anon_vma_prepare: properly lock even newly allocated entries

The anon_vma code is very subtle, and we end up doing optimistic lookups
of anon_vmas under RCU in page_lock_anon_vma() with no locking.  Other
CPU's can also see the newly allocated entry immediately after we've
exposed it by setting "vma->anon_vma" to the new value.

We protect against the anon_vma being destroyed by having the SLAB
marked as SLAB_DESTROY_BY_RCU, so the RCU lookup can depend on the
allocation not being destroyed - but it might still be free'd and
re-allocated here to a new vma.

As a result, we should not do the anon_vma list ops on a newly allocated
vma without proper locking.

Acked-by: Nick Piggin <npiggin@suse.de>
Acked-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -55,7 +55,33 @@
 
 struct kmem_cache *anon_vma_cachep;
 
-/* This must be called under the mmap_sem. */
+/**
+ * anon_vma_prepare - attach an anon_vma to a memory region
+ * @vma: the memory region in question
+ *
+ * This makes sure the memory mapping described by 'vma' has
+ * an 'anon_vma' attached to it, so that we can associate the
+ * anonymous pages mapped into it with that anon_vma.
+ *
+ * The common case will be that we already have one, but if
+ * if not we either need to find an adjacent mapping that we
+ * can re-use the anon_vma from (very common when the only
+ * reason for splitting a vma has been mprotect()), or we
+ * allocate a new one.
+ *
+ * Anon-vma allocations are very subtle, because we may have
+ * optimistically looked up an anon_vma in page_lock_anon_vma()
+ * and that may actually touch the spinlock even in the newly
+ * allocated vma (it depends on RCU to make sure that the
+ * anon_vma isn't actually destroyed).
+ *
+ * As a result, we need to do proper anon_vma locking even
+ * for the new allocation. At the same time, we do not want
+ * to do any locking for the common case of already having
+ * an anon_vma.
+ *
+ * This must be called with the mmap_sem held for reading.
+ */
 int anon_vma_prepare(struct vm_area_struct *vma)
 {
 	struct anon_vma *anon_vma = vma->anon_vma;
@@ -63,20 +89,17 @@ int anon_vma_prepare(struct vm_area_struct *vma)
 	might_sleep();
 	if (unlikely(!anon_vma)) {
 		struct mm_struct *mm = vma->vm_mm;
-		struct anon_vma *allocated, *locked;
+		struct anon_vma *allocated;
 
 		anon_vma = find_mergeable_anon_vma(vma);
-		if (anon_vma) {
-			allocated = NULL;
-			locked = anon_vma;
-			spin_lock(&locked->lock);
-		} else {
+		allocated = NULL;
+		if (!anon_vma) {
 			anon_vma = anon_vma_alloc();
 			if (unlikely(!anon_vma))
 				return -ENOMEM;
 			allocated = anon_vma;
-			locked = NULL;
 		}
+		spin_lock(&anon_vma->lock);
 
 		/* page_table_lock to protect against threads */
 		spin_lock(&mm->page_table_lock);
@@ -87,8 +110,7 @@ int anon_vma_prepare(struct vm_area_struct *vma)
 		}
 		spin_unlock(&mm->page_table_lock);
 
-		if (locked)
-			spin_unlock(&locked->lock);
+		spin_unlock(&anon_vma->lock);
 		if (unlikely(allocated))
 			anon_vma_free(allocated);
 	}