Re: [PATCH] mm/rmap: convert anon_vma.refcount from atomic_t to refcount_t

[Andrew Morton]

On Thu, 11 Mar 2021 13:56:15 +0800 Yejune Deng  wrote:

> refcount_t type should be used instead of atomic_t when the variable
> is used as a reference counter. This is because the implementation of
> refcount_t can prevent overflows and detect possible use-after-free.

The use of refcount_t comes at a cost:

q:/usr/src/25> size mm/rmap.o
   textdata bss dec hex filename
  311142147  32   33293820d mm/rmap.o
  315582147  32   3373783c9 mm/rmap.o

That's a bunch more instructions to execute on some fairly hot
code paths.

I guess the debugging/checking features are nice, but this is pretty
mature code.  I'm quite unsure that this tradeoff is a favorable one.

[PATCH] mm/rmap: convert anon_vma.refcount from atomic_t to refcount_t

[Yejune Deng]

refcount_t type should be used instead of atomic_t when the variable
is used as a reference counter. This is because the implementation of
refcount_t can prevent overflows and detect possible use-after-free.

Signed-off-by: Yejune Deng 
---
 include/linux/rmap.h |  7 ---
 mm/rmap.c| 14 +++---
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index def5c62c93b3..0d6dfea7afa6 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -11,6 +11,7 @@
 #include 
 #include 
 #include 
+#include 
 
 /*
  * The anon_vma heads a list of private "related" vmas, to scan if
@@ -36,7 +37,7 @@ struct anon_vma {
 * the reference is responsible for clearing up the
 * anon_vma if they are the last user on release
 */
-   atomic_t refcount;
+   refcount_t refcount;
 
/*
 * Count of child anon_vmas and VMAs which points to this anon_vma.
@@ -103,14 +104,14 @@ enum ttu_flags {
 #ifdef CONFIG_MMU
 static inline void get_anon_vma(struct anon_vma *anon_vma)
 {
-   atomic_inc(&anon_vma->refcount);
+   refcount_inc(&anon_vma->refcount);
 }
 
 void __put_anon_vma(struct anon_vma *anon_vma);
 
 static inline void put_anon_vma(struct anon_vma *anon_vma)
 {
-   if (atomic_dec_and_test(&anon_vma->refcount))
+   if (refcount_dec_and_test(&anon_vma->refcount))
__put_anon_vma(anon_vma);
 }
 
diff --git a/mm/rmap.c b/mm/rmap.c
index b0fc27e77d6d..6aea12f3ac5c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -88,7 +88,7 @@ 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);
+   refcount_set(&anon_vma->refcount, 1);
anon_vma->degree = 1;   /* Reference for first vma */
anon_vma->parent = anon_vma;
/*
@@ -103,7 +103,7 @@ static inline struct anon_vma *anon_vma_alloc(void)
 
 static inline void anon_vma_free(struct anon_vma *anon_vma)
 {
-   VM_BUG_ON(atomic_read(&anon_vma->refcount));
+   VM_BUG_ON(refcount_read(&anon_vma->refcount));
 
/*
 * Synchronize against page_lock_anon_vma_read() such that
@@ -445,7 +445,7 @@ static void anon_vma_ctor(void *data)
struct anon_vma *anon_vma = data;
 
init_rwsem(&anon_vma->rwsem);
-   atomic_set(&anon_vma->refcount, 0);
+   refcount_set(&anon_vma->refcount, 0);
anon_vma->rb_root = RB_ROOT_CACHED;
 }
 
@@ -495,7 +495,7 @@ struct anon_vma *page_get_anon_vma(struct page *page)
goto out;
 
anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
-   if (!atomic_inc_not_zero(&anon_vma->refcount)) {
+   if (!refcount_inc_not_zero(&anon_vma->refcount)) {
anon_vma = NULL;
goto out;
}
@@ -554,7 +554,7 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
}
 
/* trylock failed, we got to sleep */
-   if (!atomic_inc_not_zero(&anon_vma->refcount)) {
+   if (!refcount_inc_not_zero(&anon_vma->refcount)) {
anon_vma = NULL;
goto out;
}
@@ -569,7 +569,7 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
rcu_read_unlock();
anon_vma_lock_read(anon_vma);
 
-   if (atomic_dec_and_test(&anon_vma->refcount)) {
+   if (refcount_dec_and_test(&anon_vma->refcount)) {
/*
 * Oops, we held the last refcount, release the lock
 * and bail -- can't simply use put_anon_vma() because
@@ -1810,7 +1810,7 @@ void __put_anon_vma(struct anon_vma *anon_vma)
struct anon_vma *root = anon_vma->root;
 
anon_vma_free(anon_vma);
-   if (root != anon_vma && atomic_dec_and_test(&root->refcount))
+   if (root != anon_vma && refcount_dec_and_test(&root->refcount))
anon_vma_free(root);
 }
 
-- 
2.29.0