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
