Re: [PATCH 3/4] mm, memcg: reset memcg's memory.{min, low} for reclaiming itself

[Chris Down <chris@chrisdown.name>]

Hi Yafang,

Yafang Shao writes:
>memory.{emin, elow} are set in mem_cgroup_protected(), and the values of
>them won't be changed until next recalculation in this function. After
>either or both of them are set, the next reclaimer to relcaim this memcg
>may be a different reclaimer, e.g. this memcg is also the root memcg of
>the new reclaimer, and then in mem_cgroup_protection() in get_scan_count()
>the old values of them will be used to calculate scan count, that is not
>proper. We should reset them to zero in this case.

If the memcg in question is passed as "root" to mem_cgroup_protected with a 
child as the new "memcg" argument, then I still don't see what is wrong. 
mem_cgroup_protected must be called top-down from the root of the hierarchy in 
order to work already, which we already do in shrink_node_memcgs. This will 
already update the tree's cached effective protections properly, as far as I 
can see.

As such I'm not sure I understand what you mean in the changelog or in the 
patch. emin/elow as a mechanism is already intended to be racy/best-effort, 
since by the time we get to doing work it's always possible that reclaim 
eligibility state changed, and callers have to consider that.

Could you please explain further the situation you're trying to guard against? 
Thanks.

>Cc: Chris Down <chris@chrisdown.name>
>Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
>---
> mm/memcontrol.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
>diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>index f35fcca..234370c 100644
>--- a/mm/memcontrol.c
>+++ b/mm/memcontrol.c
>@@ -6287,8 +6287,17 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
>
> 	if (!root)
> 		root = root_mem_cgroup;
>-	if (memcg == root)
>+	if (memcg == root) {
>+		/*
>+		 * Reset memory.(emin, elow) for reclaiming the memcg
>+		 * itself.
>+		 */
>+		if (memcg != root_mem_cgroup) {
>+			memcg->memory.emin = 0;
>+			memcg->memory.emin = 0;
>+		}
> 		return MEMCG_PROT_NONE;
>+	}
>
> 	usage = page_counter_read(&memcg->memory);
> 	if (!usage)
>-- 
>1.8.3.1
>