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

Nico Pache
2025-01-02T21:59:25Z

On Mon, Dec 16, 2024 at 10:31 AM Dev Jain <dev.jain@arm.com> wrote:
>
> +Nico, apologies, forgot to CC you.
Hey Dev,

Happy New Year!

Thanks! I'm trying to apply/test your patches, but am failing to apply
them due to mm-unstable which has "unstable" sha values, making
applying them difficult.
Could you share a public git repo to your patches?

Also, have you seen any issues with your patches? My version of
khugepaged mTHP support was mostly done before the holidays but I
haven't posted due to some issues with (BAD PAGE) refcount issues when
trying to reclaim pages that I haven't found the cause of yet.

-- Nico

>
> 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(-)
> >
>