Re: [PATCH v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes_none helper function

[David Hildenbrand <david@redhat.com>]

On 28.10.25 14:36, Nico Pache wrote:
> On Mon, Oct 27, 2025 at 11:54 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
>>
>> On Wed, Oct 22, 2025 at 12:37:08PM -0600, Nico Pache wrote:
>>> The current mechanism for determining mTHP collapse scales the
>>> khugepaged_max_ptes_none value based on the target order. This
>>> introduces an undesirable feedback loop, or "creep", when max_ptes_none
>>> is set to a value greater than HPAGE_PMD_NR / 2.
>>>
>>> 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 caps the max_ptes_none
>>> to HPAGE_PMD_NR / 2 - 1 (255 on 4k page size). The function also scales
>>> the max_ptes_none number by the (PMD_ORDER - target collapse order).
>>>
>>> The limits can be ignored by passing full_scan=true, this is useful for
>>> madvise_collapse (which ignores limits), or in the case of
>>> collapse_scan_pmd(), allows the full PMD to be scanned when mTHP
>>> collapse is available.
>>>
>>> Signed-off-by: Nico Pache <npache@redhat.com>
>>> ---
>>>   mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++-
>>>   1 file changed, 34 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 4ccebf5dda97..286c3a7afdee 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -459,6 +459,39 @@ void __khugepaged_enter(struct mm_struct *mm)
>>>                wake_up_interruptible(&khugepaged_wait);
>>>   }
>>>
>>> +/**
>>> + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
>>> + * @order: The folio order being collapsed to
>>> + * @full_scan: Whether this is a full scan (ignore limits)
>>> + *
>>> + * For madvise-triggered collapses (full_scan=true), all limits are bypassed
>>> + * and allow up to HPAGE_PMD_NR - 1 empty PTEs.
>>> + *
>>> + * For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the configured
>>> + * khugepaged_max_ptes_none value.
>>> + *
>>> + * For mTHP collapses, scale down the max_ptes_none proportionally to the folio
>>> + * order, but caps it at HPAGE_PMD_NR/2-1 to prevent a collapse feedback loop.
>>> + *
>>> + * Return: Maximum number of empty PTEs allowed for the collapse operation
>>> + */
>>> +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
>>> +{
>>> +     unsigned int max_ptes_none;
>>> +
>>> +     /* ignore max_ptes_none limits */
>>> +     if (full_scan)
>>> +             return HPAGE_PMD_NR - 1;
>>> +
>>> +     if (order == HPAGE_PMD_ORDER)
>>> +             return khugepaged_max_ptes_none;
>>> +
>>> +     max_ptes_none = min(khugepaged_max_ptes_none, HPAGE_PMD_NR/2 - 1);
>>
> 
> Hey Lorenzo,
> 
>> I mean not to beat a dead horse re: v11 commentary, but I thought we were going
>> to implement David's idea re: the new 'eagerness' tunable, and again we're now just
>> implementing the capping at HPAGE_PMD_NR/2 - 1 thing again?
> 
> I spoke to David and he said to continue forward with this series; the
> "eagerness" tunable will take some time, and may require further
> considerations/discussion.

Right, after talking to Johannes it got clearer that what we envisioned 
with "eagerness" would not be like swappiness, and we will really have 
to be careful here. I don't know yet when I will have time to look into 
that.

If we want to avoid the implicit capping, I think there are the 
following possible approaches

(1) Tolerate creep for now, maybe warning if the user configures it.
(2) Avoid creep by counting zero-filled pages towards none_or_zero.
(3) Have separate toggles for each THP size. Doesn't quite solve the
     problem, only shifts it.

Anything else?

IIUC, creep is less of a problem when we have the underused shrinker 
enabled: whatever we over-allocated can (unless longterm-pinned etc) get 
reclaimed again.

So maybe having underused-shrinker support for mTHP as well would be a 
solution to tackle (1) later?

-- 
Cheers

David / dhildenb