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

["David Hildenbrand (Arm)" <david@kernel.org>]

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?

-- 
Cheers,

David