Re: [RFC v2 0/9] khugepaged: mTHP support

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

On 11/02/25 6:00 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 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 will be scaled by the attempted collapse
> order to determine how full a THP must be to be eligible. If a mTHP collapse
> is attempted, but contains swapped out, or shared pages, we dont perform the
> collapse.
> 
> With the default max_ptes_none=511, the code should keep its most of its
> original behavior. To exercise mTHP collapse we need to set max_ptes_none<=255.
> With max_ptes_none > HPAGE_PMD_NR/2 you will experience collapse "creep" and
> constantly promote mTHPs to the next available size.

How does creep stop when max_ptes_none <= 255?

> 
> Patch 1:     Some refactoring to combine madvise_collapse and khugepaged
> Patch 2:     Refactor/rename hpage_collapse
> Patch 3-5:   Generalize khugepaged functions for arbitrary orders
> Patch 6-9:   The mTHP patches
> 
> ---------
>   Testing
> ---------
> - Built for x86_64, aarch64, ppc64le, and s390x
> - selftests mm
> - I created a test script that I used to push khugepaged to its limits while
>     monitoring a number of stats and tracepoints. The code is available
>     here[1] (Run in legacy mode for these changes and set mthp sizes to inherit)
>     The summary from my testings was that there was no significant regression
>     noticed through this test. In some cases my changes had better collapse
>     latencies, and was able to scan more pages in the same amount of time/work,
>     but for the most part the results were consistant.
> - redis testing. I tested these changes along with my defer changes
>    (see followup post for more details).
> - some basic testing on 64k page size.
> - lots of general use. These changes have been running in my VM for some time.
> 
> Changes since V1 [2]:
> - Minor bug fixes discovered during review and testing
> - removed dynamic allocations for bitmaps, and made them stack based
> - Adjusted bitmap offset from u8 to u16 to support 64k pagesize.
> - Updated trace events to include collapsing order info.
> - Scaled max_ptes_none by order rather than scaling to a 0-100 scale.
> - No longer require a chunk to be fully utilized before setting the bit. Use
>     the same max_ptes_none scaling principle to achieve this.
> - Skip mTHP collapse that requires swapin or shared handling. This helps prevent
>     some of the "creep" that was discovered in v1.
> 
> [1] - https://gitlab.com/npache/khugepaged_mthp_test
> [2] - https://lore.kernel.org/lkml/20250108233128.14484-1-npache@redhat.com/
> 
> Nico Pache (9):
>    introduce khugepaged_collapse_single_pmd to unify khugepaged and
>      madvise_collapse
>    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: improve tracepoints for mTHP orders
>    khugepaged: skip collapsing mTHP to smaller orders
> 
>   include/linux/khugepaged.h         |   4 +
>   include/trace/events/huge_memory.h |  34 ++-
>   mm/khugepaged.c                    | 422 +++++++++++++++++++----------
>   3 files changed, 306 insertions(+), 154 deletions(-)
>