On Wed, Dec 20, 2017 at 3:34 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > On Wed 20-12-17 14:32:19, Andrey Ryabinin wrote: >> On 12/20/2017 01:33 PM, Michal Hocko wrote: >> > On Wed 20-12-17 13:24:28, Andrey Ryabinin 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. >> >> >> >> It's 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 trying to free small amount of pages, it's much more >> >> reasonable to free 'usage - limit' pages. >> > >> > But that only makes the issue less probable. It doesn't fix it because >> > if (curusage >= oldusage) >> > retry_count--; >> > can still be true because allocator might be faster than the reclaimer. >> > Wouldn't it be more reasonable to simply remove the retry count and keep >> > trying until interrupted or we manage to update the limit. >> >> But does it makes sense to continue reclaiming even if reclaimer can't >> make any progress? I'd say no. "Allocator is faster than reclaimer" >> may be not the only reason for failed reclaim. E.g. we could try to >> set limit lower than amount of mlock()ed memory in cgroup, retrying >> reclaim would be just a waste of machine's resources. Or we simply >> don't have any swap, and anon > new_limit. Should be burn the cpu in >> that case? > > We can check the number of reclaimed pages and go EBUSY if it is 0. > >> > Another option would be to commit the new limit and allow temporal overcommit >> > of the hard limit. New allocations and the limit update paths would >> > reclaim to the hard limit. >> > >> >> It sounds a bit fragile and tricky to me. I wouldn't go that way >> without unless we have a very good reason for this. > > I haven't explored this, to be honest, so there may be dragons that way. > I've just mentioned that option for completness. > We already do this for cgroup-v2's memory.max. So, I don't think it is fragile or tricky. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
