[RFC 00/11] khugepaged: mTHP support

[Nico Pache <npache@redhat.com>]

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.

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(-)

-- 
2.47.1