Message-ID: <CAA1CXcBPyhs6kO4ucFFOjc5g+ywY+pKqd3QuYq_a2VYzoo72yw@mail.gmail.com>
Date: Thu, 9 Jan 2025 18:28:02 -0700
From: Nico Pache <npache@...hat.com>
To: Dev Jain <dev.jain@....com>
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Subject: Re: [RFC 00/11] khugepaged: mTHP support

On Wed, Jan 8, 2025 at 11:27 PM Dev Jain <dev.jain@....com> wrote:
>
>
> On 09/01/25 5:01 am, Nico Pache wrote:
> > The following series provides khugepaged and madvise collapse with the
> > capability to collapse regions to mTHPs.
> >
> > To achieve this we generalize the khugepaged functions to no longer depend
> > on PMD_ORDER. Then during the PMD scan, we keep track of chunks of pages
> > (defined by MTHP_MIN_ORDER) that are fully utilized. This info is tracked
> > using a bitmap. 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. max_ptes_none is mapped to a 0-100 range to
> > determine how full a mTHP order needs to be before collapsing it.
> >
> > Some design choices to note:
> >   - bitmap structures are allocated dynamically because on some arch's
> >      (like PowerPC) the value of MTHP_BITMAP_SIZE cannot be computed at
> >      compile time leading to warnings.
> >   - The recursion is masked through a stack structure.
> >   - A MTHP_MIN_ORDER was added to compress the bitmap, and ensure it was
> >      64bit on x86. This provides some optimization on the bitmap operations.
> >      if other arches/configs that have larger than 512 PTEs per PMD want to
> >      compress their bitmap further we can change this value per arch.
> >
> > Patch 1-2:  Some refactoring to combine madvise_collapse and khugepaged
> > Patch 3:    A minor "fix"/optimization
> > Patch 4:    Refactor/rename hpage_collapse
> > Patch 5-7:  Generalize khugepaged functions for arbitrary orders
> > Patch 8-11: The mTHP patches
> >
> > This series acts as an alternative to Dev Jain's approach [1]. The two
> > series differ in a few ways:
> >    - My approach uses a bitmap to store the state of the linear scan_pmd to
> >      then determine potential mTHP batches. Devs incorporates his directly
> >      into the scan, and will try each available order.
> >    - Dev is attempting to optimize the locking, while my approach keeps the
> >      locking changes to a minimum. I believe his changes are not safe for
> >      uffd.
> >    - Dev's changes only work for khugepaged not madvise_collapse (although
> >      i think that was by choice and it could easily support madvise)
> >    - Dev scales all khugepaged sysfs tunables by order, while im removing
> >      the restriction of max_ptes_none and converting it to a scale to
> >      determine a (m)THP threshold.
> >    - Dev turns on khugepaged if any order is available while mine still
> >      only runs if PMDs are enabled. I like Dev's approach and will most
> >      likely do the same in my PATCH posting.
> >    - mTHPs need their ref count updated to 1<<order, which Dev is missing.
>
> Well, I did not miss it :)
Sorry! I missed that in my initial review of your code. Seeing that
would have saved me a few hours of debugging xD

>
> int nr_pages = folio_nr_pages(folio);
> folio_ref_add(folio, nr_pages - 1);

Once I found the fix I forgot to cross reference with your series.
Missing this ref update was causing the issue I alluded to in your RFC
thread. When you said you ran into some issues on the debug configs I
figured it was the same one.


>
> >
> > Patch 11 was inspired by one of Dev's changes.
> >
> > [1] https://lore.kernel.org/lkml/20241216165105.56185-1-dev.jain@arm.com/
> >
> > Nico Pache (11):
> >    introduce khugepaged_collapse_single_pmd to collapse a single pmd
> >    khugepaged: refactor madvise_collapse and khugepaged_scan_mm_slot
> >    khugepaged: Don't allocate khugepaged mm_slot early
> >    khugepaged: rename hpage_collapse_* to khugepaged_*
> >    khugepaged: generalize hugepage_vma_revalidate for mTHP support
> >    khugepaged: generalize alloc_charge_folio for mTHP support
> >    khugepaged: generalize __collapse_huge_page_* for mTHP support
> >    khugepaged: introduce khugepaged_scan_bitmap for mTHP support
> >    khugepaged: add mTHP support
> >    khugepaged: remove max_ptes_none restriction on the pmd scan
> >    khugepaged: skip collapsing mTHP to smaller orders
> >
> >   include/linux/khugepaged.h |   4 +-
> >   mm/huge_memory.c           |   3 +-
> >   mm/khugepaged.c            | 436 +++++++++++++++++++++++++------------
> >   3 files changed, 306 insertions(+), 137 deletions(-)
> >
>

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