Re: [PATCH] mm: avoid bothering interrupted task when charge memcg in softirq

[Yafang Shao <laoar.shao@gmail.com>]

On Sun, Jul 15, 2018 at 10:10 AM, Yafang Shao <laoar.shao@gmail.com> wrote:
> On Sat, Jul 14, 2018 at 11:38 PM, Shakeel Butt <shakeelb@google.com> wrote:
>> On Sat, Jul 14, 2018 at 1:32 AM Yafang Shao <laoar.shao@gmail.com> 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.
>
> Pls. let me know if I miss something.
>
>

Maybe bellow change is better,
@@ -2123,6 +2123,9 @@ static int try_charge(struct mem_cgroup *memcg,
gfp_t gfp_mask,
                goto retry;
        }

+       if (!gfpflags_allow_blocking(gfp_mask))
+               goto nomem;
+
        /*
         * Unlike in global OOM situations, memcg is not in a physical
         * memory shortage.  Allow dying and OOM-killed tasks to
@@ -2146,9 +2149,6 @@ static int try_charge(struct mem_cgroup *memcg,
gfp_t gfp_mask,
        if (unlikely(task_in_memcg_oom(current)))
                goto nomem;

-       if (!gfpflags_allow_blocking(gfp_mask))
-               goto nomem;

Thanks
Yafang