Subject: Re: [PATCH 0/5] blkcg: Limit maximum number of aio requests available
 for cgroup
To: Jeff Moyer <jmoyer@redhat.com>
Cc: Tejun Heo <tj@kernel.org>, axboe@kernel.dk, bcrl@kvack.org,
        viro@zeniv.linux.org.uk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-aio@kvack.org, oleg@redhat.com
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 <6eaa11a6-a087-42ab-df65-9142b59bf726@virtuozzo.com>
 <x49a7yyw14n.fsf@segfault.boston.devel.redhat.com>
From: Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <707ca8fa-aee1-f068-b8ab-de5004d3d7ac@virtuozzo.com>
Date: Tue, 5 Dec 2017 02:14:54 +0300
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On 05.12.2017 01:59, Jeff Moyer wrote:
> 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.

A request of struct aio_kiocb type consumes much more memory, than
struct io_event does. Shouldn't we account it too?

Kirill