Re: [PATCH v6 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

["Michal Koutný" <mkoutny@suse.com>]

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On Mon, Apr 14, 2025 at 12:47:21PM -0400, Johannes Weiner <hannes@cmpxchg.org> wrote:
> It's not a functional change to the protection semantics or the
> reclaim behavior.

Yes, that's how I understand it, therefore I'm wondering what does it
change.

If this is taken:
               if (!mem_cgroup_usage(memcg, false))
                       continue;

this would've been taken too:
                if (mem_cgroup_below_min(target_memcg, memcg))
                        continue;
(unless target_memcg == memcg but that's not interesting for the events
here)

> The problem is if we go into low_reclaim and encounter an empty group,
> we'll issue "low-protected group is being reclaimed" events,

How can this happen when
	page_counter_read(&memcg->memory) <= memcg->memory.emin
? (I.e. in this case 0 <= emin and emin >= 0.)

> which is kind of absurd (nothing will be reclaimed) and thus confusing
> to users (I didn't even configure any protection!)

Yes.
 
> I suggested, instead of redefining the protection definitions for that
> special case, to bypass all the checks and the scan count calculations
> when we already know the group is empty and none of this applies.
> 
> https://lore.kernel.org/linux-mm/20250404181308.GA300138@cmpxchg.org/

Is this non-functional change to make shrink_node_memcgs() robust
against possible future redefinitions of mem_cgroup_below_*()?


Michal

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