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Date:   Wed, 11 May 2022 15:06:56 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Michal =?iso-8859-1?Q?Koutn=FD?= <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
Message-ID: <YnwJUL90fuoHs3YW@cmpxchg.org>
References: <20220510152847.230957-1-hannes@cmpxchg.org>
 <20220510152847.230957-7-hannes@cmpxchg.org>
 <20220511173218.GB31592@blackbody.suse.cz>
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On Wed, May 11, 2022 at 07:32:18PM +0200, Michal Koutn? 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.