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

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Date:   Wed, 5 May 2021 11:18:47 -0700
From:   Roman Gushchin <guro@...com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Waiman Long <longman@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Shakeel Butt <shakeelb@...gle.com>,
        <linux-kernel@...r.kernel.org>, <cgroups@...r.kernel.org>,
        <linux-mm@...ck.org>
Subject: Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of
 kmalloc-cg-<n> caches

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 */

Oh, interesting... I wonder if there is (still) a good reason for that? Maybe
we can remove this limitation and save some memory?

> 
> It wouldn't hurt though to create the kmalloc-cg-* caches with SLAB_ACCOUNT flag
> to prevent accidental merging in case the above is ever removed. It would also
> better reflect reality, and ensure that the array is allocated immediately with
> the page, AFAICS.

That wouldn't be enough, because a !SLAB_ACCOUNT cache can still have accounted
allocations and be merged with kmalloc-* cache. What we might wanna do is to
keep the no-merging rule for kmalloc-*, but relax it for kmalloc-cg-* caches.

But we can do it later, as a separate change.

Thanks!