Re: [PATCH 4/6] memcg: simplify mem_cgroup_page_stat()

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Nov 12, 2010 at 12:40:22PM -0800, Greg Thelen wrote:
> Johannes Weiner <hannes@cmpxchg.org> writes:
> 
> > On Tue, Nov 09, 2010 at 01:24:29AM -0800, Greg Thelen wrote:
> >> The cgroup given to mem_cgroup_page_stat() is no allowed to be
> >> NULL or the root cgroup.  So there is no need to complicate the code
> >> handling those cases.
> >> 
> >> Signed-off-by: Greg Thelen <gthelen@google.com>
> >> ---
> >>  mm/memcontrol.c |   48 ++++++++++++++++++++++--------------------------
> >>  1 files changed, 22 insertions(+), 26 deletions(-)
> >> 
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index eb621ee..f8df350 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -1364,12 +1364,10 @@ memcg_hierarchical_free_pages(struct mem_cgroup *mem)
> >>  
> >>  /*
> >>   * mem_cgroup_page_stat() - get memory cgroup file cache statistics
> >> - * @mem:	optional memory cgroup to query.  If NULL, use current task's
> >> - *		cgroup.
> >> + * @mem:	memory cgroup to query
> >>   * @item:	memory statistic item exported to the kernel
> >>   *
> >> - * Return the accounted statistic value or negative value if current task is
> >> - * root cgroup.
> >> + * Return the accounted statistic value.
> >>   */
> >>  long mem_cgroup_page_stat(struct mem_cgroup *mem,
> >>  			  enum mem_cgroup_nr_pages_item item)
> >> @@ -1377,29 +1375,27 @@ long mem_cgroup_page_stat(struct mem_cgroup *mem,
> >>  	struct mem_cgroup *iter;
> >>  	long value;
> >>  
> >> +	VM_BUG_ON(!mem);
> >> +	VM_BUG_ON(mem_cgroup_is_root(mem));
> >> +
> >>  	get_online_cpus();
> >> -	rcu_read_lock();
> >> -	if (!mem)
> >> -		mem = mem_cgroup_from_task(current);
> >> -	if (__mem_cgroup_has_dirty_limit(mem)) {
> >
> > What about mem->use_hierarchy that is checked in
> > __mem_cgroup_has_dirty_limit()?  Is it no longer needed?
> 
> It is no longer needed because the callers of mem_cgroup_page_stat()
> call __mem_cgroup_has_dirty_limit().  In the current implementation, if
> use_hierarchy=1 then the cgroup does not have dirty limits, so calls
> into mem_cgroup_page_stat() are avoided.  Specifically the callers of
> mem_cgroup_page_stat() are:
> 
> 1. mem_cgroup_dirty_info() which calls __mem_cgroup_has_dirty_limit()
>    and returns false if use_hierarchy=1.
> 
> 2. throttle_vm_writeout() which calls mem_dirty_info() ->
>    mem_cgroup_dirty_info() -> __mem_cgroup_has_dirty_limit() will fall
>    back to global limits if use_hierarchy=1.

Thanks for the clarification.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>