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Date: Wed, 3 Jan 2024 18:07:43 -0700
From: Yu Zhao <yuzhao@google.com>
To: Dan Schatzberg <schatzberg.dan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
	cgroups@vger.kernel.org, linux-mm@kvack.org,
	Yosry Ahmed <yosryahmed@google.com>, Michal Hocko <mhocko@suse.com>,
	David Rientjes <rientjes@google.com>, Chris Li <chrisl@kernel.org>,
	Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Jonathan Corbet <corbet@lwn.net>, Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Shakeel Butt <shakeelb@google.com>,
	Muchun Song <muchun.song@linux.dev>,
	David Hildenbrand <david@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Kefeng Wang <wangkefeng.wang@huawei.com>,
	Yue Zhao <findns94@gmail.com>, Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v6 2/2] mm: add swapiness= arg to memory.reclaim
Message-ID: <ZZYE36e0BFFzi0X3@google.com>
References: <20240103164841.2800183-1-schatzberg.dan@gmail.com>
 <20240103164841.2800183-3-schatzberg.dan@gmail.com>
 <CAOUHufZ-hTwdiy7eYgJWo=CHyPbdxTX60hxjPmwa9Ox6FXMYQQ@mail.gmail.com>
 <ZZWlT5wmDaMceSlQ@dschatzberg-fedora-PC0Y6AEN>
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In-Reply-To: <ZZWlT5wmDaMceSlQ@dschatzberg-fedora-PC0Y6AEN>

On Wed, Jan 03, 2024 at 01:19:59PM -0500, Dan Schatzberg wrote:
> On Wed, Jan 03, 2024 at 10:19:40AM -0700, Yu Zhao wrote:
> [...]
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index d91963e2d47f..394e0dd46b2e 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -92,6 +92,11 @@ struct scan_control {
> > >         unsigned long   anon_cost;
> > >         unsigned long   file_cost;
> > >
> > > +#ifdef CONFIG_MEMCG
> > > +       /* Swappiness value for proactive reclaim. Always use sc_swappiness()! */
> > > +       int *proactive_swappiness;
> > > +#endif
> > 
> > Why is proactive_swappiness still a pointer? The whole point of the
> > previous conversation is that sc->proactive can tell whether
> > sc->swappiness is valid or not, and that's less awkward than using a
> > pointer.
> 
> It's the same reason as before - zero initialization ensures that the
> pointer is NULL which tells us if it's valid or not. Proactive reclaim
> might not set swappiness and you need to distinguish swappiness of 0
> and not-set. See this discussion with Michal:
> 
> https://lore.kernel.org/linux-mm/ZZUizpTWOt3gNeqR@tiehlicka/

 static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
                              size_t nbytes, loff_t off)
 {
        struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
        unsigned int nr_retries = MAX_RECLAIM_RETRIES;
        unsigned long nr_to_reclaim, nr_reclaimed = 0;
+       int swappiness = -1;
...
                reclaimed = try_to_free_mem_cgroup_pages(memcg,
                                        min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
-                                       GFP_KERNEL, reclaim_options);
+                                       GFP_KERNEL, reclaim_options,
+                                       swappiness);

...

+static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
+{
+       return sc->proactive && sc->proactive_swappiness > -1 ?
+              sc->proactive_swappiness : mem_cgroup_swappiness(memcg);
+}