[PATCH v2 01/15] mm/memory_hotplug: fix possible race in scan_movable_pages()

[Lorenzo Stoakes (Oracle)]

On Fri, Mar 20, 2026 at 11:13:33PM +0100, David Hildenbrand (Arm) wrote:
> If a hugetlb folio gets freed while we are in scan_movable_pages(),
> folio_nr_pages() could return 0, resulting in or'ing "0 - 1 = -1"
> to the PFN, resulting in PFN = -1. We're not holding any locks or
> references that would prevent that.
>
> for_each_valid_pfn() would then search for the next valid PFN, and could
> return a PFN that is outside of the range of the original requested
> range. do_migrate_page() would then try to migrate quite a big range,
> which is certainly undesirable.
>
> To fix it, simply test for valid folio_nr_pages() values. While at it,
> as PageHuge() really just does a page_folio() internally, we can just
> use folio_test_hugetlb() on the folio directly.
>
> scan_movable_pages() is expected to be fast, and we try to avoid taking
> locks or grabbing references. We cannot use folio_try_get() as that does
> not work for free hugetlb folios. We could grab the hugetlb_lock, but
> that just adds complexity.
>
> The race is unlikely to trigger in practice, so we won't be CCing
> stable.
>
> Fixes: 16540dae959d ("mm/hugetlb: mm/memory_hotplug: use a folio in scan_movable_pages()")
> Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>

Logic looks right to me, though some nits below. With those accounted for:

Reviewed-by: Lorenzo Stoakes (Oracle) <ljs at kernel.org>

> ---
>  mm/memory_hotplug.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 86d3faf50453..969cd7ddf68f 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1747,6 +1747,7 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>  	unsigned long pfn;
>
>  	for_each_valid_pfn(pfn, start, end) {
> +		unsigned long nr_pages;
>  		struct page *page;
>  		struct folio *folio;
>
> @@ -1763,9 +1764,9 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>  		if (PageOffline(page) && page_count(page))
>  			return -EBUSY;
>
> -		if (!PageHuge(page))

Yeah interesting to see this is folio_test_hugetlb(page_folio(page)) :))

So this is a nice change for sure.

> -			continue;
>  		folio = page_folio(page);
> +		if (!folio_test_hugetlb(folio))
> +			continue;
>  		/*
>  		 * This test is racy as we hold no reference or lock.  The
>  		 * hugetlb page could have been free'ed and head is no longer
> @@ -1775,7 +1776,11 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>  		 */
>  		if (folio_test_hugetlb_migratable(folio))
>  			goto found;
> -		pfn |= folio_nr_pages(folio) - 1;
> +		nr_pages = folio_nr_pages(folio);
> +		if (unlikely(nr_pages < 1 || nr_pages > MAX_FOLIO_NR_PAGES ||

NIT: since nr_pages is an unsigned long, would this be better as !nr_pages || ...?

> +			     !is_power_of_2(nr_pages)))

Could the latter two conditions ever really happen? I guess some weird tearing
or something maybe?

It would also be nice to maybe separate this out as is_valid_nr_pages() or
something, but then again, I suppose given this is a rare case of us
checking this under circumstances where the value might not be valid, maybe
not worth it.

> +			continue;
> +		pfn |= nr_pages - 1;
>  	}
>  	return -ENOENT;
>  found:
>
> --
> 2.43.0
>

Cheers, Lorenzo