linux-kernel - Re: [PATCH V4 2/2] mm: add swapiness= arg to memory.reclaim

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Date: Fri, 15 Dec 2023 09:50:04 +0100
From: Michal Hocko <mhocko@...e.com>
To: Dan Schatzberg <schatzberg.dan@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Yosry Ahmed <yosryahmed@...gle.com>, Huan Yang <link@...o.com>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org, Tejun Heo <tj@...nel.org>,
	Zefan Li <lizefan.x@...edance.com>,
	Jonathan Corbet <corbet@....net>,
	Shakeel Butt <shakeelb@...gle.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Matthew Wilcox <willy@...radead.org>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Yue Zhao <findns94@...il.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH V4 2/2] mm: add swapiness= arg to memory.reclaim

On Thu 14-12-23 13:22:29, Dan Schatzberg wrote:
> On Thu, Dec 14, 2023 at 09:38:55AM +0100, Michal Hocko wrote:
[...]
> > While the review can point those out it is quite easy to break and you
> > will only learn about that very indirectly. I think it would be easier
> > to review and maintain if you go with a pointer that would fallback to
> > mem_cgroup_swappiness() if NULL which will be the case for every
> > existing reclaimer except memory.reclaim with swappiness value.
> 
> I agree. My initial implementation used a pointer for this
> reason. I'll switch this back. Just to be clear - I still need to
> initialize scan_control.swappiness in all these other places right?

No. They will just get initialized to 0. All you need to make sure is
that the swappiness is used consistently. I would propose something like
scan_control_swappiness() (or sc_swappiness) which would actually do

	if (sc->swappiness)
		return sc->swappiness;
	return mem_cgroup_swappiness(sc->target_mem_cgroup);

and then make sure that mem_cgroup_swappiness is never used directly.

-- 
Michal Hocko
SUSE Labs