On Mon 30-07-18 16:42:27, David Hildenbrand wrote: > On 30.07.2018 16:10, Michal Hocko wrote: > > On Mon 30-07-18 15:51:45, David Hildenbrand wrote: > >> On 30.07.2018 15:30, Pavel Tatashin wrote: > > [...] > >>> Hi David, > >>> > >>> Have you figured out why we access struct pages during hot-unplug for > >>> offlined memory? Also, a panic trace would be useful in the patch. > >> > >> __remove_pages() needs a zone as of now (e.g. to recalculate if the zone > >> is contiguous). This zone is taken from the first page of memory to be > >> removed. If the struct pages are uninitialized that value is random and > >> we might even get an invalid zone. > >> > >> The zone is also used to locate pgdat. > >> > >> No stack trace available so far, I'm just reading the code and try to > >> understand how this whole memory hotplug/unplug machinery works. > > > > Yes this is a mess (evolution of the code called otherwise ;) [1]. > > So I guess I should not feel bad if I am having problems understanding > all the details? ;) > > > Functionality has been just added on top of not very well thought > > through bases. This is a nice example of it. We are trying to get a zone > > to 1) special case zone_device 2) recalculate zone state. The first > > shouldn't be really needed because we should simply rely on altmap. > > Whether it is used for zone device or not. 2) shouldn't be really needed > > if the section is offline and we can check that trivially. > > > > About 2, I am not sure if this is the case and that easy. To me it looks > more like remove_pages() fixes up things that should be done in > offline_pages(). Especially, if the same memory was onlined/offlined to > different zones we might be in trouble (looking at code on a very high > level view). Well, this might be possible. Hotplug remove path was on my todo list for a long time. I didn't get that far TBH. shrink_zone_span begs for some attention. > "if the section is offline and we can check that trivially" is not was > is being used here. It is "of the section was online and is now offline". yes. > Accessing a zone when removing memory sounds very wrong. offline_pages() > should cleanup everything that online_pages() did. > > Removing memory with pages that are still online should not be allowed. > And I think this is already enforced via check_memblock_offlined_cb(). yes. -- Michal Hocko SUSE Labs
