Messages in this thread | | | Subject | Re: [PATCH 0/2 v8] oom: capture unreclaimable slab info in oom message | From | "Yang Shi" <> | Date | Mon, 02 Oct 2017 23:40:11 +0800 |
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On 9/30/17 4:00 AM, Tetsuo Handa wrote: > Yang Shi wrote: >> On 9/28/17 1:45 PM, Tetsuo Handa wrote: >>> Yang Shi wrote: >>>> On 9/28/17 12:57 PM, Tetsuo Handa wrote: >>>>> Yang Shi wrote: >>>>>> On 9/27/17 9:36 PM, Tetsuo Handa wrote: >>>>>>> On 2017/09/28 6:46, Yang Shi wrote: >>>>>>>> Changelog v7 -> v8: >>>>>>>> * Adopted Michal’s suggestion to dump unreclaim slab info when unreclaimable slabs amount > total user memory. Not only in oom panic path. >>>>>>> >>>>>>> Holding slab_mutex inside dump_unreclaimable_slab() was refrained since V2 >>>>>>> because there are >>>>>>> >>>>>>> mutex_lock(&slab_mutex); >>>>>>> kmalloc(GFP_KERNEL); >>>>>>> mutex_unlock(&slab_mutex); >>>>>>> >>>>>>> users. If we call dump_unreclaimable_slab() for non OOM panic path, aren't we >>>>>>> introducing a risk of crash (i.e. kernel panic) for regular OOM path? >>>>>> >>>>>> I don't see the difference between regular oom path and oom path other >>>>>> than calling panic() at last. >>>>>> >>>>>> And, the slab dump may be called by panic path too, it is for both >>>>>> regular and panic path. >>>>> >>>>> Calling a function that might cause kerneloops immediately before calling panic() >>>>> would be tolerable, for the kernel will panic after all. But calling a function >>>>> that might cause kerneloops when there is no plan to call panic() is a bug. >>>> >>>> I got your point. slab_mutex is used to protect the list of all the >>>> slabs, since we are already in oom, there should be not kmem cache >>>> destroy happen during the list traverse. And, list_for_each_entry() has >>>> been replaced to list_for_each_entry_safe() to make the traverse more >>>> robust. >>> >>> I consider that OOM event and kmem chache destroy event can run concurrently >>> because slab_mutex is not held by OOM event (and unfortunately cannot be held >>> due to possibility of deadlock) in order to protect the list of all the slabs. >>> >>> I don't think replacing list_for_each_entry() with list_for_each_entry_safe() >>> makes the traverse more robust, for list_for_each_entry_safe() does not defer >>> freeing of memory used by list element. Rather, replacing list_for_each_entry() >>> with list_for_each_entry_rcu() (and making relevant changes such as >>> rcu_read_lock()/rcu_read_unlock()/synchronize_rcu()) will make the traverse safe. >> >> I'm not sure if rcu could satisfy this case. rcu just can protect >> slab_caches_to_rcu_destroy list, which is used by SLAB_TYPESAFE_BY_RCU >> slabs. > > I'm not sure why you are talking about SLAB_TYPESAFE_BY_RCU. > What I meant is that > > Upon registration: > > // do initialize/setup stuff here > synchronize_rcu(); // <= for dump_unreclaimable_slab() > list_add_rcu(&kmem_cache->list, &slab_caches); > > Upon unregistration: > > list_del_rcu(&kmem_cache->list); > synchronize_rcu(); // <= for dump_unreclaimable_slab() > // do finalize/cleanup stuff here > > then (if my understanding is correct) > > rcu_read_lock(); > list_for_each_entry_rcu(s, &slab_caches, list) { > if (!is_root_cache(s) || (s->flags & SLAB_RECLAIM_ACCOUNT)) > continue; > > memset(&sinfo, 0, sizeof(sinfo)); > get_slabinfo(s, &sinfo); > > if (sinfo.num_objs > 0) > pr_info("%-17s %10luKB %10luKB\n", cache_name(s), > (sinfo.active_objs * s->size) / 1024, > (sinfo.num_objs * s->size) / 1024); > } > rcu_read_unlock(); > > will make dump_unreclaimable_slab() safe.
Thanks for the detailed description. However, it sounds this change is too much for slub, I'm not sure if this may change the subtle behavior of slub.
trylock sounds like a good alternative.
Yang
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