Re: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes

[Andrey Ryabinin]

On 01/12/2018 03:21 AM, Andrew Morton wrote:
> On Thu, 11 Jan 2018 14:59:23 +0300 Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
> 
>> On 01/11/2018 01:31 AM, Andrew Morton wrote:
>>> On Wed, 10 Jan 2018 15:43:17 +0300 Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>>>
>>>> mem_cgroup_resize_[memsw]_limit() tries to free only 32 (SWAP_CLUSTER_MAX)
>>>> pages on each iteration. This makes practically impossible to decrease
>>>> limit of memory cgroup. Tasks could easily allocate back 32 pages,
>>>> so we can't reduce memory usage, and once retry_count reaches zero we return
>>>> -EBUSY.
>>>>
>>>> Easy to reproduce the problem by running the following commands:
>>>>
>>>>   mkdir /sys/fs/cgroup/memory/test
>>>>   echo $$ >> /sys/fs/cgroup/memory/test/tasks
>>>>   cat big_file > /dev/null &
>>>>   sleep 1 && echo $((100*1024*1024)) > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
>>>>   -bash: echo: write error: Device or resource busy
>>>>
>>>> Instead of relying on retry_count, keep retrying the reclaim until
>>>> the desired limit is reached or fail if the reclaim doesn't make
>>>> any progress or a signal is pending.
>>>>
>>>
>>> Is there any situation under which that mem_cgroup_resize_limit() can
>>> get stuck semi-indefinitely in a livelockish state?  It isn't very
>>> obvious that we're protected from this, so perhaps it would help to
>>> have a comment which describes how loop termination is assured?
>>>
>>
>> We are not protected from this. If tasks in cgroup *indefinitely* generate reclaimable memory at high rate
>> and user asks to set unreachable limit, like 'echo 4096 > memory.limit_in_bytes', than
>> try_to_free_mem_cgroup_pages() will return non-zero indefinitely.
>>
>> Is that a big deal? At least loop can be interrupted by a signal, and we don't hold any locks here.
> 
> It may be better to detect this condition, give up and return an error?
> 

That's basically what how v1 worked, "if (curusage >= oldusage)" used to be
the way to detect this potential livelock.
So we can just go back to it?