Re: [PATCH mm-unstable v1 2/5] mm: introduce is_pmd_order helper

Dev Jain
2026-02-20T10:38:28Z

On 12/02/26 7:48 am, Nico Pache wrote:
> In order to add mTHP support to khugepaged, we will often be checking if a
> given order is (or is not) a PMD order. Some places in the kernel already
> use this check, so lets create a simple helper function to keep the code
> clean and readable.
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
>
> arches# pick b3ace8be204f # mm/khugepaged: rename hpage_collapse_* to collapse_*
> ---

Thanks, this is useful.

In drivers/dax/device.c and fs/dax.c, we have order == PMD_ORDER check,
not HPAGE_PMD_ORDER. Therefore,

1) Do we have a bug here, in that these codepaths are fault
handlers, therefore should be using HPAGE_PMD_ORDER since the
definition of this macro zeroes out on !CONFIG_PGTABLE_HAS_HUGE_LEAVES?

2) If the distinction between HPAGE_PMD_ORDER and PMD_ORDER is real,
then the helper should be named is_hpage_pmd_order?

> include/linux/huge_mm.h | 5 +++++
> mm/huge_memory.c | 2 +-
> mm/khugepaged.c | 4 ++--
> mm/mempolicy.c | 2 +-
> mm/page_alloc.c | 2 +-
> 5 files changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index a4d9f964dfde..bd7f0e1d8094 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -771,6 +771,11 @@ static inline bool pmd_is_huge(pmd_t pmd)
> }
> #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>
> +static inline bool is_pmd_order(unsigned int order)
> +{
> + return order == HPAGE_PMD_ORDER;
> +}
> +
> static inline int split_folio_to_list_to_order(struct folio *folio,
> struct list_head *list, int new_order)
> {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 44ff8a648afd..5eae85818635 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4097,7 +4097,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> i_mmap_unlock_read(mapping);
> out:
> xas_destroy(&xas);
> - if (old_order == HPAGE_PMD_ORDER)
> + if (is_pmd_order(old_order))
> count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
> count_mthp_stat(old_order, !ret ? MTHP_STAT_SPLIT : MTHP_STAT_SPLIT_FAILED);
> return ret;
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index fa1e57fd2c46..c362b3b2e08a 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2000,7 +2000,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> * we locked the first folio, then a THP might be there already.
> * This will be discovered on the first iteration.
> */
> - if (folio_order(folio) == HPAGE_PMD_ORDER &&
> + if (is_pmd_order(folio_order(folio)) &&
> folio->index == start) {
> /* Maybe PMD-mapped */
> result = SCAN_PTE_MAPPED_HUGEPAGE;
> @@ -2329,7 +2329,7 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
> continue;
> }
>
> - if (folio_order(folio) == HPAGE_PMD_ORDER &&
> + if (is_pmd_order(folio_order(folio)) &&
> folio->index == start) {
> /* Maybe PMD-mapped */
> result = SCAN_PTE_MAPPED_HUGEPAGE;
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index dbd48502ac24..3802e52b01fc 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2450,7 +2450,7 @@ static struct page *alloc_pages_mpol(gfp_t gfp, unsigned int order,
>
> if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> /* filter "hugepage" allocation, unless from alloc_pages() */
> - order == HPAGE_PMD_ORDER && ilx != NO_INTERLEAVE_INDEX) {
> + is_pmd_order(order) && ilx != NO_INTERLEAVE_INDEX) {
> /*
> * For hugepage allocation and non-interleave policy which
> * allows the current node (or other explicitly preferred
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c2e96ac35636..2acf22f97ae5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -719,7 +719,7 @@ static inline bool pcp_allowed_order(unsigned int order)
> if (order <= PAGE_ALLOC_COSTLY_ORDER)
> return true;
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> - if (order == HPAGE_PMD_ORDER)
> + if (is_pmd_order(order))
> return true;
> #endif
> return false;