The following series provides khugepaged and madvise collapse 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 chunks of pages (defined by KHUGEPAGED_MTHP_MIN_ORDER) that are utilized. 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. With the default max_ptes_none=511, the code should keep its most of its original behavior. When enabling multiple adjacent (m)THP sizes 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. This is due the fact that a collapse will introduce at least 2x the number of pages, and on a future scan will satisfy the promotion condition once again. Patch 1: Refactor/rename hpage_collapse Patch 2: Some refactoring to combine madvise_collapse and khugepaged Patch 3-5: Generalize khugepaged functions for arbitrary orders Patch 6-9: The mTHP patches Patch 10-12: Allow khugepaged to operate without PMD enabled Patch 13-14: Tracing/stats 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 [3] 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. V8 Changes: - Fix mishandled conflict with shmem config changes (Baolin) - Add Baolin's patches for allowing collapse without PMD enabled - Add additional patch for allowing madvise_collapse without PMD enabled - Documentations nits (Randy) - Simplify SCAN_ANY_PROCESS lock jumbling (Liam) - Add a BUG_ON to the mTHP collapse similar to PMD (Dev) - Remove doc comment about khugepaged PMD only limitation (Dev) - Change revalidation function to accept multiple orders - Handled conflicts introduced by Lorenzo's madvise changes V7 (RESEND) [2] V6 Changes: - Dont release the anon_vma_lock early (like in the PMD case), as not all pages are isolated. - Define the PTE as null to avoid a uninitilized condition - minor nits and newline cleanup - make sure to unmap and unlock the pte for the swapin case - change the revalidation to always check the PMD order (as this will make sure that no other VMA spans it) V5 Changes: - switched the order of patches 1 and 2 - fixed some edge cases on the unified madvise_collapse and khugepaged - Explained the "creep" some more in the docs - fix EXCEED_SHARED vs EXCEED_SWAP accounting issue - fix potential highmem issue caused by a early unmap of the PTE V4 Changes: - Rebased onto mm-unstable - small changes to Documentation V3 Changes: - corrected legacy behavior for khugepaged and madvise_collapse - added proper mTHP stat tracking - Minor changes to prevent a nested lock on non-split-lock arches - Took Devs version of alloc_charge_folio as it has the proper stats - Skip cases were trying to collapse to a lower order would still fail - Fixed cases were the bitmap was not being updated properly - Moved Documentation update to this series instead of the defer set - Minor bugs discovered during testing and review - Minor "nit" cleanup V2 Changes: - 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. 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/20250515032226.128900-1-npa...@redhat.com/ [3] - https://lore.kernel.org/lkml/20250515033857.132535-1-npa...@redhat.com/ Baolin Wang (2): khugepaged: allow khugepaged to check all anonymous mTHP orders khugepaged: kick khugepaged for enabling none-PMD-sized mTHPs Dev Jain (1): khugepaged: generalize alloc_charge_folio() Nico Pache (12): khugepaged: rename hpage_collapse_* to khugepaged_* introduce khugepaged_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 khugepaged_scan_bitmap for mTHP support khugepaged: add mTHP support khugepaged: skip collapsing mTHP to smaller orders khugepaged: avoid unnecessary mTHP collapse attempts khugepaged: allow madvise_collapse to check all anonymous mTHP orders khugepaged: improve tracepoints for mTHP orders khugepaged: add per-order mTHP khugepaged stats Documentation: mm: update the admin guide for mTHP collapse Documentation/admin-guide/mm/transhuge.rst | 19 +- include/linux/huge_mm.h | 5 + include/linux/khugepaged.h | 4 + include/trace/events/huge_memory.h | 34 +- mm/huge_memory.c | 11 + mm/khugepaged.c | 510 ++++++++++++++------- 6 files changed, 406 insertions(+), 177 deletions(-) -- 2.49.0
