On 5/5/21 12:17 PM, Shakeel Butt wrote:
On Wed, May 5, 2021 at 8:47 AM Waiman Long <longman@xxxxxxxxxx> wrote:
There are currently two problems in the way the objcg pointer array
(memcg_data) in the page structure is being allocated and freed.
On its allocation, it is possible that the allocated objcg pointer
array comes from the same slab that requires memory accounting. If this
happens, the slab will never become empty again as there is at least
one object left (the obj_cgroup array) in the slab.
When it is freed, the objcg pointer array object may be the last one
in its slab and hence causes kfree() to be called again. With the
right workload, the slab cache may be set up in a way that allows the
recursive kfree() calling loop to nest deep enough to cause a kernel
stack overflow and panic the system.
One way to solve this problem is to split the kmalloc-<n> caches
(KMALLOC_NORMAL) into two separate sets - a new set of kmalloc-<n>
(KMALLOC_NORMAL) caches for non-accounted objects only and a new set of
kmalloc-cg-<n> (KMALLOC_CGROUP) caches for accounted objects only. All
the other caches can still allow a mix of accounted and non-accounted
objects.
With this change, all the objcg pointer array objects will come from
KMALLOC_NORMAL caches which won't have their objcg pointer arrays. So
both the recursive kfree() problem and non-freeable slab problem are
gone. Since both the KMALLOC_NORMAL and KMALLOC_CGROUP caches no longer
have mixed accounted and unaccounted objects, this will slightly reduce
the number of objcg pointer arrays that need to be allocated and save
a bit of memory.
The new KMALLOC_CGROUP is added between KMALLOC_NORMAL and
KMALLOC_RECLAIM so that the first for loop in create_kmalloc_caches()
will include the newly added caches without change.
Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
One nit below and after incorporating Vlastimil's suggestions:
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
---
include/linux/slab.h | 42 ++++++++++++++++++++++++++++++++++--------
mm/slab_common.c | 23 +++++++++++++++--------
2 files changed, 49 insertions(+), 16 deletions(-)
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 0c97d788762c..f2d9ebc34f5c 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -305,9 +305,16 @@ static inline void __check_heap_object(const void *ptr, unsigned long n,
/*
* Whenever changing this, take care of that kmalloc_type() and
* create_kmalloc_caches() still work as intended.
+ *
+ * KMALLOC_NORMAL is for non-accounted objects only whereas KMALLOC_CGROUP
+ * is for accounted objects only.
I think you can say "KMALLOC_CGROUP is for accounted and unreclaimable
objects only".
Thanks for the suggestion. Will incorporate that.
Cheers,
Longman
