Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches
From: Shakeel Butt <hidden>
Date: 2021-05-05 16:17:58
Also in:
linux-mm, lkml
On Wed, May 5, 2021 at 8:47 AM Waiman Long [off-list ref] 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 <redacted> Signed-off-by: Waiman Long <redacted>
One nit below and after incorporating Vlastimil's suggestions: Reviewed-by: Shakeel Butt <redacted>
quoted hunk ↗ jump to hunk
--- 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".