Re: [PATCH] mm, memcg: fix wrong mem cgroup protection

[Yafang Shao <laoar.shao@gmail.com>]

On Fri, Apr 24, 2020 at 8:18 PM Chris Down <chris@chrisdown.name> wrote:
>
> Yafang Shao writes:
> >If the author can't understand deeply in the code worte by
> >himself/herself, I think the author should do more test on his/her
> >patches.
> >Regarding the issue in this case, my understanding is you know the
> >benefit of proportional reclaim, but I'm wondering that do you know
> >the loss if the proportional is not correct ?
> >I don't mean to affend you, while I just try to explain how the
> >community should cooperate.
>
> I'm pretty sure that since multiple people on mm list have already expressed
> confusion at this patch, this isn't a question of testing, but of lack of
> clarity in usage :-)
>
> Promoting "testing" as a panacea for this issue misses a significant part of
> the real problem: that the intended semantics and room for allowed races is
> currently unclear, which is why there is a general sense of confusion around
> your proposed patch and what it solves. If more testing would help, then the
> benefit of your patch should be patently obvious -- but it isn't.

I have shown a testcase in my commit log.
Bellow is the result without my patch,

[  601.811428] vmscan: protection 1048576 memcg /foo target memcg /foo
[  601.811429] vmscan:
[  601.811429] vmscan: protection 1048576 memcg /foo target memcg /foo
[  601.811430] vmscan:
[  601.811430] vmscan: protection 1048576 memcg /foo target memcg /foo
[  601.811431] vmscan:
[  602.452791] vmscan: protection 1048576 memcg /foo target memcg /foo
[  602.452795] vmscan:
[  602.452796] vmscan: protection 1048576 memcg /foo target memcg /foo
[  602.452805] vmscan:
[  602.452805] vmscan: protection 1048576 memcg /foo target memcg /foo
[  602.452806] vmscan:
[  602.452807] vmscan: protection 1048576 memcg /foo target memcg /foo
[  602.452808] vmscan:


Here's patch to print the above info.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b06868fc4926..7525665d7cec 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2344,10 +2344,18 @@ static void get_scan_count(struct lruvec
*lruvec, struct scan_control *sc,
                unsigned long lruvec_size;
                unsigned long scan;
                unsigned long protection;
+               struct mem_cgroup *target = sc->target_mem_cgroup;

                lruvec_size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
                protection = mem_cgroup_protection(memcg,
                                                   sc->memcg_low_reclaim);
+               if (memcg && memcg != root_mem_cgroup && target) {
+                       pr_info("protection %lu memcg ", protection);
+                       pr_cont_cgroup_path(memcg->css.cgroup);
+                       pr_cont(" target memcg ");
+                       pr_cont_cgroup_path(target->css.cgroup);
+                       pr_info("\n");
+               }

                if (protection) {


So my question is that do you think the protection in these log is okay ?
Can you answer me ?

Hint: what should protection be if memcg is the target memcg ?

-- 
Thanks
Yafang