linux-kernel - Re: [PATCH v6 17/19] mm: memcg/slab: use a single set of kmem

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Date:   Thu, 18 Jun 2020 09:33:08 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christoph Lameter <cl@...ux.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v6 17/19] mm: memcg/slab: use a single set of kmem_caches
 for all allocations

On 6/18/20 2:35 AM, Roman Gushchin wrote:
> On Wed, Jun 17, 2020 at 04:35:28PM -0700, Andrew Morton wrote:
>> On Mon, 8 Jun 2020 16:06:52 -0700 Roman Gushchin <guro@...com> wrote:
>> 
>> > Instead of having two sets of kmem_caches: one for system-wide and
>> > non-accounted allocations and the second one shared by all accounted
>> > allocations, we can use just one.
>> > 
>> > The idea is simple: space for obj_cgroup metadata can be allocated
>> > on demand and filled only for accounted allocations.
>> > 
>> > It allows to remove a bunch of code which is required to handle
>> > kmem_cache clones for accounted allocations. There is no more need
>> > to create them, accumulate statistics, propagate attributes, etc.
>> > It's a quite significant simplification.
>> > 
>> > Also, because the total number of slab_caches is reduced almost twice
>> > (not all kmem_caches have a memcg clone), some additional memory
>> > savings are expected. On my devvm it additionally saves about 3.5%
>> > of slab memory.
>> > 
>> 
>> This ran afoul of Vlastimil's "mm, slab/slub: move and improve
>> cache_from_obj()"
>> (http://lkml.kernel.org/r/20200610163135.17364-10-vbabka@suse.cz).  I
>> resolved things as below.  Not too sure about slab.c's
>> cache_from_obj()...
> 
> It can actually be as simple as:
> static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> {
> 	return s;
> }
> 
> But I wonder if we need it at all, or maybe we wanna rename it to
> something like obj_check_kmem_cache(void *obj, struct kmem_cache *s),
> because it has now only debug purposes.
> 
> Let me and Vlastimil figure it out and send a follow-up patch.
> Your version is definitely correct.

Well, Kees wants to restore the common version of cache_from_obj() [1] for SLAB
hardening.

To prevent all that back and forth churn entering git history, I think the best
is for me to send a -fix to my patch that is functionally same while keeping the
common function, and then this your patch should only have a minor conflict and
Kees can rebase his patches on top to become much smaller?

[1] https://lore.kernel.org/linux-mm/20200617195349.3471794-1-keescook@chromium.org/

> Thanks!
>