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

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, May 18, 2026 at 03:16:11PM +0200, David Hildenbrand (Arm) wrote:
> On 5/14/26 05:10, Wei Yang wrote:
> > 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.
>
> At this point I'll prefer to not return errors from collapse_max_ptes_none().
> It's just rather awkward to return an error deep down in collapse code for a
> configuration problem.
>
> For mthp collapse, we only support max_ptes_none==0 and
> max_ptes_none=="HPAGE_PMD_NR - 1" (default).
>
> If another value is specified while collapsing mTHP, print a warning and treat
> it as 0 (save value, no creep, no memory waste).
>
> In a sense, this is similar to how we handle max_ptes_shared + max_ptes_swap:
> for mTHP: we always treat them as being 0 for mTHP collapse (and don't issue a
> warning, because we would issue a warning with the default settings).
>
> @Lorenzo, fine with you?

Yes 100%, this sounds sensible both in terms of the error and the default. Let's
keep our lives simple(-ish) please :)

>
> --
> Cheers,
>
> David

Cheers, Lorenzo