(2012/11/14 0:30), Michal Hocko wrote: > per-zone per-priority iterator is aimed at coordinating concurrent > reclaimers on the same hierarchy (or the global reclaim when all > groups are reclaimed) so that all groups get reclaimed evenly as > much as possible. iter->position holds the last css->id visited > and iter->generation signals the completed tree walk (when it is > incremented). > Concurrent reclaimers are supposed to provide a reclaim cookie which > holds the reclaim priority and the last generation they saw. If cookie's > generation doesn't match the iterator's view then other concurrent > reclaimer already did the job and the tree walk is done for that > priority. > > This scheme works nicely in most cases but it is not raceless. Two > racing reclaimers can see the same iter->position and so bang on the > same group. iter->generation increment is not serialized as well so a > reclaimer can see an updated iter->position with and old generation so > the iteration might be restarted from the root of the hierarchy. > > The simplest way to fix this issue is to synchronise access to the > iterator by a lock. This implementation uses per-zone per-priority > spinlock which linearizes only directly racing reclaimers which use > reclaim cookies so the effect of the new locking should be really > minimal. > > I have to note that I haven't seen this as a real issue so far. The > primary motivation for the change is different. The following patch > will change the way how the iterator is implemented and css->id > iteration will be replaced cgroup generic iteration which requires > storing mem_cgroup pointer into iterator and that requires reference > counting and so concurrent access will be a problem. > > Signed-off-by: Michal Hocko <mhocko@xxxxxxx> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> -- 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>