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

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Message-ID: <20200618003515.GB24694@carbon.DHCP.thefacebook.com>
Date:   Wed, 17 Jun 2020 17:35:15 -0700
From:   Roman Gushchin <guro@...com>
To:     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>,
        Vlastimil Babka <vbabka@...e.cz>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 17/19] mm: memcg/slab: use a single set of kmem_caches
 for all allocations

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.

Thanks!