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/[email protected]
>>[2] -
>>https://lore.kernel.org/all/[email protected]
>>
>>Co-developed-by: Dev Jain <[email protected]>
>>Signed-off-by: Dev Jain <[email protected]>
>>Signed-off-by: Nico Pache <[email protected]>
>>---
>> 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 <[email protected]>
>
>>+ return -EINVAL;
--
Wei Yang
Help you, Help me
