On Thu, Jul 14, 2016 at 10:32:09AM +0200, Vlastimil Babka wrote: > On 07/14/2016 07:23 AM, Joonsoo Kim wrote: > >On Fri, Jul 08, 2016 at 11:11:47AM +0100, Mel Gorman wrote: > >>On Fri, Jul 08, 2016 at 11:44:47AM +0900, Joonsoo Kim wrote: > >> > >>It doesn't stop reclaiming for the lower zones. It's reclaiming the LRU > >>for the whole node that may or may not have lower zone pages at the end > >>of the LRU. If it does, then the allocation request will be satisfied. > >>If it does not, then kswapd will think the node is balanced and get > >>rewoken to do a zone-constrained reclaim pass. > > > >If zone-constrained request could go direct reclaim pass, there would > >be no problem. But, please assume that request is zone-constrained > >without __GFP_DIRECT_RECLAIM which is common for some device driver > >implementation. And, please assume one more thing that this request > >always comes with zone-unconstrained allocation request. In this case, > >your max() logic will set kswapd_classzone_idx to highest zone index > >and re-worken kswapd would not balance for low zone again. In the end, > >zone-constrained allocation request without __GFP_DIRECT_RECLAIM could > >fail. > > I don't think there's a problem in the scenario? Kswapd will keep > being woken up and reclaim from the node lru. It will hit and free > any low zone pages that are on the lru, even though it doesn't > "balance for low zone". Eventually it will either satisfy the > constrained allocation by reclaiming those low-zone pages during the > repeated wakeups, or the low-zone wakeups will stop coming together > with higher-zone wakeups and then it will reclaim the low-zone pages > in a single low-zone wakeup. If the zone-constrained request is not Yes, probability of this would be low. > allowed to fail, then it will just keep waking up kswapd and waiting > for the progress. If it's allowed to fail (i.e. not __GFP_NOFAIL), > but not allowed to direct reclaim, it goes "goto nopage" rather > quickly in __alloc_pages_slowpath(), without any waiting for > kswapd's progress, so there's not really much difference whether the > kswapd wakeup picked up a low classzone or not. Note the Hmm... Even if allocation could fail, we should do our best to prevent failure. Relying on luck isn't good idea to me. Thanks. > __GFP_NOFAIL but ~__GFP_DIRECT_RECLAIM is a WARN_ON_ONCE() scenario, > so definitely not common... > > >Thanks. > > > > -- > 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> -- 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>