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



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