On Thu 27-09-12 15:31:57, Glauber Costa wrote: > On 09/26/2012 07:51 PM, Michal Hocko wrote: > > On Tue 18-09-12 18:04:03, Glauber Costa wrote: [...] > >> + *_memcg = NULL; > >> + rcu_read_lock(); > >> + p = rcu_dereference(current->mm->owner); > >> + memcg = mem_cgroup_from_task(p); > > > > mem_cgroup_from_task says it can return NULL. Do we care here? If not > > then please put VM_BUG_ON(!memcg) here. > > > >> + rcu_read_unlock(); > >> + > >> + if (!memcg_can_account_kmem(memcg)) > >> + return true; > >> + > >> + mem_cgroup_get(memcg); > > > > I am confused. Why do we take a reference to memcg rather than css_get > > here? Ahh it is because we keep the reference while the page is > > allocated, right? Comment please. > ok. > > > > > I am still not sure whether we need css_get here as well. How do you > > know that the current is not moved in parallel and it is a last task in > > a group which then can go away? > > the reference count aquired by mem_cgroup_get will still prevent the > memcg from going away, no? Yes but you are outside of the rcu now and we usually do css_get before we rcu_unlock. mem_cgroup_get just makes sure the group doesn't get deallocated but it could be gone before you call it. Or I am just confused - these 2 levels of ref counting is really not nice. Anyway, I have just noticed that __mem_cgroup_try_charge does VM_BUG_ON(css_is_removed(&memcg->css)) on a given memcg so you should keep css ref count up as well. > >> + /* The page allocation failed. Revert */ > >> + if (!page) { > >> + memcg_uncharge_kmem(memcg, PAGE_SIZE << order); > >> + return; > >> + } > >> + > >> + pc = lookup_page_cgroup(page); > >> + lock_page_cgroup(pc); > >> + pc->mem_cgroup = memcg; > >> + SetPageCgroupUsed(pc); > >> + unlock_page_cgroup(pc); > >> +} > >> + > >> +void __memcg_kmem_uncharge_page(struct page *page, int order) > >> +{ > >> + struct mem_cgroup *memcg = NULL; > >> + struct page_cgroup *pc; > >> + > >> + > >> + pc = lookup_page_cgroup(page); > >> + /* > >> + * Fast unlocked return. Theoretically might have changed, have to > >> + * check again after locking. > >> + */ > >> + if (!PageCgroupUsed(pc)) > >> + return; > >> + > >> + lock_page_cgroup(pc); > >> + if (PageCgroupUsed(pc)) { > >> + memcg = pc->mem_cgroup; > >> + ClearPageCgroupUsed(pc); > >> + } > >> + unlock_page_cgroup(pc); > >> + > >> + /* > >> + * Checking if kmem accounted is enabled won't work for uncharge, since > >> + * it is possible that the user enabled kmem tracking, allocated, and > >> + * then disabled it again. > > > > disabling cannot happen, right? > > > not anymore, right. I can update the comment, yes, it is confusing > but I still believe it is a lot saner to trust information in > page_cgroup. I have no objections against that. PageCgroupUsed test and using pc->mem_cgroup is fine. > >> +#ifdef CONFIG_MEMCG_KMEM > >> +int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp, u64 size) > >> +{ > >> + struct res_counter *fail_res; > >> + struct mem_cgroup *_memcg; > >> + int ret; > >> + bool may_oom; > >> + bool nofail = false; > >> + > >> + may_oom = (gfp & __GFP_WAIT) && (gfp & __GFP_FS) && > >> + !(gfp & __GFP_NORETRY); > > > > A comment please? Why __GFP_IO is not considered for example? > > > > > > Actually, I believe testing for GFP_WAIT and !GFP_NORETRY would be enough. > > The rationale here is, of course, under which circumstance would it be > valid to call the oom killer? Which is, if the allocation can wait, and > can retry. Yes __GFP_WAIT is clear because memcg OOM can wait for arbitrary amount of time (wait for userspace action on oom_control). __GFP_NORETRY couldn't get to oom before because oom was excluded explicitely for THP and migration didn't go through the charging path to reach the oom. But I do agree that __GFP_NORETRY allocations shouldn't cause the OOM because we should rather fail the allocation from kernel rather than shoot something. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html