On Sun, Oct 27, 2013 at 10:30:17AM -0700, Greg Thelen wrote: > As of v3.11-9444-g3ea67d0 "memcg: add per cgroup writeback pages > accounting" memcg counter errors are possible when moving charged > memory to a different memcg. Charge movement occurs when processing > writes to memory.force_empty, moving tasks to a memcg with > memcg.move_charge_at_immigrate=1, or memcg deletion. An example > showing error after memory.force_empty: > $ cd /sys/fs/cgroup/memory > $ mkdir x > $ rm /data/tmp/file > $ (echo $BASHPID >> x/tasks && exec mmap_writer /data/tmp/file 1M) & > [1] 13600 > $ grep ^mapped x/memory.stat > mapped_file 1048576 > $ echo 13600 > tasks > $ echo 1 > x/memory.force_empty > $ grep ^mapped x/memory.stat > mapped_file 4503599627370496 > > mapped_file should end with 0. > 4503599627370496 == 0x10,0000,0000,0000 == 0x100,0000,0000 pages > 1048576 == 0x10,0000 == 0x100 pages > > This issue only affects the source memcg on 64 bit machines; the > destination memcg counters are correct. So the rmdir case is not too > important because such counters are soon disappearing with the entire > memcg. But the memcg.force_empty and > memory.move_charge_at_immigrate=1 cases are larger problems as the > bogus counters are visible for the (possibly long) remaining life of > the source memcg. > > The problem is due to memcg use of __this_cpu_from(.., -nr_pages), > which is subtly wrong because it subtracts the unsigned int nr_pages > (either -1 or -512 for THP) from a signed long percpu counter. When > nr_pages=-1, -nr_pages=0xffffffff. On 64 bit machines > stat->count[idx] is signed 64 bit. So memcg's attempt to simply > decrement a count (e.g. from 1 to 0) boils down to: > long count = 1 > unsigned int nr_pages = 1 > count += -nr_pages /* -nr_pages == 0xffff,ffff */ > count is now 0x1,0000,0000 instead of 0 > > The fix is to subtract the unsigned page count rather than adding its > negation. This only works once "percpu: fix this_cpu_sub() subtrahend > casting for unsigneds" is applied to fix this_cpu_sub(). > > Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx> > Acked-by: Tejun Heo <tj@xxxxxxxxxx> Huh, it looked so innocent... At first I thought 2/3 would fix this case as well but the cast happens only after the negation, so the sign extension does not happen. Alright, then. Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- 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>