On Tue 21-01-14 13:18:42, Hugh Dickins wrote: [...] > We do have a confusing situation. The hang goes back to 3.10 but takes > two different forms, because of intervening changes: in 3.10 and 3.11 > mem_cgroup_iter repeatedly returns root memcg to its caller, in 3.12 and > 3.13 mem_cgroup_iter repeatedly gets NULL memcg from mem_cgroup_iter_next > and cannot return to its caller. > > Patch 1/2 is what's needed to fix 3.10 and 3.11 (and applies correctly > to 3.11, but will have to be rediffed for 3.10 because of rearrangement > in between). I will backport it when it reaches stable queue. > Patch 2/2 is what's needed to fix 3.12 and 3.13 (but applies > correctly to neither of them because it's diffed on top of my CSS_ONLINE > fix). Patch 1/2 is correct but unnecessary in 3.12 and 3.13: I'm unclear > whether Michal is claiming that it would also fix the hang in 3.12 and > 3.13 if we didn't have 2/2: I doubt that, and haven't tested that. Actually both patches are needed. If we had only 2/2 then we wouldn't endless loop inside mem_cgroup_iter but we could still return root to caller all the time because mem_cgroup_iter_load would return NULL on css_tryget failure on the cached root. Or am I missing something that would prevent that? > Given how Michal has diffed this patch on top of my CSS_ONLINE one > (mm-memcg-iteration-skip-memcgs-not-yet-fully-initialized.patch), > it would be helpful if you could mark that one also for stable 3.12+, > to save us from having to rediff this one for stable. We don't have > a concrete example of a problem it solves in the vanilla kernel, but > it makes more sense to include it than to exclude it. Yes, I think it makes sense to queue it for 3.12+ as well because it is non intrusive and potential issues would be really subtle. > (You would be right to point out that the CSS_ONLINE one fixes > something that goes back to 3.10: I'm saying 3.12+ because I'm not > motivated to rediff it for 3.10 and 3.11 when there's nothing to > go on top; but that's not a very good reason to lie - overrule me.) > > Hugh -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>