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

Dev Jain
2025-01-03T07:04:54Z

On 03/01/25 3:28 am, Nico Pache wrote:
> 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!

Happy New Year to you too!

>
> 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.

That is strange. This works for me: Clone mm from akpm, checkout to mm-unstable,
hard reset to e7e89af21ffcfd1077ca6d2188de6497db1ad84c , then apply the patches.

> 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

Did not find any obvious issues till now with debug configs on :)

>
>> 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(-)
>>>