Simplify and comment on anon_vma re-use for anon_vma_prepare()

This changes the anon_vma reuse case to require that we only reuse
simple anon_vma's - ie the case when the vma only has a single anon_vma
associated with it.

This means that a reuse of an anon_vma from an adjacent vma will always
guarantee that both vma's are associated not only with the same
anon_vma, they will also have the same anon_vma chain (of just a single
entry in this case).

And since anon_vma re-use was the only case where the same anon_vma
might be associated with different chains of anon_vma's, we now have the
case that every vma that shares the same anon_vma will always also have
the same chain.  That makes it much easier to think about merging vma's
that share the same anon_vma's: you can always just drop the other
anon_vma chain in anon_vma_merge() since you know that they are always
identical.

This also splits up the function to validate the anon_vma re-use, and
adds a lot of commentary about the possible races.
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Borislav Petkov <bp@alien8.de> [ "That didn't fix it" ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -824,6 +824,61 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 	return NULL;
 }
 
+/*
+ * Rough compatbility check to quickly see if it's even worth looking
+ * at sharing an anon_vma.
+ *
+ * They need to have the same vm_file, and the flags can only differ
+ * in things that mprotect may change.
+ *
+ * NOTE! The fact that we share an anon_vma doesn't _have_ to mean that
+ * we can merge the two vma's. For example, we refuse to merge a vma if
+ * there is a vm_ops->close() function, because that indicates that the
+ * driver is doing some kind of reference counting. But that doesn't
+ * really matter for the anon_vma sharing case.
+ */
+static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct *b)
+{
+	return a->vm_end == b->vm_start &&
+		mpol_equal(vma_policy(a), vma_policy(b)) &&
+		a->vm_file == b->vm_file &&
+		!((a->vm_flags ^ b->vm_flags) & ~(VM_READ|VM_WRITE|VM_EXEC)) &&
+		b->vm_pgoff == a->vm_pgoff + ((b->vm_start - a->vm_start) >> PAGE_SHIFT);
+}
+
+/*
+ * Do some basic sanity checking to see if we can re-use the anon_vma
+ * from 'old'. The 'a'/'b' vma's are in VM order - one of them will be
+ * the same as 'old', the other will be the new one that is trying
+ * to share the anon_vma.
+ *
+ * NOTE! This runs with mm_sem held for reading, so it is possible that
+ * the anon_vma of 'old' is concurrently in the process of being set up
+ * by another page fault trying to merge _that_. But that's ok: if it
+ * is being set up, that automatically means that it will be a singleton
+ * acceptable for merging, so we can do all of this optimistically. But
+ * we do that ACCESS_ONCE() to make sure that we never re-load the pointer.
+ *
+ * IOW: that the "list_is_singular()" test on the anon_vma_chain only
+ * matters for the 'stable anon_vma' case (ie the thing we want to avoid
+ * is to return an anon_vma that is "complex" due to having gone through
+ * a fork).
+ *
+ * We also make sure that the two vma's are compatible (adjacent,
+ * and with the same memory policies). That's all stable, even with just
+ * a read lock on the mm_sem.
+ */
+static struct anon_vma *reusable_anon_vma(struct vm_area_struct *old, struct vm_area_struct *a, struct vm_area_struct *b)
+{
+	if (anon_vma_compatible(a, b)) {
+		struct anon_vma *anon_vma = ACCESS_ONCE(old->anon_vma);
+
+		if (anon_vma && list_is_singular(&old->anon_vma_chain))
+			return anon_vma;
+	}
+	return NULL;
+}
+
 /*
  * find_mergeable_anon_vma is used by anon_vma_prepare, to check
  * neighbouring vmas for a suitable anon_vma, before it goes off
@@ -834,28 +889,16 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
  */
 struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *vma)
 {
+	struct anon_vma *anon_vma;
 	struct vm_area_struct *near;
-	unsigned long vm_flags;
 
 	near = vma->vm_next;
 	if (!near)
 		goto try_prev;
 
-	/*
-	 * Since only mprotect tries to remerge vmas, match flags
-	 * which might be mprotected into each other later on.
-	 * Neither mlock nor madvise tries to remerge at present,
-	 * so leave their flags as obstructing a merge.
-	 */
-	vm_flags = vma->vm_flags & ~(VM_READ|VM_WRITE|VM_EXEC);
-	vm_flags |= near->vm_flags & (VM_READ|VM_WRITE|VM_EXEC);
-
-	if (near->anon_vma && vma->vm_end == near->vm_start &&
- 			mpol_equal(vma_policy(vma), vma_policy(near)) &&
-			can_vma_merge_before(near, vm_flags,
-				NULL, vma->vm_file, vma->vm_pgoff +
-				((vma->vm_end - vma->vm_start) >> PAGE_SHIFT)))
-		return near->anon_vma;
+	anon_vma = reusable_anon_vma(near, vma, near);
+	if (anon_vma)
+		return anon_vma;
 try_prev:
 	/*
 	 * It is potentially slow to have to call find_vma_prev here.
@@ -868,14 +911,9 @@ try_prev:
 	if (!near)
 		goto none;
 
-	vm_flags = vma->vm_flags & ~(VM_READ|VM_WRITE|VM_EXEC);
-	vm_flags |= near->vm_flags & (VM_READ|VM_WRITE|VM_EXEC);
-
-	if (near->anon_vma && near->vm_end == vma->vm_start &&
-  			mpol_equal(vma_policy(near), vma_policy(vma)) &&
-			can_vma_merge_after(near, vm_flags,
-				NULL, vma->vm_file, vma->vm_pgoff))
-		return near->anon_vma;
+	anon_vma = reusable_anon_vma(near, near, vma);
+	if (anon_vma)
+		return anon_vma;
 none:
 	/*
 	 * There's no absolute need to look only at touching neighbours: