no adjustment done
+         * here.
+         */
+        if (!is_error_pfn(pfn) && !kvm_is_mmio_pfn(pfn) &&
+            level == PT_PAGE_TABLE_LEVEL &&
+            PageTransCompound(pfn_to_page(pfn)) &&
+            !has_wrprotected_page(vcpu->kvm, gfn, PT_DIRECTORY_LEVEL)) {
+                unsigned long mask;
+                /*
+                 * mmu_notifier_retry was successful and we hold the
+                 * mmu_lock here, so the pmd can't become splitting
+                 * from under us, and in turn
+                 * __split_huge_page_refcount() can't run from under
+                 * us and we can safely transfer the refcount from
+                 * PG_tail to PG_head as we switch the pfn to tail to
+                 * head.
+                 */
+                *levelp = level = PT_DIRECTORY_LEVEL;
+                mask = KVM_PAGES_PER_HPAGE(level) - 1;
+                VM_BUG_ON((gfn & mask) != (pfn & mask));
+                if (pfn & mask) {
+                        gfn &= ~mask;
+                        *gfnp = gfn;
+                        kvm_release_pfn_clean(pfn);
+                        pfn &= ~mask;
+                        if (!get_page_unless_zero(pfn_to_page(pfn)))
+                                BUG();
+                        *pfnp = pfn;
+                }
+        }
+}
+
 static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
                          gva_t gva, pfn_t *pfn, bool write, bool *writable);
 
@@ -2289,20 +2335,25 @@ static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, int write, gfn_t gfn,
 {
         int r;
         int level;
+        int force_pt_level;
         pfn_t pfn;
         unsigned long mmu_seq;
         bool map_writable;
 
-        level = mapping_level(vcpu, gfn);
-
-        /*
-         * This path builds a PAE pagetable - so we can map 2mb pages at
-         * maximum. Therefore check if the level is larger than that.
-         */
-        if (level > PT_DIRECTORY_LEVEL)
-                level = PT_DIRECTORY_LEVEL;
+        force_pt_level = mapping_level_dirty_bitmap(vcpu, gfn);
+        if (likely(!force_pt_level)) {
+                level = mapping_level(vcpu, gfn);
+                /*
+                 * This path builds a PAE pagetable - so we can map
+                 * 2mb pages at maximum. Therefore check if the level
+                 * is larger than that.
+                 */
+                if (level > PT_DIRECTORY_LEVEL)
+                        level = PT_DIRECTORY_LEVEL;
 
-        gfn &= ~(KVM_PAGES_PER_HPAGE(level) - 1);
+                gfn &= ~(KVM_PAGES_PER_HPAGE(level) - 1);
+        } else
+                level = PT_PAGE_TABLE_LEVEL;
 
         mmu_seq = vcpu->kvm->mmu_notifier_seq;
         smp_rmb();
@@ -2318,6 +2369,8 @@ static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, int write, gfn_t gfn,
         if (mmu_notifier_retry(vcpu, mmu_seq))
                 goto out_unlock;
         kvm_mmu_free_some_pages(vcpu);
+        if (likely(!force_pt_level))
+                transparent_hugepage_adjust(vcpu, &gfn, &pfn, &level);
         r = __direct_map(vcpu, v, write, map_writable, level, gfn, pfn,
                          prefault);
         spin_unlock(&vcpu->kvm->mmu_lock);
@@ -2655,6 +2708,7 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
         pfn_t pfn;
         int r;
         int level;
+        int force_pt_level;
         gfn_t gfn = gpa >> PAGE_SHIFT;
         unsigned long mmu_seq;
         int write = error_code & PFERR_WRITE_MASK;
@@ -2667,9 +2721,12 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
         if (r)
                 return r;
 
-        level = mapping_level(vcpu, gfn);
-
-        gfn &= ~(KVM_PAGES_PER_HPAGE(level) - 1);
+        force_pt_level = mapping_level_dirty_bitmap(vcpu, gfn);
+        if (likely(!force_pt_level)) {
+                level = mapping_level(vcpu, gfn);
+                gfn &= ~(KVM_PAGES_PER_HPAGE(level) - 1);
+        } else
+                level = PT_PAGE_TABLE_LEVEL;
 
         mmu_seq = vcpu->kvm->mmu_notifier_seq;
         smp_rmb();
@@ -2684,6 +2741,8 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
         if (mmu_notifier_retry(vcpu, mmu_seq))
                 goto out_unlock;
         kvm_mmu_free_some_pages(vcpu);
+        if (likely(!force_pt_level))
+                transparent_hugepage_adjust(vcpu, &gfn, &pfn, &level);
         r = __direct_map(vcpu, gpa, write, map_writable,
                          level, gfn, pfn, prefault);
         spin_unlock(&vcpu->kvm->mmu_lock);
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 53210f1e94c2..6bccc24c4181 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -550,6 +550,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
         int r;
         pfn_t pfn;
         int level = PT_PAGE_TABLE_LEVEL;
+        int force_pt_level;
         unsigned long mmu_seq;
         bool map_writable;
 
@@ -577,7 +578,11 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
                 return 0;
         }
 
-        if (walker.level >= PT_DIRECTORY_LEVEL) {
+        if (walker.level >= PT_DIRECTORY_LEVEL)
+                force_pt_level = mapping_level_dirty_bitmap(vcpu, walker.gfn);
+        else
+                force_pt_level = 1;
+        if (!force_pt_level) {
                 level = min(walker.level, mapping_level(vcpu, walker.gfn));
                 walker.gfn = walker.gfn & ~(KVM_PAGES_PER_HPAGE(level) - 1);
         }
@@ -599,6 +604,8 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
 
         trace_kvm_mmu_audit(vcpu, AUDIT_PRE_PAGE_FAULT);
         kvm_mmu_free_some_pages(vcpu);
+        if (!force_pt_level)
+                transparent_hugepage_adjust(vcpu, &walker.gfn, &pfn, &level);
         sptep = FNAME(fetch)(vcpu, addr, &walker, user_fault, write_fault,
                              level, &write_pt, pfn, map_writable, prefault);
         (void)sptep;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 4835cae71047..907f1605926b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -409,6 +409,18 @@ static inline void ClearPageCompound(struct page *page)
 
 #endif /* !PAGEFLAGS_EXTENDED */
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static inline int PageTransCompound(struct page *page)
+{
+        return PageCompound(page);
+}
+#else
+static inline int PageTransCompound(struct page *page)
+{
+        return 0;
+}
+#endif
+
 #ifdef CONFIG_MMU
 #define __PG_MLOCKED            (1 << PG_mlocked)
 #else
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7f686251f711..85ab7db0d366 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -104,8 +104,36 @@ static pfn_t fault_pfn;
 inline int kvm_is_mmio_pfn(pfn_t pfn)
 {
         if (pfn_valid(pfn)) {
-                struct page *page = compound_head(pfn_to_page(pfn));
-                return PageReserved(page);
+                struct page *head;
+                struct page *tail = pfn_to_page(pfn);
+                head = compound_head(tail);
+                if (head != tail) {
+                        smp_rmb();
+                        /*
+                         * head may be a dangling pointer.
+                         * __split_huge_page_refcount clears PageTail
+                         * before overwriting first_page, so if
+                         * PageTail is still there it means the head
+                         * pointer isn't dangling.
+                         */
+                        if (PageTail(tail)) {
+                                /*
+                                 * the "head" is not a dangling
+                                 * pointer but the hugepage may have
+                                 * been splitted from under us (and we
+                                 * may not hold a reference count on
+                                 * the head page so it can be reused
+                                 * before we run PageReferenced), so
+                                 * we've to recheck PageTail before
+                                 * returning what we just read.
+                                 */
+                                int reserved = PageReserved(head);
+                                smp_rmb();
+                                if (PageTail(tail))
+                                        return reserved;
+                        }
+                }
+                return PageReserved(tail);
         }
 
         return true;