On Mon, Oct 02, 2017 at 06:15:00PM +0200, Michal Hocko wrote: > On Mon 02-10-17 17:06:38, Alexandru Moise wrote: > > On Mon, Oct 02, 2017 at 04:27:17PM +0200, Michal Hocko wrote: > > > On Mon 02-10-17 16:06:33, Alexandru Moise wrote: > > > > On Mon, Oct 02, 2017 at 02:54:32PM +0200, Michal Hocko wrote: > > > > > On Mon 02-10-17 00:51:11, Alexandru Moise wrote: > > > > > > This attempts to bring more flexibility to how hugepages are allocated > > > > > > by making it possible to decide whether we want the hugepages to be > > > > > > allocated from ZONE_MOVABLE or to the zone allocated by the "kernelcore=" > > > > > > boot parameter for non-movable allocations. > > > > > > > > > > > > A new boot parameter is introduced, "hugepages_movable=", this sets the > > > > > > default value for the "hugepages_treat_as_movable" sysctl. This allows > > > > > > us to determine the zone for hugepages allocated at boot time. It only > > > > > > affects 2M hugepages allocated at boot time for now because 1G > > > > > > hugepages are allocated much earlier in the boot process and ignore > > > > > > this sysctl completely. > > > > > > > > > > > > The "hugepages_treat_as_movable" sysctl is also turned into a mandatory > > > > > > setting that all hugepage allocations at runtime must respect (both > > > > > > 2M and 1G sized hugepages). The default value is changed to "1" to > > > > > > preserve the existing behavior that if hugepage migration is supported, > > > > > > then the pages will be allocated from ZONE_MOVABLE. > > > > > > > > > > > > Note however if not enough contiguous memory is present in ZONE_MOVABLE > > > > > > then the allocation will fallback to the non-movable zone and those > > > > > > pages will not be migratable. > > > > > > > > > > This changelog doesn't explain _why_ we would need something like that. > > > > > > > > > > > > > So people shouldn't be able to choose whether their hugepages should be > > > > migratable or not? > > > > > > How are hugetlb pages any different from THP wrt. migrateability POV? Or > > > any other mapped memory to the userspace in general? > > > > THP shares more with regular userspace mapped memory than with hugetlbfs pages. > > They have separate codepaths in migrate_pages(). > > That is a mere implementation detail. You are right that THP shares more > with regular userspace memory because it is transparent from the > configuration POV but that has nothing to do with page migration AFAICS. > > > And no one ever sets the movable > > flag on a hugetlbfs mapping, so even though __PageMovable(hpage) on a hugetlbfs > > page returns false, it will still move. > > __PageMovable is a completely unrelated thing. It is for pages which are > !LRU but still movable. > > > > > > > > > > Maybe they consider some of their applications more important than > > > > others. > > > > > > I do not understand this part. > > > > > > > Say: > > > > You have a large number of correctable errors on a subpage of a compound > > > > page. So you copy the contents of the page to another hugepage, break the > > > > original page and offline the subpage. > > > > > > I suspect you have HWPoisoning in mind right? > > > > No, rather soft offlining. > > I thought this is the same thing. > > > > > But maybe you'd rather that some of > > > > your hugepages not be broken and moved because you're not that worried about > > > > memory corruption, but more about availability. > > > > > > Could you be more specific please? > > > > You can have a platform with reliable DIMM modules and a platform with less reliable > > DIMM modules. So you would prefer to inhibit hugepage migration on the platform with > > reliable DIMM modules that you know will behave ok even under a high number of > > correctable memory errors. tools like mcelog however are not hugepage aware and > > cannot be told "if this PFN is part of a hugepage, don't try to soft offline it", > > rather deciding which PFNs should be unmovable should be done in the kernel, > > but it should still be controllable by the administrator. > > This sounds like a userspace policy that should be handled outside of > the kernel. > > > For hugetlbfs pages in particular, this behavior is not present, without this patch. > > > > > > > > > Without this patch even if hugepages are in the non-movable zone, they move. > > > > > > which is ok. This is very same with any other movable allocations. > > > > So you can have movable pages in the non-movable kernel zone? > > yes. Most configuration even do not have any movable zone unless > explicitly configured. > > > > > > > The implementation is a bit dirty so obviously I'm open to suggestions > > > > > > for a better way to implement this behavior, or comments whether the whole > > > > > > idea is fundamentally __wrong__. > > > > > > > > > > To be honest I think this is just a wrong approach. hugepages_treat_as_movable > > > > > is quite questionable to be honest because it breaks the basic semantic > > > > > of the movable zone if the hugetlb pages are not really migratable which > > > > > should be the only criterion. Hugetlb pages are no different from other > > > > > migratable pages in that regards. > > > > > > > > Shouldn't hugepages allocated to unmovable zone, by definition, not be able > > > > to be migrated? With this patch, hugepages in the movable zone do move, but > > > > hugepages in the non-movable zone don't. Or am I misunderstanding the semantics > > > > completely? > > > > > > yes. movable zone is only about a guarantee to move memory around. > > > Movable allocations are still allowed to use kernel zones (aka > > > non-movable). The main reason for the movable zone these days is memory > > > hotplug which needs a semi-guarantee that the memory used can be > > > migrated elsewhere to free up the offlined memory. > > > > But isn't kernel-zone memory guaranteed not to migrate? > > No. > > > I agree that movable allocations are allowed to fallback to kernel zones. > > i.e. This is behavior is correct: > > Page A is in ZONE_MOVABLE, page B is in kernel zone. > > Page A gets soft-offlined, the contents are moved to page B. > > > > This behavior is not correct: > > Page C is in kernel zone, page D is also in kernel zone. > > Page C gets soft offlined, contents of page C get moved to page D. > > Why is this incorrect? > > > With hugepages, there is no check for whereto the migration goes because > > the pages are pre-allocated and simply dequeued from the hstate freelist. > > true > > > Thus hugepages will end up being unreserved and moved to a different > > reserved hugepage, and the administrator has no control over this behavior, > > even if they're kernel zone pages. > > I really fail to see why kernel vs. movable zones play any role here. > Zones should be mostly an implementation detail which userspace > shouldn't really care about. Ok, the whole zone approach is a bad idea. Do you think that there's any value at all to trying to make hugepages un-movable at all? Should the hugepages_treat_as_movable sysctl die and just make hugepages movable by default? ../Alex > > -- > 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>
