On 01/11/2018 01:31 AM, Andrew Morton wrote: > On Wed, 10 Jan 2018 15:43:17 +0300 Andrey Ryabinin <aryabi...@virtuozzo.com> > 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.
