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

David Hildenbrand
2025-10-08T14:44:30Z

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