Re: [PATCH mm-unstable v15 12/13] mm/khugepaged: run khugepaged for all orders

[Lance Yang <lance.yang@linux.dev>]

On Wed, Feb 25, 2026 at 08:26:50PM -0700, Nico Pache wrote:
>From: Baolin Wang <baolin.wang@linux.alibaba.com>
>
>If any order (m)THP is enabled we should allow running khugepaged to
>attempt scanning and collapsing mTHPs. In order for khugepaged to operate
>when only mTHP sizes are specified in sysfs, we must modify the predicate
>function that determines whether it ought to run to do so.
>
>This function is currently called hugepage_pmd_enabled(), this patch
>renames it to hugepage_enabled() and updates the logic to check to
>determine whether any valid orders may exist which would justify
>khugepaged running.
>
>We must also update collapse_allowable_orders() to check all orders if
>the vma is anonymous and the collapse is khugepaged.
>
>After this patch khugepaged mTHP collapse is fully enabled.
>
>Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>Signed-off-by: Nico Pache <npache@redhat.com>
>---
> mm/khugepaged.c | 30 ++++++++++++++++++------------
> 1 file changed, 18 insertions(+), 12 deletions(-)
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index 388d3f2537e2..e8bfcc1d0c9a 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -434,23 +434,23 @@ static inline int collapse_test_exit_or_disable(struct mm_struct *mm)
> 		mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm);
> }
> 
>-static bool hugepage_pmd_enabled(void)
>+static bool hugepage_enabled(void)
> {
> 	/*
> 	 * We cover the anon, shmem and the file-backed case here; file-backed
> 	 * hugepages, when configured in, are determined by the global control.
>-	 * Anon pmd-sized hugepages are determined by the pmd-size control.
>+	 * Anon hugepages are determined by its per-size mTHP control.
> 	 * Shmem pmd-sized hugepages are also determined by its pmd-size control,
> 	 * except when the global shmem_huge is set to SHMEM_HUGE_DENY.
> 	 */
> 	if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
> 	    hugepage_global_enabled())
> 		return true;
>-	if (test_bit(PMD_ORDER, &huge_anon_orders_always))
>+	if (READ_ONCE(huge_anon_orders_always))
> 		return true;
>-	if (test_bit(PMD_ORDER, &huge_anon_orders_madvise))
>+	if (READ_ONCE(huge_anon_orders_madvise))
> 		return true;
>-	if (test_bit(PMD_ORDER, &huge_anon_orders_inherit) &&
>+	if (READ_ONCE(huge_anon_orders_inherit) &&
> 	    hugepage_global_enabled())
> 		return true;
> 	if (IS_ENABLED(CONFIG_SHMEM) && shmem_hpage_pmd_enabled())
>@@ -521,8 +521,14 @@ static unsigned int collapse_max_ptes_none(unsigned int order)
> static unsigned long collapse_allowable_orders(struct vm_area_struct *vma,
> 			vm_flags_t vm_flags, bool is_khugepaged)
> {
>+	unsigned long orders;
> 	enum tva_type tva_flags = is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;
>-	unsigned long orders = BIT(HPAGE_PMD_ORDER);
>+
>+	/* If khugepaged is scanning an anonymous vma, allow mTHP collapse */
>+	if (is_khugepaged && vma_is_anonymous(vma))
>+		orders = THP_ORDERS_ALL_ANON;
>+	else
>+		orders = BIT(HPAGE_PMD_ORDER);
> 
> 	return thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
> }

IIUC, an anonymous VMA can pass collapse_allowable_orders() even if it
is smaller than 2MB ...

But collapse_scan_mm_slot() still scans only full PMD-sized windows:

		hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
		hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
		if (khugepaged_scan.address > hend) {
			cc->progress++;
			continue;
		}

and hugepage_vma_revalidate() still requires PMD suitability:

	/* Always check the PMD order to ensure its not shared by another VMA */
	if (!thp_vma_suitable_order(vma, address, PMD_ORDER))
		return SCAN_ADDRESS_RANGE;


>@@ -531,7 +537,7 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
> 			  vm_flags_t vm_flags)
> {
> 	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
>-	    hugepage_pmd_enabled()) {
>+	    hugepage_enabled()) {
> 		if (collapse_allowable_orders(vma, vm_flags, /*is_khugepaged=*/true))
> 			__khugepaged_enter(vma->vm_mm);

I wonder if we should also require at least one PMD-sized scan window
here? Not a big deal, just might be good to tighten the gate a bit :)

Apart from that, LGTM!
Reviewed-by: Lance Yang <lance.yang@linux.dev>