Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

From: Waiman Long
Date: Wed May 05 2021 - 19:06:34 EST

On 5/5/21 6:19 PM, Roman Gushchin wrote:
On Wed, May 05, 2021 at 11:29:54PM +0200, Vlastimil Babka wrote:
On 5/5/21 8:32 PM, Roman Gushchin wrote:
On Wed, May 05, 2021 at 08:02:06PM +0200, Vlastimil Babka wrote:
On 5/5/21 7:30 PM, Roman Gushchin wrote:
On Wed, May 05, 2021 at 11:46:13AM -0400, Waiman Long wrote:
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.
Unfortunately the positive effect of this change will be likely
reversed by a lower utilization due to a larger number of caches.

Btw, I wonder if we also need a change in the slab caches merging procedure?
KMALLOC_NORMAL caches should not be merged with caches which can potentially
include accounted objects.
Good point. But looks like kmalloc* caches are extempt from all merging in
create_boot_cache() via

s->refcount = -1; /* Exempt from merging for now */
Wait, s->refcount is adjusted to 1 in create_kmalloc_cache() after calling
into create_boot_cache?
Hmm I missed that

Now I wonder why all kmalloc caches on my system have 0 aliases :)
cat /sys/kernel/slab/kmalloc-*/aliases
Yeah, I noticed it too, it's a good question. And I remember a case from
the past when it wasn't true (kmalloc-32 was shared with something else).

The criteria for cache merging require close to exact match in all attributes with a size difference of no more than sizeof(void *). So it is not easy to find a close match.