On Tue, 16 Mar 2010 10:11:50 -0400
Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Tue, Mar 16, 2010 at 11:32:38AM +0900, Daisuke Nishimura wrote:
>
> [..]
> > > + * mem_cgroup_page_stat() - get memory cgroup file cache statistics
> > > + * @item: memory statistic item exported to the kernel
> > > + *
> > > + * Return the accounted statistic value, or a negative value in case of error.
> > > + */
> > > +s64 mem_cgroup_page_stat(enum mem_cgroup_read_page_stat_item item)
> > > +{
> > > + struct mem_cgroup_page_stat stat = {};
> > > + struct mem_cgroup *mem;
> > > +
> > > + rcu_read_lock();
> > > + mem = mem_cgroup_from_task(current);
> > > + if (mem && !mem_cgroup_is_root(mem)) {
> > > + /*
> > > + * 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)
> > > + stat.value = memcg_get_hierarchical_free_pages(mem);
> > > + /*
> > > + * Recursively evaluate page statistics against all cgroup
> > > + * under hierarchy tree
> > > + */
> > > + stat.item = item;
> > > + mem_cgroup_walk_tree(mem, &stat, mem_cgroup_page_stat_cb);
> > > + } else
> > > + stat.value = -EINVAL;
> > > + rcu_read_unlock();
> > > +
> > > + return stat.value;
> > > +}
> > > +
> > hmm, mem_cgroup_page_stat() can return negative value, but you place BUG_ON()
> > in [5/5] to check it returns negative value. What happens if the current is moved
> > to root between mem_cgroup_has_dirty_limit() and mem_cgroup_page_stat() ?
> > How about making mem_cgroup_has_dirty_limit() return the target mem_cgroup, and
> > passing the mem_cgroup to mem_cgroup_page_stat() ?
> >
>
> Hmm, if mem_cgroup_has_dirty_limit() retrun pointer to memcg, then one
> shall have to use rcu_read_lock() and that will look ugly.
>
agreed.
> Why don't we simply look at the return value and if it is negative, we
> fall back to using global stats and get rid of BUG_ON()?
>
> Or, modify mem_cgroup_page_stat() to return global stats if it can't
> determine per cgroup stat for some reason. (mem=NULL or root cgroup etc).
>
I don't have any objection as long as we don't hit BUG_ON.
Thanks,
Daisuke Nishimura.
> Vivek
>
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