Re: [PATCH v2 3/7] memcg: reduce memory for the lruvec and memcg stats

From: "T.J. Mercier" <tjmercier@google.com>
To: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	 Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	 Muchun Song <muchun.song@linux.dev>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/7] memcg: reduce memory for the lruvec and memcg stats
Date: Mon, 29 Apr 2024 15:23:28 -0700	[thread overview]
Message-ID: <CABdmKX0OQM65dRK29XCAUrnJt2yv=p7i00Cpc9V9FGuOEfuZ_g@mail.gmail.com> (raw)
In-Reply-To: <j4dsapl5hu2enzoq5c7y3z2bqksk6fpygkk5t45ok7d5v3gdt2@5ygpjaj2hiva>

On Mon, Apr 29, 2024 at 1:13 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Mon, Apr 29, 2024 at 10:35:38AM -0700, T.J. Mercier wrote:
> > On Fri, Apr 26, 2024 at 5:37 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > >
> [...]
> > > +
> > > +static const unsigned int memcg_stat_items[] = {
> > > +       MEMCG_SWAP,
> > > +       MEMCG_SOCK,
> > > +       MEMCG_PERCPU_B,
> > > +       MEMCG_VMALLOC,
> > > +       MEMCG_KMEM,
> > > +       MEMCG_ZSWAP_B,
> > > +       MEMCG_ZSWAPPED,
> > > +};
> >
> > Unsigned for these? All the values are positive now, but I don't think
> > we'll get a build warning if a negative one ever got added, just a
> > crash or corruption. BUG_ON in init_memcg_stats if a
> > memcg_stat_items[i] < 0?
>
> We are depending on NR_VM_NODE_STAT_ITEMS to tell the number of elements
> for vmstats. So, I think there is an implicit assumption that there are
> no negative enums in enum node_stat_item. So, if we want to verify those
> assumptions then we should be adding such warnings/build-bugs in vmstat
> first.

Ok fair. I guess this if we get C23:

enum node_stat_item : unsigned {