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

[Shakeel Butt]

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.

If the network buffers is allocated through kmem interface, the
charging is bypassed altogether (through memcg_kmem_bypass()) for
interrupt context.

regards,
Shakeel