On Sun, Jul 15, 2018 at 1:02 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote: > > On Sun, Jul 15, 2018 at 2:34 PM, Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > On Sat, Jul 14, 2018 at 10:26 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote: > >> > >> On Sun, Jul 15, 2018 at 12:25 PM, Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > >> > On Sat, Jul 14, 2018 at 7:10 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote: > >> >> > >> >> On Sat, Jul 14, 2018 at 11:38 PM, Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > >> >> > On Sat, Jul 14, 2018 at 1:32 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote: > >> >> >> > >> >> >> try_charge maybe executed in packet receive path, which is in interrupt > >> >> >> context. > >> >> >> In this situation, the 'current' is the interrupted task, which may has > >> >> >> no relation to the rx softirq, So it is nonsense to use 'current'. > >> >> >> > >> >> > > >> >> > Have you actually seen this occurring? > >> >> > >> >> Hi Shakeel, > >> >> > >> >> I'm trying to produce this issue, but haven't find it occur yet. > >> >> > >> >> > I am not very familiar with the > >> >> > network code but I can think of two ways try_charge() can be called > >> >> > from network code. Either through kmem charging or through > >> >> > mem_cgroup_charge_skmem() and both locations correctly handle > >> >> > interrupt context. > >> >> > > >> >> > >> >> Why do you say that mem_cgroup_charge_skmem() correctly hanle > >> >> interrupt context ? > >> >> > >> >> Let me show you why mem_cgroup_charge_skmem isn't hanling interrupt > >> >> context correctly. > >> >> > >> >> mem_cgroup_charge_skmem() is calling try_charge() twice. > >> >> The first one is with GFP_NOWAIT as the gfp_mask, and the second one > >> >> is with (GFP_NOWAIT | __GFP_NOFAIL) as the gfp_mask. > >> >> > >> >> If page_counter_try_charge() failes at the first time, -ENOMEM is returned. > >> >> Then mem_cgroup_charge_skmem() will call try_charge() once more with > >> >> __GFP_NOFAIL set, and this time if If page_counter_try_charge() failes > >> >> again the ' > >> >> force' label in try_charge() will be executed and 0 is returned. > >> >> > >> >> No matter what, the 'current' will be used and touched, that is > >> >> meaning mem_cgroup_charge_skmem() isn't hanling the interrupt context > >> >> correctly. > >> >> > >> > > >> > Hi Yafang, > >> > > >> > If you check mem_cgroup_charge_skmem(), the memcg passed is not > >> > 'current' but is from the sock object i.e. sk->sk_memcg for which the > >> > network buffer is allocated for. > >> > > >> > >> That's correct, the memcg if from the sock object. > >> But the point is, in this situation why 'current' is used in try_charge() ? > >> As 'current' is not related with the memcg, which is just a interrupted task. > >> > > > > Hmm so you mean the behavior of memcg charging in the interrupt > > context depends on the state of the interrupted task. > > Yes. > > > As you have > > noted, mem_cgroup_charge_skmem() tries charging again with > > __GFP_NOFAIL and the charge succeeds. Basically the memcg charging by > > mem_cgroup_charge_skmem() will always succeed irrespective of the > > state of the interrupted task. However mem_cgroup_charge_skmem() can > > return true if the interrupted task was exiting or a fatal signal is > > pending or oom victim or reclaiming memory. Can you please explain why > > this is bad? > > > > Let me show you the possible issues cause by this behavoir. > 1. In mem_cgroup_oom(), some members in 'current' is set. > That means an innocent task will be in task_in_memcg_oom state. > But this task may be in a different memcg, I mean the memcg of > the 'current' may be differenct with the sk->sk_memcg. > Then when this innocent 'current' do try_charge it will hit "if > (unlikely(task_in_memcg_oom(current)))" and -ENOMEM is returned, > While there're maybe some free memory (or some memory could be freed ) > in the memcg of the innocent 'task'. > No memory will be freed as try_charge() is in interrupt context. > 2. If the interrupted task was exiting or a fatal signal is pending > or oom victim, > it will directly goto force and 0 is returned, and then > mem_cgroup_charge_skmem() will return true. > But mem_cgroup_charge_skmem() maybe need to try the second time > and return false. > > That are all unexpected behavoir. > Yes, this is inconsistent behavior. Can you explain how this will affect network traffic? Basically mem_cgroup_charge_skmem() was supposed to return false but sometime based on the interrupted task, mem_cgroup_charge_skmem() returns true. How is this behavior bad for network traffic? Please note that I am not against this patch. I just want that the motivation/reason behind it is very clear. thanks, Shakeel
