[PATCH v2 2/3] mm: prevent vm_area_struct::anon_name refcount saturation

[Suren Baghdasaryan]

A deep process chain with many vmas could grow really high.
With default sysctl_max_map_count (64k) and default pid_max (32k)
the max number of vmas in the system is 2147450880 and the
refcounter has headroom of 1073774592 before it reaches
REFCOUNT_SATURATED (3221225472). Therefore it's unlikely that
an anonymous name refcounter will overflow with these defaults.
Currently the max for pid_max is PID_MAX_LIMIT (4194304) and
for sysctl_max_map_count it's INT_MAX (2147483647). In this
configuration anon_vma_name refcount overflow becomes
theoretically possible (that still require heavy sharing of
that anon_vma_name between processes).
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 refcounter.
To ensure anon_vma_name refcount does not overflow, stop anon_vma_name
sharing when the refcount reaches REFCOUNT_MAX (2147483647), which
still leaves INT_MAX/2 (1073741823) values before the counter reaches
REFCOUNT_SATURATED. This should provide enough headroom for raising
the refcounts temporarily.

Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
changes in v2:
- Updated description to include calculation details, per Michal Hocko

 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 4bad32507570..f82085ff8a6b 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -161,15 +161,25 @@ static inline void anon_vma_name_put(struct anon_vma_name *anon_name)
 		kref_put(&anon_name->kref, anon_vma_name_free);
 }
 
+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) < REFCOUNT_MAX) {
+		anon_vma_name_get(anon_name);
+		return anon_name;
+
+	}
+	return anon_vma_name_alloc(anon_name->name);
+}
+
 static inline void dup_anon_vma_name(struct vm_area_struct *orig_vma,
 				     struct vm_area_struct *new_vma)
 {
 	struct anon_vma_name *anon_name = anon_vma_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_anon_vma_name(struct vm_area_struct *vma)
diff --git a/mm/madvise.c b/mm/madvise.c
index 081b1cded21e..1f2693dccf7b 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -113,8 +113,7 @@ static int replace_anon_vma_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