Re: [PATCH v13 mm-new 05/16] khugepaged: introduce is_mthp_order helper

On 13 Jan 2026, at 21:38, Nico Pache wrote:

On Tue, Dec 2, 2025 at 8:13 PM Zi Yan [off-list ref] wrote:

quoted

On 1 Dec 2025, at 12:46, Nico Pache wrote:

quoted

In order to add mTHP support, we will often be checking if a given order
is a mTHP or PMD order. Lets create a simple helper function to keep the
code clean and readable.

Suggested-by: Lorenzo Stoakes <redacted>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 8599c7fa112e..9c041141b2e3 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c

@@ -338,6 +338,11 @@ static bool pte_none_or_zero(pte_t pte)
      return pte_present(pte) && is_zero_pfn(pte_pfn(pte));
 }

+static bool is_mthp_order(unsigned int order)
+{
+     return order != HPAGE_PMD_ORDER;
+}
+

The code at the bottom of __folio_split() in mm/huge_memory.c
can use this helper.

The code in deferred_split_folio() uses folio_test_pmd_mappable()
instead of order == HPAGE_PMD_ORDER, but the code can be changed
to use the helper.

static inline bool folio_test_pmd_mappable(struct folio *folio)
{
return folio_order(folio) >= HPAGE_PMD_ORDER;
}

Should we make the is_mthp_order() check all orders that are less than
HPAGE_PMD_ORDER.

ie)

+static bool is_mthp_order(unsigned int order)
+{
+     return order < HPAGE_PMD_ORDER;
}

This way it's true to its mTHP nature being smaller than the PMD.

I am not sure. It really depends on in the future whether we call

9 order (can be mapped by multiple PMDs) large folio mTHP or not.

My current feeling is that THP is a legacy name for PMD THP and
any other large folios are mTHP. So your current definition of
is_mthp_order() looks right to me. This is just my feeling and
I am open to other opinions.

folio_test_pmd_mappable() is different, since as long as a folio's
order is greater than HPAGE_PMD_ORDER, it indeed can be mapped
by PMDs.

--
Best Regards,
Yan, Zi