The following series provides khugepaged with the capability to collapse anonymous memory regions to mTHPs.
To achieve this we generalize the khugepaged functions to no longer depend on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual pages that are occupied (!none/zero). 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. When no mTHP size is enabled, the legacy behavior of khugepaged is maintained. max_ptes_none will be scaled by the attempted collapse order to determine how full a mTHP must be to be eligible for the collapse to occur. If a mTHP collapse is attempted, but contains swapped out, or shared pages, we don't perform the collapse. It is now also possible to collapse to mTHPs without requiring the PMD THP size to be enabled. When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for mTHP collapses to prevent collapse "creep" behavior. This prevents constantly promoting mTHPs to the next available size, which would occur because a collapse introduces more non-zero pages that would satisfy the promotion condition on subsequent scans. Patch 1: Refactor/rename hpage_collapse Patch 2: Refactoring to combine madvise_collapse and khugepaged Patch 3-7: Generalize khugepaged functions for arbitrary orders Patch 8: skip collapsing mTHP to smaller orders Patch 9-10: Add per-order mTHP statistics and tracepoints Patch 11: Introduce collapse_allowable_orders Patch 12-14: Introduce bitmap and mTHP collapse support Patch 15: Documentation --------- 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 consistent. - redis testing. I tested these changes along with my defer changes (see followup [2] post for more details). We've decided to get the mTHP changes merged first before attempting the defer series. - some basic testing on 64k page size. - lots of general use. V11 Changes: - Minor nits and cleanup/refactoring - commit messages updates - split patches to make them easier to review - added helper functions for checking allowable orders and max_ptes_none - simplified anon_vma locking tracking to improve code readability - Added automatic cap to max_ptes_none for mTHP collapse to prevent "creep" - changed bitmap tracking to use page-level granularity instead of chunk-based this removes inaccuracies/complexity - Improved documentation and comments throughout the series - Refactored collapse_single_pmd() for better readability - Added proper WARN_ON_ONCE() instead of BUG_ON() for better error handling - reordered patches to make them easier to review - merge Baolins patch to run khugepaged for all orders now that collapse_allowable_orders is available V10: https://lore.kernel.org/lkml/20250819134205.622806-1-npa...@redhat.com/ V9 : https://lore.kernel.org/lkml/20250714003207.113275-1-npa...@redhat.com/ V8 : https://lore.kernel.org/lkml/20250702055742.102808-1-npa...@redhat.com/ V7 : https://lore.kernel.org/lkml/20250515032226.128900-1-npa...@redhat.com/ V6 : https://lore.kernel.org/lkml/20250515030312.125567-1-npa...@redhat.com/ V5 : https://lore.kernel.org/lkml/20250428181218.85925-1-npa...@redhat.com/ V4 : https://lore.kernel.org/lkml/20250417000238.74567-1-npa...@redhat.com/ V3 : https://lore.kernel.org/lkml/20250414220557.35388-1-npa...@redhat.com/ V2 : https://lore.kernel.org/lkml/20250211003028.213461-1-npa...@redhat.com/ V1 : https://lore.kernel.org/lkml/20250108233128.14484-1-npa...@redhat.com/ A big thanks to everyone that has reviewed, tested, and participated in the development process. Its been a great experience working with all of you on this long endeavour. [1] - https://gitlab.com/npache/khugepaged_mthp_test [2] - https://lore.kernel.org/lkml/20250515033857.132535-1-npa...@redhat.com/ Baolin Wang (1): khugepaged: run khugepaged for all orders Dev Jain (1): khugepaged: generalize alloc_charge_folio() Nico Pache (13): khugepaged: rename hpage_collapse_* to collapse_* introduce collapse_single_pmd to unify khugepaged and madvise_collapse khugepaged: generalize hugepage_vma_revalidate for mTHP support khugepaged: generalize __collapse_huge_page_* for mTHP support khugepaged: introduce collapse_max_ptes_none helper function khugepaged: generalize collapse_huge_page for mTHP collapse khugepaged: skip collapsing mTHP to smaller orders khugepaged: add per-order mTHP collapse failure statistics khugepaged: improve tracepoints for mTHP orders khugepaged: introduce collapse_allowable_orders helper function khugepaged: Introduce mTHP collapse support khugepaged: avoid unnecessary mTHP collapse attempts Documentation: mm: update the admin guide for mTHP collapse Documentation/admin-guide/mm/transhuge.rst | 91 ++- include/linux/huge_mm.h | 5 + include/linux/khugepaged.h | 2 + include/trace/events/huge_memory.h | 34 +- mm/huge_memory.c | 11 + mm/khugepaged.c | 612 ++++++++++++++------- mm/mremap.c | 2 +- 7 files changed, 535 insertions(+), 222 deletions(-) -- 2.51.0
