On Mon, May 14, 2012 at 01:25:04PM +0900, Minchan Kim wrote: > > <SNIP> > > > > If CMA decide they want to alter mlocked pages in this way, it's sortof > > ok. While CMA is being used, there are no expectations on the RT > > behaviour of the system - stalls are expected. In their use cases, CMA > > failing is far worse than access latency to an mlocked page being > > variable while CMA is running. > > > > Compaction on the other hand is during the normal operation of the > > machine. There are applications that assume that if anonymous memory > > is mlocked() then access to it is close to zero latency. They are > > not RT-critical processes (or they would disable THP) but depend on > > this. Allowing compaction to migrate mlocked() pages will result in bugs > > being reported by these people. > > > > I've received one bug this year about access latency to mlocked() regions but > > it turned out to be a file-backed region and related to when the write-fault > > is incurred. The ultimate fix was in the application but we'll get new bug > > reports if anonymous mlocked pages do not preserve the current guarantees > > on access latency. > > > > If so, what do you think about migration of mlocked pages by migrate_pages, cpuset_migrate_mm and memcg? migrate_pages() is a core function used by a variety of different callers. It *optionally* could move mlocked pages and it would be up to the caller to specify if that was allowed. cpuset_migrate_mm() should be allowed to move mlocked() pages because it's called in a path where the pages are on a node that should not longer be accessible to the processes. In this case, the latency hit is unavoidable and a bug reporter that says "there is an unexpected latency accessing memory while a process moves memory to another node" will be told to get a clue. Where does memcg call migrate_pages()? > I think they all is done by under user's control while compaction happens regardless of user. > So do you think that's why compaction shouldn't migrate mlocked page? > Yes. If the user takes an explicit action that causes latencies when accessing an mlocked anonymous region while the pages are migrated, that's fine. I still do not think that THP and khugepaged should cause unexpected latencies accessing mlocked anonymous regions because it is beyond the control of the application. -- Mel Gorman SUSE Labs -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>