Re: [PATCH 0/5] blkcg: Limit maximum number of aio requests available for cgroup

[Jeff Moyer <jmoyer@redhat.com>]

Kirill Tkhai <ktkhai@virtuozzo.com> writes:

> On 05.12.2017 00:52, Tejun Heo wrote:
>> Hello, Kirill.
>> 
>> On Tue, Dec 05, 2017 at 12:44:00AM +0300, Kirill Tkhai wrote:
>>>> Can you please explain how this is a fundamental resource which can't
>>>> be controlled otherwise?
>>>
>>> Currently, aio_nr and aio_max_nr are global. In case of containers this
>>> means that a single container may occupy all aio requests, which are
>>> available in the system, and to deprive others possibility to use aio
>>> at all. This may happen because of evil intentions of the container's
>>> user or because of the program error, when the user makes this occasionally.
>> 
>> Hmm... I see.  It feels really wrong to me to make this a first class
>> resource because there is a system wide limit.  The only reason I can
>> think of for the system wide limit is to prevent too much kernel
>> memory consumed by creating a lot of aios but that squarely falls
>> inside cgroup memory controller protection.  If there are other
>> reasons why the number of aios should be limited system-wide, please
>> bring them up.
>>
>> If the only reason is kernel memory consumption protection, the only
>> thing we need to do is making sure that memory used for aio commands
>> are accounted against cgroup kernel memory consumption and
>> relaxing/removing system wide limit.
>
> So, we just use GFP_KERNEL_ACCOUNT flag for allocation of internal aio
> structures and pages, and all the memory will be accounted in kmem and
> limited by memcg. Looks very good.
>
> One detail about memory consumption. io_submit() calls primitives
> file_operations::write_iter and read_iter. It's not clear for me whether
> they consume the same memory as if writev() or readv() system calls
> would be used instead. writev() may delay the actual write till dirty
> pages limit will be reached, so it seems logic of the accounting should
> be the same. So aio mustn't use more not accounted system memory in file
> system internals, then simple writev().
>
> Could you please to say if you have thoughts about this?

I think you just need to account the completion ring.

Cheers,
Jeff