Re: [PATCH v5 2/3] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

[Waiman Long]

On 5/12/21 10:51 AM, Waiman Long 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 unaccounted 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 unaccounted
> 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. On the other hand, creating a new set of kmalloc caches does
> have the effect of reducing cache utilization. So it is properly a wash.
>
> 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.
>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Acked-by: Roman Gushchin <guro@xxxxxx>
> ---
>   include/linux/slab.h | 42 +++++++++++++++++++++++++++++++++---------
>   mm/slab_common.c     | 25 +++++++++++++++++--------
>   2 files changed, 50 insertions(+), 17 deletions(-)

The following are the diff's from previous version. It turns out that 
the previous patch doesn't work if CONFIG_ZONE_DMA isn't defined.

diff --git a/include/linux/slab.h b/include/linux/slab.h
index a51cad5f561c..aa7f6c222a60 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -312,16 +312,17 @@ static inline void __check_heap_object(const void 
*ptr, un
signed long n,
  */
 enum kmalloc_cache_type {
     KMALLOC_NORMAL = 0,
-#ifdef CONFIG_MEMCG_KMEM
-    KMALLOC_CGROUP,
-#else
+#ifndef CONFIG_ZONE_DMA
+    KMALLOC_DMA = KMALLOC_NORMAL,
+#endif
+#ifndef CONFIG_MEMCG_KMEM
     KMALLOC_CGROUP = KMALLOC_NORMAL,
+#else
+    KMALLOC_CGROUP,
 #endif
     KMALLOC_RECLAIM,
 #ifdef CONFIG_ZONE_DMA
     KMALLOC_DMA,
-#else
-    KMALLOC_DMA = KMALLOC_NORMAL,
 #endif
     NR_KMALLOC_TYPES
 };

Cheers,
Longman