On Thu 29-08-19 14:08:48, David Hildenbrand wrote: > On 29.08.19 13:43, David Hildenbrand wrote: > > On 29.08.19 13:33, David Hildenbrand wrote: > >> On 29.08.19 10:23, Michal Hocko wrote: > >>> On Thu 29-08-19 09:00:08, David Hildenbrand wrote: > >>>> This is the successor of "[PATCH v2 0/6] mm/memory_hotplug: Consider all > >>>> zones when removing memory". I decided to go one step further and finally > >>>> factor out the shrinking of zones from memory removal code. Zones are now > >>>> fixed up when offlining memory/onlining of memory fails/before removing > >>>> ZONE_DEVICE memory. > >>> > >>> I was about to say Yay! but then reading... > >> > >> Almost ;) > >> > >>> > >>>> Example: > >>>> > >>>> :/# cat /proc/zoneinfo > >>>> Node 1, zone Movable > >>>> spanned 0 > >>>> present 0 > >>>> managed 0 > >>>> :/# echo "online_movable" > /sys/devices/system/memory/memory41/state > >>>> :/# echo "online_movable" > /sys/devices/system/memory/memory43/state > >>>> :/# cat /proc/zoneinfo > >>>> Node 1, zone Movable > >>>> spanned 98304 > >>>> present 65536 > >>>> managed 65536 > >>>> :/# echo 0 > /sys/devices/system/memory/memory43/online > >>>> :/# cat /proc/zoneinfo > >>>> Node 1, zone Movable > >>>> spanned 32768 > >>>> present 32768 > >>>> managed 32768 > >>>> :/# echo 0 > /sys/devices/system/memory/memory41/online > >>>> :/# cat /proc/zoneinfo > >>>> Node 1, zone Movable > >>>> spanned 0 > >>>> present 0 > >>>> managed 0 > >>> > >>> ... this made me realize that you are trying to fix it instead. Could > >>> you explain why do we want to do that? Why don't we simply remove all > >>> that crap? Why do we even care about zone boundaries when offlining or > >>> removing memory? Zone shrinking was mostly necessary with the previous > >>> onlining semantic when the zone type could be only changed on the > >>> boundary or unassociated memory. We can interleave memory zones now > >>> arbitrarily. > >> > >> Last time I asked whether we can just drop all that nasty > >> zone->contiguous handling I was being told that it does have a > >> significant performance impact and is here to stay. The boundaries are a > >> key component to detect whether a zone is contiguous. > >> > >> So yes, while we allow interleaved memory zones, having contiguous zones > >> is beneficial for performance. That's why also memory onlining code will > >> try to online memory as default to the zone that will keep/make zones > >> contiguous. > >> > >> Anyhow, I think with this series most of the zone shrinking code becomes > >> "digestible". Except minor issues with ZONE_DEVICE - which is acceptable. > >> > > > > Also, there are plenty of other users of > > node_spanned_pages/zone_spanned_pages etc.. I don't think this can go - > > not that easy :) > > > > ... re-reading, your suggestion is to drop the zone _shrinking_ code > only, sorry :) That makes more sense. > > This would mean that once a zone was !contiguous, it will always remain > like that. Also, even empty zones after unplug would not result in > zone_empty() == true. exactly. We only need to care about not declaring zone !contigious when offlining from ends but that should be trivial. > I can see that some users of *_spanned_pages make certain assumptions > based on the size (snapshot, oom killer, ...), but that would already be > wrong in case the zone is very sparse. at least oom killer usage is certainly wrong. I will have a look. > I'll prepare something, then we can discuss. Thanks! -- Michal Hocko SUSE Labs
