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.
Patch 11 was inspired by one of Dev's changes.
[1] https://lore.kernel.org/lkml/20241216165105.56185-1-dev.jain@xxxxxxx/
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(-)
Before I take a proper look at your series, can you please include any testing
you may have done?
