Re: [RFC 00/11] khugepaged: mTHP support

[Dev Jain <dev.jain@arm.com>]

On 09/01/25 5:01 am, Nico Pache wrote:
> The following series provides khugepaged and madvise collapse with the
> capability to collapse regions to mTHPs.
>
> To achieve this we generalize the khugepaged functions to no longer depend
> on PMD_ORDER. Then during the PMD scan, we keep track of chunks of pages
> (defined by MTHP_MIN_ORDER) that are fully utilized. This info is tracked
> using a bitmap. After the PMD scan is done, we do binary recursion on the
> bitmap to find the optimal mTHP sizes for the PMD range. The restriction
> on max_ptes_none is removed during the scan, to make sure we account for
> the whole PMD range. max_ptes_none is mapped to a 0-100 range to
> determine how full a mTHP order needs to be before collapsing it.
>
> Some design choices to note:
>   - bitmap structures are allocated dynamically because on some arch's
>      (like PowerPC) the value of MTHP_BITMAP_SIZE cannot be computed at
>      compile time leading to warnings.
>   - The recursion is masked through a stack structure.
>   - A MTHP_MIN_ORDER was added to compress the bitmap, and ensure it was
>      64bit on x86. This provides some optimization on the bitmap operations.
>      if other arches/configs that have larger than 512 PTEs per PMD want to
>      compress their bitmap further we can change this value per arch.
>
> Patch 1-2:  Some refactoring to combine madvise_collapse and khugepaged
> Patch 3:    A minor "fix"/optimization
> Patch 4:    Refactor/rename hpage_collapse
> Patch 5-7:  Generalize khugepaged functions for arbitrary orders
> Patch 8-11: The mTHP patches
>
> This series acts as an alternative to Dev Jain's approach [1]. The two
> series differ in a few ways:
>    - My approach uses a bitmap to store the state of the linear scan_pmd to
>      then determine potential mTHP batches. Devs incorporates his directly
>      into the scan, and will try each available order.
>    - Dev is attempting to optimize the locking, while my approach keeps the
>      locking changes to a minimum. I believe his changes are not safe for
>      uffd.
>    - Dev's changes only work for khugepaged not madvise_collapse (although
>      i think that was by choice and it could easily support madvise)
>    - Dev scales all khugepaged sysfs tunables by order, while im removing
>      the restriction of max_ptes_none and converting it to a scale to
>      determine a (m)THP threshold.
>    - Dev turns on khugepaged if any order is available while mine still
>      only runs if PMDs are enabled. I like Dev's approach and will most
>      likely do the same in my PATCH posting.
>    - mTHPs need their ref count updated to 1<<order, which Dev is missing.

Well, I did not miss it :)

int nr_pages = folio_nr_pages(folio);
folio_ref_add(folio, nr_pages - 1);

>
> Patch 11 was inspired by one of Dev's changes.
>
> [1] https://lore.kernel.org/lkml/20241216165105.56185-1-dev.jain@arm.com/
>
> Nico Pache (11):
>    introduce khugepaged_collapse_single_pmd to collapse a single pmd
>    khugepaged: refactor madvise_collapse and khugepaged_scan_mm_slot
>    khugepaged: Don't allocate khugepaged mm_slot early
>    khugepaged: rename hpage_collapse_* to khugepaged_*
>    khugepaged: generalize hugepage_vma_revalidate for mTHP support
>    khugepaged: generalize alloc_charge_folio for mTHP support
>    khugepaged: generalize __collapse_huge_page_* for mTHP support
>    khugepaged: introduce khugepaged_scan_bitmap for mTHP support
>    khugepaged: add mTHP support
>    khugepaged: remove max_ptes_none restriction on the pmd scan
>    khugepaged: skip collapsing mTHP to smaller orders
>
>   include/linux/khugepaged.h |   4 +-
>   mm/huge_memory.c           |   3 +-
>   mm/khugepaged.c            | 436 +++++++++++++++++++++++++------------
>   3 files changed, 306 insertions(+), 137 deletions(-)
>