Re: [PATCH v2 06/11] memcg: kmem controller infrastructure

[Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

>>> Perhaps we're just trying to take a conservative initial implementation
>>> which is consistent with user visible pages.
>>>
>>
>> The way I see it, is not about being conservative, but rather about my
>> physical safety. It is quite easy and natural to assume that "all
>> modifications to page cgroup are done under lock". So someone modifying
>> this later will likely find out about this exception in a rather
>> unpleasant way. They know where I live, and guns for hire are everywhere.
>>
>> Note that it is not unreasonable to believe that we can modify this
>> later. This can be a way out, for example, for the memcg lifecycle problem.
>>
>> I agree with your analysis and we can ultimately remove it, but if we
>> cannot pinpoint any performance problems to here, maybe consistency
>> wins. Also, the locking operation itself is a bit expensive, but the
>> biggest price is the actual contention. If we'll have nobody contending
>> for the same page_cgroup, the problem - if exists - shouldn't be that
>> bad. And if we ever have, the lock is needed.
> 
> Sounds reasonable. Another reason we might have to eventually revisit
> this lock is the fact that lock_page_cgroup() is not generally irq_safe.
> I assume that slab pages may be freed in softirq and would thus (in an
> upcoming patch series) call __memcg_kmem_free_page.  There are a few
> factors that might make it safe to grab this lock here (and below in
> __memcg_kmem_free_page) from hard/softirq context:
> * the pc lock is a per page bit spinlock.  So we only need to worry
>   about interrupting a task which holds the same page's lock to avoid
>   deadlock.
> * for accounted kernel pages, I am not aware of other code beyond
>   __memcg_kmem_charge_page and __memcg_kmem_free_page which grab pc
>   lock.  So we shouldn't find __memcg_kmem_free_page() called from a
>   context which interrupted a holder of the page's pc lock.
> 

All very right.