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


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