Re: [PATCH] Add file based RSS accounting for memory resource controller (v3)

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 17 Apr 2009 19:48:38 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

>
> ...
>
> We currently don't track file RSS, the RSS we report is actually anon RSS.
> All the file mapped pages, come in through the page cache and get accounted
> there. This patch adds support for accounting file RSS pages. It should
> 
> 1. Help improve the metrics reported by the memory resource controller
> 2. Will form the basis for a future shared memory accounting heuristic
>    that has been proposed by Kamezawa.
> 
> Unfortunately, we cannot rename the existing "rss" keyword used in memory.stat
> to "anon_rss". We however, add "mapped_file" data and hope to educate the end
> user through documentation.
> 
> Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
>
> ...
>
> @@ -1096,6 +1135,10 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
>  	struct mem_cgroup_per_zone *from_mz, *to_mz;
>  	int nid, zid;
>  	int ret = -EBUSY;
> +	struct page *page;
> +	int cpu;
> +	struct mem_cgroup_stat *stat;
> +	struct mem_cgroup_stat_cpu *cpustat;
>  
>  	VM_BUG_ON(from == to);
>  	VM_BUG_ON(PageLRU(pc->page));
> @@ -1116,6 +1159,23 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
>  
>  	res_counter_uncharge(&from->res, PAGE_SIZE);
>  	mem_cgroup_charge_statistics(from, pc, false);
> +
> +	page = pc->page;
> +	if (page_is_file_cache(page) && page_mapped(page)) {
> +		cpu = smp_processor_id();
> +		/* Update mapped_file data for mem_cgroup "from" */
> +		stat = &from->stat;
> +		cpustat = &stat->cpustat[cpu];
> +		__mem_cgroup_stat_add_safe(cpustat, MEM_CGROUP_STAT_MAPPED_FILE,
> +						-1);
> +
> +		/* Update mapped_file data for mem_cgroup "to" */
> +		stat = &to->stat;
> +		cpustat = &stat->cpustat[cpu];
> +		__mem_cgroup_stat_add_safe(cpustat, MEM_CGROUP_STAT_MAPPED_FILE,
> +						1);
> +	}

This function (mem_cgroup_move_account()) does a trylock_page_cgroup()
and if that fails it will bale out, and the newly-added code will not
be executed.

What are the implications of this?  Does the missed accounting later get
performed somewhere, or does the error remain in place?

That trylock_page_cgroup() really sucks - trylocks usually do.  Could
someone please raise a patch which completely documents the reasons for
its presence, and for any other uncommented/unobvious trylocks?

Where appropriate, the comment should explain why the trylock isn't
simply a bug - why it is safe and correct to omit the operations which
we wished to perform.

Thanks.