Re: [PATCH] memcg: make mem_cgroup_read_stat() unsigned

[Andrew Morton]

On Tue, 22 Sep 2015 15:16:32 -0700 Greg Thelen <gthelen@xxxxxxxxxx> wrote:

> mem_cgroup_read_stat() returns a page count by summing per cpu page
> counters.  The summing is racy wrt. updates, so a transient negative sum
> is possible.  Callers don't want negative values:
> - mem_cgroup_wb_stats() doesn't want negative nr_dirty or nr_writeback.
> - oom reports and memory.stat shouldn't show confusing negative usage.
> - tree_usage() already avoids negatives.
> 
> Avoid returning negative page counts from mem_cgroup_read_stat() and
> convert it to unsigned.

Someone please remind me why this code doesn't use the existing
percpu_counter library which solved this problem years ago.

>  	for_each_possible_cpu(cpu)

and which doesn't iterate across offlined CPUs.


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