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Date:   Tue, 5 Dec 2017 19:02:00 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     axboe@...nel.dk, oleg@...hat.com, bcrl@...ck.org, tj@...nel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-aio@...ck.org, jmoyer@...hat.com
Subject: Re: [PATCH] aio: Add memcg accounting of user used data

On 05.12.2017 18:43, Michal Hocko wrote:
> On Tue 05-12-17 18:34:59, Kirill Tkhai wrote:
>> On 05.12.2017 18:15, Michal Hocko wrote:
>>> On Tue 05-12-17 13:00:54, Kirill Tkhai wrote:
>>>> Currently, number of available aio requests may be
>>>> limited only globally. There are two sysctl variables
>>>> aio_max_nr and aio_nr, which implement the limitation
>>>> and request accounting. They help to avoid
>>>> the situation, when all the memory is eaten in-flight
>>>> requests, which are written by slow block device,
>>>> and which can't be reclaimed by shrinker.
>>>>
>>>> This meets the problem in case of many containers
>>>> are used on the hardware node. Since aio_max_nr is
>>>> a global limit, any container may occupy the whole
>>>> available aio requests, and to deprive others the
>>>> possibility to use aio at all. The situation may
>>>> happen because of evil intentions of the container's
>>>> user or because of the program error, when the user
>>>> makes this occasionally
>>>>
>>>> The patch allows to fix the problem. It adds memcg
>>>> accounting of user used aio data (the biggest is
>>>> the bunch of aio_kiocb; ring buffer is the second
>>>> biggest), so a user of a certain memcg won't be able
>>>> to allocate more aio requests memory, then the cgroup
>>>> allows, and he will bumped into the limit.
>>>
>>> So what happens when we hit the hard limit and oom kill somebody?
>>> Are those charged objects somehow bound to a process context?
>>
>> There is exit_aio() called from __mmput(), which waits till
>> the charged objects complete and decrement reference counter.
> 
> OK, so it is bound to _a_ process context. The oom killer will not know
> about which process has consumed those objects but the effect will be at
> least reduced to a memcg.
> 
>> If there was a problem with oom in memcg, there would be
>> the same problem on global oom, as it can be seen there is
>> no __GFP_NOFAIL flags anywhere in aio code.
>>
>> But it seems everything is safe.
> 
> Could you share your testing scenario and the way how the system behaved
> during a heavy aio?
> 
> I am not saying the patch is wrong, I am just trying to undestand all
> the consequences.

My test is simple program, which creates aio context and then starts
infinity io_submit() cycle. I've tested the cases, when certain stages
fail: io_setup() meets oom, io_submit() meets oom, io_getevents() meets
oom. This was simply tested by inserting sleep() before the stage, and
moving the task to appropriate cgroup with low memory limit. The most
cases, I get bash killed (I moved it to cgroup too). Also, I've executed
the test in parallel.

If you want I can send you the source code, but I don't think it will be
easy to use it if you are not the author.

Kirill

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