Re: [PATCH v4] page_alloc: allow migration of smaller hugepages during contig_alloc

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Dec 03, 2025 at 01:30:04AM -0500, Gregory Price wrote:
> We presently skip regions with hugepages entirely when trying to do
> contiguous page allocation.  This will cause otherwise-movable
> 2MB HugeTLB pages to be considered unmovable, and will make 1GB
> hugepages more difficult to allocate on systems utilizing both.
> 
> Instead, if hugepage migration is enabled, consider regions with
> hugepages smaller than the target contiguous allocation request
> as valid targets for allocation.
> 
> isolate_migrate_pages_block() has similar logic, and the hugetlb code
> does a migratable check in folio_isolate_hugetlb() during isolation.
> So the code servicing the subsequent allocaiton and migration already
> supports this exact use case (it's just unreachable).
> 
> To test, allocate a bunch of 2MB HugeTLB pages (in this case 48GB)
> and then attempt to allocate some 1G HugeTLB pages (in this case 4GB)
> (Scale to your machine's memory capacity).
> 
> echo 24576 > .../hugepages-2048kB/nr_hugepages
> echo 4 > .../hugepages-1048576kB/nr_hugepages
> 
> Prior to this patch, the 1GB page allocation can fail if no contiguous
> 1GB pages remain.  After this patch, the kernel will try to move 2MB
> pages and successfully allocate the 1GB pages (assuming overall
> sufficient memory is available).
> 
> folio_alloc_gigantic() is the primary user of alloc_contig_pages(),
> other users are debug or init-time allocations and largely unaffected.
> - ppc/memtrace is a debugfs interface
> - x86/tdx memory allocation occurs once on module-init
> - kfence/core happens once on module (late) init
> - THP uses it in debug_vm_pgtable_alloc_huge_page at __init time
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Link: https://lore.kernel.org/linux-mm/6fe3562d-49b2-4975-aa86-e139c535ad00@redhat.com/
> Signed-off-by: Gregory Price <gourry@gourry.net>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Acked-by: David Rientjes <rientjes@google.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Tested-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> ---
>  mm/page_alloc.c | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 95d8b812efd0..8ca3273f734a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7069,8 +7069,27 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>  		if (PageReserved(page))
>  			return false;
>  
> -		if (PageHuge(page))
> -			return false;
> +		/*
> +		 * Only consider ranges containing hugepages if those pages are
> +		 * smaller than the requested contiguous region.  e.g.:
> +		 *     Move 2MB pages to free up a 1GB range.

This one makes sense to me.

> +		 *     Don't move 1GB pages to free up a 2MB range.

This one I might be missing something. We don't use cma for 2M pages,
so I don't see how we can end up in this path for 2M allocations.

The reason I'm bringing this up is because this function overall looks
kind of unnecessary. Page isolation checks all of these conditions
already, and arbitrates huge pages on hugepage_migration_supported() -
which seems to be the semantics you also desire here.

Would it make sense to just remove pfn_range_valid_contig()?