linux-kernel - Re: [PATCH v12 mm-new 13/15] khugepaged: avoid unnecessary mTHP collapse attempts

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Message-ID: <CAA1CXcBMWc04Z+7mdDAZHkSj5WGDAMc+-4p7JiTme3xCqtNXFg@mail.gmail.com>
Date: Wed, 26 Nov 2025 16:29:32 -0700
From: Nico Pache <npache@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
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Subject: Re: [PATCH v12 mm-new 13/15] khugepaged: avoid unnecessary mTHP
 collapse attempts

On Wed, Nov 19, 2025 at 5:06 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Wed, Oct 22, 2025 at 12:37:15PM -0600, Nico Pache wrote:
> > There are cases where, if an attempted collapse fails, all subsequent
> > orders are guaranteed to also fail. Avoid these collapse attempts by
> > bailing out early.
> >
> > Signed-off-by: Nico Pache <npache@...hat.com>
> > ---
> >  mm/khugepaged.c | 31 ++++++++++++++++++++++++++++++-
> >  1 file changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index e2319bfd0065..54f5c7888e46 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1431,10 +1431,39 @@ static int collapse_scan_bitmap(struct mm_struct *mm, unsigned long address,
> >                       ret = collapse_huge_page(mm, address, referenced,
> >                                                unmapped, cc, mmap_locked,
> >                                                order, offset);
> > -                     if (ret == SCAN_SUCCEED) {
> > +
> > +                     /*
> > +                      * Analyze failure reason to determine next action:
> > +                      * - goto next_order: try smaller orders in same region
> > +                      * - continue: try other regions at same order
>
> The stack is a DFS, so this isn't correct, you may have pushed a bunch of higher
> order candidate mTHPs (I don't like plain 'region') which you will also true.
>
> > +                      * - break: stop all attempts (system-wide failure)
> > +                      */
>
> This comment isn't hugely helpful, just put the relevant comments next to each
> of the goto, continue, break (soon to be return re: review below) statements
> please.
>
> > +                     switch (ret) {
> > +                     /* Cases were we should continue to the next region */
> > +                     case SCAN_SUCCEED:
> >                               collapsed += 1UL << order;
> > +                             fallthrough;
> > +                     case SCAN_PTE_MAPPED_HUGEPAGE:
> >                               continue;
>
> Would we not run into trouble potentially in the previous patch's implementation
> of this examing candidate mTHPs that are part of an already existing huge page,
> or would a folio check in the collapse just make this wasted work?

whoops almost missed this comment.

There is a folio check in the __collapse_huge_page_isolate function
that handles this. I think Dev has some plans to try and add
partially-mapped support as the todo comment suggests (I think he
already has some patches from earlier mTHP work).

/*
* TODO: In some cases of partially-mapped folios, we'd actually
* want to collapse.
*/

>
> > +                     /* Cases were lower orders might still succeed */
> > +                     case SCAN_LACK_REFERENCED_PAGE:
> > +                     case SCAN_EXCEED_NONE_PTE:
>
> How can we, having checked the max_pte_none, still fail due to this?
>
> > +                     case SCAN_EXCEED_SWAP_PTE:
> > +                     case SCAN_EXCEED_SHARED_PTE:
> > +                     case SCAN_PAGE_LOCK:
> > +                     case SCAN_PAGE_COUNT:
> > +                     case SCAN_PAGE_LRU:
> > +                     case SCAN_PAGE_NULL:
> > +                     case SCAN_DEL_PAGE_LRU:
> > +                     case SCAN_PTE_NON_PRESENT:
> > +                     case SCAN_PTE_UFFD_WP:
> > +                     case SCAN_ALLOC_HUGE_PAGE_FAIL:
> > +                             goto next_order;
> > +                     /* All other cases should stop collapse attempts */
>
> I don't love us having a catch-all, plase spell out all cases.
>
> > +                     default:
> > +                             break;
> >                       }
> > +                     break;
>
> _Hate_ this double break. Just return collapsed please.
>
> >               }
> >
> >  next_order:
> > --
> > 2.51.0
> >
>