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    SubjectRe: [PATCH -mmotm 4/5] memcg: dirty pages accounting and limiting infrastructure
    On Tue, Mar 16, 2010 at 10:11:50AM -0400, Vivek Goyal wrote:
    > On Tue, Mar 16, 2010 at 11:32:38AM +0900, Daisuke Nishimura wrote:
    >
    > [..]
    > > > + * mem_cgroup_page_stat() - get memory cgroup file cache statistics
    > > > + * @item: memory statistic item exported to the kernel
    > > > + *
    > > > + * Return the accounted statistic value, or a negative value in case of error.
    > > > + */
    > > > +s64 mem_cgroup_page_stat(enum mem_cgroup_read_page_stat_item item)
    > > > +{
    > > > + struct mem_cgroup_page_stat stat = {};
    > > > + struct mem_cgroup *mem;
    > > > +
    > > > + rcu_read_lock();
    > > > + mem = mem_cgroup_from_task(current);
    > > > + if (mem && !mem_cgroup_is_root(mem)) {
    > > > + /*
    > > > + * If we're looking for dirtyable pages we need to evaluate
    > > > + * free pages depending on the limit and usage of the parents
    > > > + * first of all.
    > > > + */
    > > > + if (item == MEMCG_NR_DIRTYABLE_PAGES)
    > > > + stat.value = memcg_get_hierarchical_free_pages(mem);
    > > > + /*
    > > > + * Recursively evaluate page statistics against all cgroup
    > > > + * under hierarchy tree
    > > > + */
    > > > + stat.item = item;
    > > > + mem_cgroup_walk_tree(mem, &stat, mem_cgroup_page_stat_cb);
    > > > + } else
    > > > + stat.value = -EINVAL;
    > > > + rcu_read_unlock();
    > > > +
    > > > + return stat.value;
    > > > +}
    > > > +
    > > hmm, mem_cgroup_page_stat() can return negative value, but you place BUG_ON()
    > > in [5/5] to check it returns negative value. What happens if the current is moved
    > > to root between mem_cgroup_has_dirty_limit() and mem_cgroup_page_stat() ?
    > > How about making mem_cgroup_has_dirty_limit() return the target mem_cgroup, and
    > > passing the mem_cgroup to mem_cgroup_page_stat() ?
    > >
    >
    > Hmm, if mem_cgroup_has_dirty_limit() retrun pointer to memcg, then one
    > shall have to use rcu_read_lock() and that will look ugly.
    >
    > Why don't we simply look at the return value and if it is negative, we
    > fall back to using global stats and get rid of BUG_ON()?

    I vote for this one. IMHO the caller of mem_cgroup_page_stat() should
    fallback to the equivalent global stats. This allows to keep the things
    separated and put in mm/memcontrol.c only the memcg stuff.

    >
    > Or, modify mem_cgroup_page_stat() to return global stats if it can't
    > determine per cgroup stat for some reason. (mem=NULL or root cgroup etc).
    >
    > Vivek

    Thanks,
    -Andrea


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