Revert "Revert "mm/rmap: Fix anon_vma->degree ambiguity leading to double-reuse""

This reverts commit 4f35cec76058557d9eaec0d501d03c7657eb56b4 and does so
in an abi-safe way.

This is done by adding the new fields only to the end of the structure
and this structure is only passed around to other functions as a
pointer, the internal structure layout is only touched by the core
kernel, so adding it to the end is safe.

ABI differences manually updated:

Leaf changes summary: 1 artifact changed
Changed leaf types summary: 1 leaf type changed
Removed/Changed/Added functions summary: 0 Removed, 0 Changed, 0 Added function
Removed/Changed/Added variables summary: 0 Removed, 0 Changed, 0 Added variable

'struct anon_vma at rmap.h:29:1' changed:
  type size changed from 704 to 832 (in bits)
  2 data member insertions:
    'unsigned long int num_children', at offset 704 (in bits) at rmap.h:70:1
    'unsigned long int num_active_vmas', at offset 768 (in bits) at rmap.h:72:1
  761 impacted interfaces

Bug: 260678056
Bug: 253167854
Change-Id: Ib1d45625cbc2e0b21330ca3dc2aa7aff34666d31
Signed-off-by: Lee Jones <joneslee@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
(cherry picked from commit d3e1a50cba092fa9c56fc642ee74f360c4b40a17)

include/linux/rmap.h

@@ -38,13 +38,7 @@ struct anon_vma {
 	 */
 	atomic_t refcount;
 
-	/*
-	 * Count of child anon_vmas and VMAs which points to this anon_vma.
-	 *
-	 * This counter is used for making decision about reusing anon_vma
-	 * instead of forking new one. See comments in function anon_vma_clone.
-	 */
-	unsigned degree;
+	unsigned degree;		/* ANDROID: KABI preservation, DO NOT USE! */
 
 	struct anon_vma *parent;	/* Parent of this anon_vma */
 
@@ -59,6 +53,25 @@ struct anon_vma {
 
 	/* Interval tree of private "related" vmas */
 	struct rb_root_cached rb_root;
+
+	/*
+	 * ANDROID: KABI preservation, it's safe to put these at the end of this structure as it's
+	 * only passed by a pointer everywhere, the size and internal structures are local to the
+	 * core kernel.
+	 */
+#ifndef __GENKSYMS__
+	/*
+	 * Count of child anon_vmas. Equals to the count of all anon_vmas that
+	 * have ->parent pointing to this one, including itself.
+	 *
+	 * This counter is used for making decision about reusing anon_vma
+	 * instead of forking new one. See comments in function anon_vma_clone.
+	 */
+	unsigned long num_children;
+	/* Count of VMAs whose ->anon_vma pointer points to this object. */
+	unsigned long num_active_vmas;
+#endif
+
 };
 
 /*

mm/rmap.c

@@ -82,7 +82,8 @@ static inline struct anon_vma *anon_vma_alloc(void)
 	anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
 	if (anon_vma) {
 		atomic_set(&anon_vma->refcount, 1);
-		anon_vma->degree = 1;	/* Reference for first vma */
+		anon_vma->num_children = 0;
+		anon_vma->num_active_vmas = 0;
 		anon_vma->parent = anon_vma;
 		/*
 		 * Initialise the anon_vma root to point to itself. If called
@@ -190,6 +191,7 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
 		anon_vma = anon_vma_alloc();
 		if (unlikely(!anon_vma))
 			goto out_enomem_free_avc;
+		anon_vma->num_children++; /* self-parent link for new root */
 		allocated = anon_vma;
 	}
 
@@ -199,8 +201,7 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
 	if (likely(!vma->anon_vma)) {
 		vma->anon_vma = anon_vma;
 		anon_vma_chain_link(vma, avc, anon_vma);
-		/* vma reference or self-parent link for new root */
-		anon_vma->degree++;
+		anon_vma->num_active_vmas++;
 		allocated = NULL;
 		avc = NULL;
 	}
@@ -279,19 +280,19 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 		anon_vma_chain_link(dst, avc, anon_vma);
 
 		/*
-		 * Reuse existing anon_vma if its degree lower than two,
-		 * that means it has no vma and only one anon_vma child.
+		 * Reuse existing anon_vma if it has no vma and only one
+		 * anon_vma child.
 		 *
-		 * Do not chose parent anon_vma, otherwise first child
-		 * will always reuse it. Root anon_vma is never reused:
+		 * Root anon_vma is never reused:
 		 * it has self-parent reference and at least one child.
 		 */
-		if (!dst->anon_vma && anon_vma != src->anon_vma &&
-				anon_vma->degree < 2)
+		if (!dst->anon_vma && src->anon_vma &&
+		    anon_vma->num_children < 2 &&
+		    anon_vma->num_active_vmas == 0)
 			dst->anon_vma = anon_vma;
 	}
 	if (dst->anon_vma)
-		dst->anon_vma->degree++;
+		dst->anon_vma->num_active_vmas++;
 	unlock_anon_vma_root(root);
 	return 0;
 
@@ -341,6 +342,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
 	anon_vma = anon_vma_alloc();
 	if (!anon_vma)
 		goto out_error;
+	anon_vma->num_active_vmas++;
 	avc = anon_vma_chain_alloc(GFP_KERNEL);
 	if (!avc)
 		goto out_error_free_anon_vma;
@@ -361,7 +363,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
 	vma->anon_vma = anon_vma;
 	anon_vma_lock_write(anon_vma);
 	anon_vma_chain_link(vma, avc, anon_vma);
-	anon_vma->parent->degree++;
+	anon_vma->parent->num_children++;
 	anon_vma_unlock_write(anon_vma);
 
 	return 0;
@@ -393,7 +395,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 		 * to free them outside the lock.
 		 */
 		if (RB_EMPTY_ROOT(&anon_vma->rb_root.rb_root)) {
-			anon_vma->parent->degree--;
+			anon_vma->parent->num_children--;
 			continue;
 		}
 
@@ -401,7 +403,8 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 		anon_vma_chain_free(avc);
 	}
 	if (vma->anon_vma)
-		vma->anon_vma->degree--;
+		vma->anon_vma->num_active_vmas--;
+
 	unlock_anon_vma_root(root);
 
 	/*
@@ -412,7 +415,8 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
 		struct anon_vma *anon_vma = avc->anon_vma;
 
-		VM_WARN_ON(anon_vma->degree);
+		VM_WARN_ON(anon_vma->num_children);
+		VM_WARN_ON(anon_vma->num_active_vmas);
 		put_anon_vma(anon_vma);
 
 		list_del(&avc->same_vma);