Re: [PATCH mm-unstable v17 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, May 12, 2026 at 03:42:02PM +0800, Lance Yang wrote:
>
>On Mon, May 11, 2026 at 12:58:04PM -0600, Nico Pache wrote:
>>generalize the order of the __collapse_huge_page_* and collapse_max_*
>>functions to support future mTHP collapse.
>>
>>The current mechanism for determining collapse with the
>>khugepaged_max_ptes_none value is not designed with mTHP in mind. This
>>raises a key design issue: if we support user defined max_pte_none values
>>(even those scaled by order), a collapse of a lower order can introduces
>>an feedback loop, or "creep", when max_ptes_none is set to a value greater
>>than HPAGE_PMD_NR / 2. [1]
>>
>>With this configuration, a successful collapse to order N will populate
>>enough pages to satisfy the collapse condition on order N+1 on the next
>>scan. This leads to unnecessary work and memory churn.
>>
>>To fix this issue introduce a helper function that will limit mTHP
>>collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
>>This effectively supports two modes: [2]
>>
>>- max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE
>>  that maps the shared zeropage. Consequently, no memory bloat.
>>- max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
>>  available mTHP order.
>>
>>This removes the possiblilty of "creep", while not modifying any uAPI
>>expectations. A warning will be emitted if any non-supported
>>max_ptes_none value is configured with mTHP enabled.
>>
>>mTHP collapse will not honor the khugepaged_max_ptes_shared or
>>khugepaged_max_ptes_swap parameters, and will fail if it encounters a
>>shared or swapped entry.
>>
>>No functional changes in this patch; however it defines future behavior
>>for mTHP collapse.
>>
>>[1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com
>>[2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com
>>
>>Co-developed-by: Dev Jain <dev.jain@arm.com>
>>Signed-off-by: Dev Jain <dev.jain@arm.com>
>>Signed-off-by: Nico Pache <npache@redhat.com>
>>---
>> include/trace/events/huge_memory.h |   3 +-
>> mm/khugepaged.c                    | 117 ++++++++++++++++++++---------
>> 2 files changed, 85 insertions(+), 35 deletions(-)
>>
>>diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
>>index bcdc57eea270..443e0bd13fdb 100644
>>--- a/include/trace/events/huge_memory.h
>>+++ b/include/trace/events/huge_memory.h
>>@@ -39,7 +39,8 @@
>> 	EM( SCAN_STORE_FAILED,		"store_failed")			\
>> 	EM( SCAN_COPY_MC,		"copy_poisoned_page")		\
>> 	EM( SCAN_PAGE_FILLED,		"page_filled")			\
>>-	EMe(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")
>>+	EM(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")	\
>>+	EMe(SCAN_INVALID_PTES_NONE,	"invalid_ptes_none")
>> 
>> #undef EM
>> #undef EMe
>>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>index f68853b3caa7..27465161fa6d 100644
>>--- a/mm/khugepaged.c
>>+++ b/mm/khugepaged.c
>>@@ -61,6 +61,7 @@ enum scan_result {
>> 	SCAN_COPY_MC,
>> 	SCAN_PAGE_FILLED,
>> 	SCAN_PAGE_DIRTY_OR_WRITEBACK,
>>+	SCAN_INVALID_PTES_NONE,
>> };
>> 
>> #define CREATE_TRACE_POINTS
>>@@ -353,37 +354,60 @@ static bool pte_none_or_zero(pte_t pte)
>>  * PTEs for the given collapse operation.
>>  * @cc: The collapse control struct
>>  * @vma: The vma to check for userfaultfd
>>+ * @order: The folio order being collapsed to
>>  *
>>  * Return: Maximum number of none-page or zero-page PTEs allowed for the
>>  * collapse operation.
>>  */
>>-static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
>>-		struct vm_area_struct *vma)
>>+static int collapse_max_ptes_none(struct collapse_control *cc,
>>+		struct vm_area_struct *vma, unsigned int order)
>> {
>>+	unsigned int max_ptes_none = khugepaged_max_ptes_none;
>> 	// If the vma is userfaultfd-armed, allow no none-page or zero-page PTEs.
>
>One thing I still want to call out: kernel code usually uses C-style
>comments :)
>
>> 	if (vma && userfaultfd_armed(vma))
>> 		return 0;
>> 	// for MADV_COLLAPSE, allow any none-page or zero-page PTEs.
>> 	if (!cc->is_khugepaged)
>> 		return HPAGE_PMD_NR;
>>-	// For all other cases repect the user defined maximum.
>>-	return khugepaged_max_ptes_none;
>>+	// for PMD collapse, respect the user defined maximum.
>>+	if (is_pmd_order(order))
>>+		return max_ptes_none;
>>+	/* Zero/non-present collapse disabled. */
>>+	if (!max_ptes_none)
>>+		return 0;
>>+	// for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT,
>>+	// scale the maximum number of PTEs to the order of the collapse.
>>+	if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
>>+		return (1 << order) - 1;
>>+
>>+	// We currently only support max_ptes_none values of 0 or KHUGEPAGED_MAX_PTES_LIMIT.
>>+	// Emit a warning and return -EINVAL.
>>+	pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %u\n",
>>+		      KHUGEPAGED_MAX_PTES_LIMIT);
>
>Maybe fallback to 0 instead, as David suggested earlier?
>

It looks reasonable to fallback to 0.

But as the updated Document says in patch 14:

  For mTHP collapse, only 0 or (HPAGE_PMD_NR - 1) are supported. Any other
  value will emit a warning and no mTHP collapse will be attempted.

This is why it does like this now.

    mthp_collapse()
        max_ptes_none = collapse_max_ptes_none();
        if (max_ptes_none < 0)
            return collapsed;

>max_ptes_none is mostly legacy PMD THP behavior. mTHP is new, and any
>intermediate value in (0, KHUGEPAGED_MAX_PTES_LIMIT) would implicitly
>disable it :(
>

So it depends on what we want to do here :-)

For me, I would vote for fallback to 0.

>Treating those values as 0 feels like the least surprising behavior,
>IMHO. It also gives mTHP a cleaner staring point, rather than carry over
>all the old PMD knob semantics :)
>
>Otherwise, LGTM!
>Reviewed-by: Lance Yang <lance.yang@linux.dev>
>
>>+	return -EINVAL;

-- 
Wei Yang
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