linux-kernel - Re: [PATCH v2 6/6] zswap: memcg accounting

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Message-ID: <YnwJUL90fuoHs3YW@cmpxchg.org>
Date:   Wed, 11 May 2022 15:06:56 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH v2 6/6] zswap: memcg accounting

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@...xchg.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.