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? > 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? > 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? > 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. > > > 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. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
