Re: [RFC PATCH 00/12] khugepaged: Asynchronous mTHP collapse

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

+Nico, apologies, forgot to CC you.

On 16/12/24 10:20 pm, Dev Jain wrote:
> This patchset extends khugepaged from collapsing only PMD-sized THPs to
> collapsing anonymous mTHPs.
>
> mTHPs were introduced in the kernel to improve memory management by allocating
> chunks of larger memory, so as to reduce number of page faults, TLB misses (due
> to TLB coalescing), reduce length of LRU lists, etc. However, the mTHP property
> is often lost due to CoW, swap-in/out, and when the kernel just cannot find
> enough physically contiguous memory to allocate on fault. Henceforth, there is a
> need to regain mTHPs in the system asynchronously. This work is an attempt in
> this direction, starting with anonymous folios.
>
> In the fault handler, we select the THP order in a greedy manner; the same has
> been used here, along with the same sysfs interface to control the order of
> collapse. In contrast to PMD-collapse, we (hopefully) get rid of the mmap_write_lock().
>
> ---------------------------------------------------------
> Testing
> ---------------------------------------------------------
>
> The set has been build tested on x86_64.
> For Aarch64,
> 1. mm-selftests: No regressions.
> 2. Analyzing with tools/mm/thpmaps on different userspace programs mapping
>     aligned VMAs of a large size, faulting in basepages/mTHPs (according to sysfs),
>     and then madvise()'ing the VMA, khugepaged is able to 100% collapse the VMAs.
>
> This patchset is rebased on mm-unstable (e7e89af21ffcfd1077ca6d2188de6497db1ad84c).
>
> Some points to be noted:
> 1. Some stats like pages_collapsed for khugepaged have not been extended for mTHP.
>     I'd welcome suggestions on any updation, or addition to the sysfs interface.
> 2. Please see patch 9 for lock handling.
>
> Dev Jain (12):
>    khugepaged: Rename hpage_collapse_scan_pmd() -> ptes()
>    khugepaged: Generalize alloc_charge_folio()
>    khugepaged: Generalize hugepage_vma_revalidate()
>    khugepaged: Generalize __collapse_huge_page_swapin()
>    khugepaged: Generalize __collapse_huge_page_isolate()
>    khugepaged: Generalize __collapse_huge_page_copy_failed()
>    khugepaged: Scan PTEs order-wise
>    khugepaged: Abstract PMD-THP collapse
>    khugepaged: Introduce vma_collapse_anon_folio()
>    khugepaged: Skip PTE range if a larger mTHP is already mapped
>    khugepaged: Enable sysfs to control order of collapse
>    selftests/mm: khugepaged: Enlighten for mTHP collapse
>
>   include/linux/huge_mm.h                 |   2 +
>   mm/huge_memory.c                        |   4 +
>   mm/khugepaged.c                         | 445 +++++++++++++++++-------
>   tools/testing/selftests/mm/khugepaged.c |   5 +-
>   4 files changed, 319 insertions(+), 137 deletions(-)
>