Re: [PATCH v2 6/6] zswap: memcg accounting

From: Johannes Weiner <hannes@cmpxchg.org>
To: "Michal Koutný" <mkoutny@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>, Roman Gushchin <guro@fb.com>,
	Shakeel Butt <shakeelb@google.com>,
	Seth Jennings <sjenning@redhat.com>,
	Dan Streetman <ddstreet@ieee.org>,
	Minchan Kim <minchan@kernel.org>,
	linux-mm@kvack.org, cgroups@vger.kernel.org,
	linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH v2 6/6] zswap: memcg accounting
Date: Wed, 11 May 2022 15:06:56 -0400	[thread overview]
Message-ID: <YnwJUL90fuoHs3YW@cmpxchg.org> (raw)
In-Reply-To: <20220511173218.GB31592@blackbody.suse.cz>

On Wed, May 11, 2022 at 07:32:18PM +0200, Michal Koutný wrote:
> Hello.
> 
> On Tue, May 10, 2022 at 11:28:47AM -0400, Johannes Weiner <hannes@cmpxchg.org> wrote:
> > +void obj_cgroup_charge_zswap(struct obj_cgroup *objcg, size_t size)
> > +{
> > +	struct mem_cgroup *memcg;
> > +
> > +	VM_WARN_ON_ONCE(!(current->flags & PF_MEMALLOC));
> > +
> > +	/* PF_MEMALLOC context, charging must succeed */
> > +	if (obj_cgroup_charge(objcg, GFP_KERNEL, size))
> > +		VM_WARN_ON_ONCE(1);
> 
> IIUC, the objcg is derived from the compressed page, i.e. same memcg
> (reparenting neglected for now). This memcg's memory.current is then
> charged with the compressed object size.

Correct. After which the uncompressed page is reclaimed and uncharged.
So the zswapout process will reduce the charge bottom line.

> Do I get it right that memory.zswap.current is a subset of memory.current?
>
> (And that zswap is limited both by memory.max and memory.zswap.max?)

Yes. Zswap is a memory consumer, and we want the compressed part of a
workload's memory to show up in the total memory footprint.

memory.zswap.* are there to configure zswap policy, within the
boundaries of available memory - it's by definition a subset.