On Wed 04-05-16 23:57:50, Joonsoo Kim wrote: > 2016-05-04 17:56 GMT+09:00 Michal Hocko <mhocko@xxxxxxxxxx>: > > On Wed 04-05-16 15:31:12, Joonsoo Kim wrote: > >> On Wed, May 04, 2016 at 03:01:24PM +0900, Joonsoo Kim wrote: > >> > On Wed, Apr 20, 2016 at 03:47:25PM -0400, Michal Hocko wrote: > > [...] > >> > > @@ -3408,6 +3456,17 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > >> > > no_progress_loops)) > >> > > goto retry; > >> > > > >> > > + /* > >> > > + * It doesn't make any sense to retry for the compaction if the order-0 > >> > > + * reclaim is not able to make any progress because the current > >> > > + * implementation of the compaction depends on the sufficient amount > >> > > + * of free memory (see __compaction_suitable) > >> > > + */ > >> > > + if (did_some_progress > 0 && > >> > > + should_compact_retry(order, compact_result, > >> > > + &migration_mode, compaction_retries)) > >> > > >> > Checking did_some_progress on each round have subtle corner case. Think > >> > about following situation. > >> > > >> > round, compaction, did_some_progress, compaction > >> > 0, defer, 1 > >> > 0, defer, 1 > >> > 0, defer, 1 > >> > 0, defer, 1 > >> > 0, defer, 0 > >> > >> Oops...Example should be below one. > >> > >> 0, defer, 1 > >> 1, defer, 1 > >> 2, defer, 1 > >> 3, defer, 1 > >> 4, defer, 0 > > > > I am not sure I understand. The point of the check is that if the > > reclaim doesn't make _any_ progress then checking the result of the > > compaction after it didn't lead to a successful allocation just doesn't > > make any sense. > > Even if this round (#4) doesn't reclaim any pages, previous rounds > (#0, #1, #2, #3) would reclaim enough pages to succeed future > compaction attempt. Then the compaction shouldn't back off and I would consider it a compaction bug. I haven't see this happening though. Vlastimil is already working on patches which would simply guarantee that really important allocations will not defer. So unless I can see an example of a real issue with this I think it is just a theoretical issue which shouldn't block the patch as is. -- Michal Hocko 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>