On Tue, 28 Feb 2012 14:45:07 -0800
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 28 Feb 2012 22:00:26 +0800
> Fengguang Wu <fengguang.wu@xxxxxxxxx> wrote:
>
> > From: Greg Thelen <gthelen@xxxxxxxxxx>
> >
> > 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?
>
Because for_each_mem_cgroup_tree() uses rcu_read_lock() and referernce counting
internally, it's not required to take any lock in callers.
One rule is the caller shoud call mem_cgroup_iter_break() if he want to break
the loop.
Thanks,
-Kame
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