On Mon, Jul 18, 2016 at 08:51:16AM +0200, Vlastimil Babka wrote: > On 07/18/2016 07:07 AM, Joonsoo Kim wrote: > >On Thu, Jul 14, 2016 at 10:32:09AM +0200, Vlastimil Babka wrote: > >>On 07/14/2016 07:23 AM, Joonsoo Kim wrote: > >> > >>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. > > But "Doing our best" has to have some sane limits. Allocation, that Ensuring to do something for the requested zone at least once isn't insane. > cannot direct reclaim, already relies on luck. And we are not really > changing this. The allocation will "goto nopage" before kswapd can > even wake up and start doing something, regardless of classzone_idx > used. But, this patch makes things worse. Even if next allocation comes after kswapd is waking up and doing something, low zone would not be balanced due to max classzone_idx and allocation could fail. It is what this patch changes and I worry. 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>