Re: [PATCH 4/9] memcg: dirty page accounting support routines

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 28 Feb 2012 22:00:26 +0800
Fengguang Wu <fengguang.wu@intel.com> wrote:

> From: Greg Thelen <gthelen@google.com>
> 
> Added memcg dirty page accounting support routines.  These routines are
> used by later changes to provide memcg aware writeback and dirty page
> limiting.  A mem_cgroup_dirty_info() tracepoint is is also included to
> allow for easier understanding of memcg writeback operation.
> 
> ...
>
> +/*
> + * Return the number of additional pages that the @memcg cgroup could allocate.
> + * If use_hierarchy is set, then this involves checking parent mem cgroups to
> + * find the cgroup with the smallest free space.
> + */

Comment needs revisting - use_hierarchy does not exist.

> +static unsigned long
> +mem_cgroup_hierarchical_free_pages(struct mem_cgroup *memcg)
> +{
> +	u64 free;
> +	unsigned long min_free;
> +
> +	min_free = global_page_state(NR_FREE_PAGES);
> +
> +	while (memcg) {
> +		free = mem_cgroup_margin(memcg);
> +		min_free = min_t(u64, min_free, free);
> +		memcg = parent_mem_cgroup(memcg);
> +	}
> +
> +	return min_free;
> +}
> +
> +/*
> + * mem_cgroup_page_stat() - get memory cgroup file cache statistics
> + * @memcg:     memory cgroup to query
> + * @item:      memory statistic item exported to the kernel
> + *
> + * Return the accounted statistic value.
> + */
> +unsigned long mem_cgroup_page_stat(struct mem_cgroup *memcg,
> +				   enum mem_cgroup_page_stat_item item)
> +{
> +	struct mem_cgroup *iter;
> +	s64 value;
> +
> +	/*
> +	 * If we're looking for dirtyable pages we need to evaluate free pages
> +	 * depending on the limit and usage of the parents first of all.
> +	 */
> +	if (item == MEMCG_NR_DIRTYABLE_PAGES)
> +		value = mem_cgroup_hierarchical_free_pages(memcg);
> +	else
> +		value = 0;
> +
> +	/*
> +	 * Recursively evaluate page statistics against all cgroup under
> +	 * hierarchy tree
> +	 */
> +	for_each_mem_cgroup_tree(iter, memcg)
> +		value += mem_cgroup_local_page_stat(iter, item);

What's the locking rule for for_each_mem_cgroup_tree()?  It's unobvious
from the code and isn't documented?

> +	/*
> +	 * Summing of unlocked per-cpu counters is racy and may yield a slightly
> +	 * negative value.  Zero is the only sensible value in such cases.
> +	 */
> +	if (unlikely(value < 0))
> +		value = 0;
> +
> +	return value;
> +}
> +
>
> ...
>

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