Re: [PATCH] mm,hugetlb,migration: don't migrate kernelcore hugepages

[Alexandru Moise <00moses.alexander00@gmail.com>]

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

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