Re: [PATCH] Revert "mm, hugetlb: remove hugepages_treat_as_movable sysctl"

[David Hildenbrand]

On 08.10.25 16:18, Gregory Price wrote:
> On Wed, Oct 08, 2025 at 10:58:23AM +0200, David Hildenbrand wrote:
> > On 07.10.25 23:44, Gregory Price wrote:
> > I mean, this is as ugly as it gets.
> >
> > Can't we just let that old approach RIP where it belongs? :)
>
> Definitely - just found this previously existed and wanted to probe for
> how offensive reintroducing it would be. Seems the answer is essentially
> "lets do it a little differently".
>
> > Something I could sympathize is is treaing gigantic pages that are actually
> > migratable as movable.
> ...
> > -       gfp |= hugepage_movable_supported(h) ? GFP_HIGHUSER_MOVABLE : GFP_HIGHUSER;
> > +       gfp |= hugepage_migration_supported(h) ? GFP_HIGHUSER_MOVABLE : GFP_HIGHUSER;
> >
> > Assume you want to offline part of the ZONE_MOVABLE there might still be sufficient
> > space to possibly allocate a 1 GiB area elsewhere and actually move the gigantic page.
> >
> > IIRC, we do the same for memory offlining already.
>
> This is generally true of other page sizes as well, though, isn't it?
> If the system is truly so pressured that it can't successfully move a
> 2MB page - offline may still fail.  So allowing 1GB pages is only a risk
> in the sense that they're harder to allocate new targets.

Right, but memory defragmentation works on pageblock level, so 2 MiB is 
much MUCH more reliable :)

> It matters more if your system has 64GB than it does if it has 4TB.
>
> > Now, maybe we want to make the configurable. But then, I would much rather tweak the
> > hstate_is_gigantic() check in hugepage_movable_supported(). And the parameter
> > would need a much better name than some "treat as movable".
>
> Makes sense - I think the change is logically equivalent.
>
> So it would look like...
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 42f374e828a2..36b1eec58e6f 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -924,7 +924,7 @@ static inline bool hugepage_movable_supported(struct hstate *h)
>          if (!hugepage_migration_supported(h))
>                  return false;
>
> -       if (hstate_is_gigantic(h))
> +       if (hstate_is_gigantic(h) && !movable_gigantic_pages)
>                  return false;
>          return true;
>   }
>
> And adjust documentation accordingly.
>
> I'm running some tests in QEMU atm, but it's taking a bit.  Will report
> back if I see issues with migration when this is turned on.
>
> If that's acceptable, I'll hack this up.

That looks better to me indeed.

Maybe we can export this toggle only if the arch supports migration? 
Then there is also nothing odd to document.

--
Cheers

David / dhildenb