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? -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
