On Tue, Aug 23, 2022 at 2:36 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Mon 22-08-22 17:20:17, Yu Zhao wrote: > > On Mon, Aug 22, 2022 at 5:16 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Mon, 22 Aug 2022 16:59:29 -0600 Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > > > > > > > > @@ -4109,7 +4109,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned > > > > > > long start, unsigned long end, > > > > > > > > > > > > walk_pmd_range(&val, addr, next, args); > > > > > > > > > > > > - if (mm_is_oom_victim(args->mm)) > > > > > > + if (test_bit(MMF_OOM_REAP_QUEUED, &args->mm->flags)) > > > > > > return 1; > > > > > > > > > > > > /* a racy check to curtail the waiting time */ > > > > > > > > > > Oh. Why? What does this change do? > > > > > > > > The MMF_OOM_REAP_QUEUED flag is similar to the deleted MMF_OOM_VICTIM > > > > flag, but it's set at a later stage during an OOM kill. > > > > > > > > When either is set, the OOM reaper is probably already freeing the > > > > memory of this mm_struct, or at least it's going to. So there is no > > > > need to dwell on it in the reclaim path, hence not about correctness. > > > > > > Thanks. That sounds worthy of some code comments? > > > > Will do. Thanks. > > I would rather not see this abuse. I understand where you're coming from, however, I don't share this POV. I see it as cooperation -- the page reclaim and the oom/reaper can't (or at least shouldn't) operate in isolation. > You cannot really make any > assumptions about oom_reaper and how quickly it is going to free the > memory. Agreed. But here we are talking about heuristics, not dependencies on certain behaviors. Assume we are playing a guessing game: there are multiple mm_structs available for reclaim, would the oom-killed ones be more profitable on average? I'd say no, because I assume it's more likely than unlikely that the oom reaper is doing/to do its work. Note that the assumption is about likelihood, hence arguably valid. > If this is really worth it (and I have to say I doubt it) then > it should be a separate patch with numbers justifying it. I definitely can artificially create a test case that runs oom a few times per second, to prove this two-liner is beneficial to that scenario. Then there is the question how much it would benefit the real-world scenarios. I'd recommend keeping this two-liner if we still had mm_is_oom_victim(), because it's simple, clear and intuitive. With MMF_OOM_REAP_QUEUED, I don't have a strong opinion. Since you do, I'll just delete it.
