Re: [PATCH v2 0/3] mm: split underused anonymous mTHP folios

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

> Kiryl, would your rework allow all values for mTHP collapse or would
> it stil only allow HPAGE_PMD_NR = 1 and 0?
> 
>> Would a per-order value make more sense here?
>>
> 
> I feel like per-order might be too many knobs? What about a percentage
> that might apply to all mTHP orders?

We discussed all that in the past, and concluded that we will have to support
the existing toggle. I played with using a percentage, but having two toggles
possibly mean the same thing (and some values not being possible) turned ugly.

So the conclusion for me was: no new toggles.

> 
>> An unset value could preserve the current behavior, while an explicit
>> value could control both mTHP collapse and underused splitting. That
> 
> Ah are you proposing that mTHP collapse can then take all values from 
> 0 to mTHP number of pages?
> I think one of the inital version of Nicos patches for mTHP collpase
> allowed that (hopefully I am not misremembering). Nico, what was the
> reason for only allowing 0 and max?

The creep problem was discussed plenty of times. That's when we decided to start
with 0 and max, and only add support for scaling later, once we actually know
which use cases would need them.  Without support for deferred shrinker, there
wasn't really the need to support other values.

Some of it can be had in:

commit 5a9c05d86683db5449b7fefd278c461cffb55234
Author: Nico Pache <nico.pache@linux.dev>
Date:   Fri Jun 5 10:14:11 2026 -0600

    mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

    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 possibility of "creep", and a warning will be emitted if
    any non-supported max_ptes_none value is configured with mTHP enabled.
    Any intermediate value will default mTHP collapse to max_ptes_none=0.

    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.


Tracing back the discussion on that patch should give you all the details.


-- 
Cheers,

David