Messages in this thread | | | Subject | Re: [PATCH 0/2 v8] oom: capture unreclaimable slab info in oom message | From | "Yang Shi" <> | Date | Sat, 30 Sep 2017 06:15:10 +0800 |
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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.
Yang
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