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

[Andrey Ryabinin]

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.