[PATCH 2/3] mm: prevent vm_area_struct::anon_name refcount saturation

[Suren Baghdasaryan <surenb@google.com>]

A deep process chain with many vmas could grow really high. kref
refcounting interface used in anon_vma_name structure will detect
a counter overflow when it reaches REFCOUNT_SATURATED value but will
only generate a warning about broken refcounting.
To ensure anon_vma_name refcount does not overflow, stop anon_vma_name
sharing when the refcount reaches INT_MAX, which still leaves INT_MAX/2
values before the counter reaches REFCOUNT_SATURATED. This should provide
enough headroom for raising the refcounts temporarily.

Suggested-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm_inline.h | 18 ++++++++++++++----
 mm/madvise.c              |  3 +--
 2 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 70b619442d56..b189e2638843 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -156,15 +156,25 @@ static inline void anon_vma_name_get(struct anon_vma_name *anon_name)
 
 extern void anon_vma_name_put(struct anon_vma_name *anon_name);
 
+static inline
+struct anon_vma_name *anon_vma_name_reuse(struct anon_vma_name *anon_name)
+{
+	/* Prevent anon_name refcount saturation early on */
+	if (kref_read(&anon_name->kref) < INT_MAX) {
+		anon_vma_name_get(anon_name);
+		return anon_name;
+
+	}
+	return anon_vma_name_alloc(anon_name->name);
+}
+
 static inline void dup_vma_anon_name(struct vm_area_struct *orig_vma,
 				     struct vm_area_struct *new_vma)
 {
 	struct anon_vma_name *anon_name = vma_anon_name(orig_vma);
 
-	if (anon_name) {
-		anon_vma_name_get(anon_name);
-		new_vma->anon_name = anon_name;
-	}
+	if (anon_name)
+		new_vma->anon_name = anon_vma_name_reuse(anon_name);
 }
 
 static inline void free_vma_anon_name(struct vm_area_struct *vma)
diff --git a/mm/madvise.c b/mm/madvise.c
index f81d62d8ce9b..a395884aeecb 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -122,8 +122,7 @@ static int replace_vma_anon_name(struct vm_area_struct *vma,
 	if (anon_vma_name_eq(orig_name, anon_name))
 		return 0;
 
-	anon_vma_name_get(anon_name);
-	vma->anon_name = anon_name;
+	vma->anon_name = anon_vma_name_reuse(anon_name);
 	anon_vma_name_put(orig_name);
 
 	return 0;
-- 
2.35.1.473.g83b2b277ed-goog