Re: [patch 2/8] mm: memcg-aware global reclaim

[Christoph Hellwig <hch@infradead.org>]

A few small nitpicks:

> +struct mem_cgroup *mem_cgroup_hierarchy_walk(struct mem_cgroup *root,
> +					     struct mem_cgroup *prev)
> +{
> +	struct mem_cgroup *mem;
> +
> +	if (mem_cgroup_disabled())
> +		return NULL;
> +
> +	if (!root)
> +		root = root_mem_cgroup;
> +	/*
> +	 * Even without hierarchy explicitely enabled in the root
> +	 * memcg, it is the ultimate parent of all memcgs.
> +	 */
> +	if (!(root == root_mem_cgroup || root->use_hierarchy))
> +		return root;

The logic here reads a bit weird, why not simply:

	 /*
	  * Even without hierarchy explicitely enabled in the root
	  * memcg, it is the ultimate parent of all memcgs.
	  */
	if (!root || root == root_mem_cgroup)
		return root_mem_cgroup;
	if (root->use_hierarchy)
		return root;


>  /*
>   * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
>   */
> -static void shrink_zone(int priority, struct zone *zone,
> -				struct scan_control *sc)
> +static void do_shrink_zone(int priority, struct zone *zone,
> +			   struct scan_control *sc)

It actually is the per-memcg shrinker now, and thus should be called
shrink_memcg.

> +		sc->mem_cgroup = mem;
> +		do_shrink_zone(priority, zone, sc);

Any passing the mem_cgroup explicitly instead of hiding it in the
scan_control would make that much more obvious.  If there's a good
reason to pass it in the structure the same probably applies to the
zone and priority, too.

Shouldn't we also have a non-cgroups stub of shrink_zone to directly
call do_shrink_zone/shrink_memcg with a NULL memcg and thus optimize
the whole loop away for it?

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