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

[Gregory Price <gourry@gourry.net>]

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.

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.

Thanks David,
~Gregory