linux-kernel - Re: [PATCH v5 09/12] khugepaged: avoid unnecessary mTHP collapse attempts

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Message-ID: <CAA1CXcAqGxVHdwos4bdZ8kR3PS=iqs2UgQ+DKdTejo5jUm_5ww@mail.gmail.com>
Date: Wed, 30 Apr 2025 12:43:01 -0600
From: Nico Pache <npache@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
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Subject: Re: [PATCH v5 09/12] khugepaged: avoid unnecessary mTHP collapse attempts

On Wed, Apr 30, 2025 at 4:12 AM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2025/4/29 02:12, 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 | 17 +++++++++++++++++
> >   1 file changed, 17 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 86d1153ce9e8..5e6732cccb86 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1365,6 +1365,23 @@ static int khugepaged_scan_bitmap(struct mm_struct *mm, unsigned long address,
> >                               collapsed += (1 << order);
> >                               continue;
> >                       }
> > +                     /*
> > +                      * Some ret values indicate all lower order will also
> > +                      * fail, dont trying to collapse smaller orders
> > +                      */
> > +                     if (ret == SCAN_EXCEED_NONE_PTE ||
> > +                             ret == SCAN_EXCEED_SWAP_PTE ||
> > +                             ret == SCAN_EXCEED_SHARED_PTE ||
> > +                             ret == SCAN_PTE_NON_PRESENT ||
> > +                             ret == SCAN_PTE_UFFD_WP ||
> > +                             ret == SCAN_ALLOC_HUGE_PAGE_FAIL ||
> > +                             ret == SCAN_CGROUP_CHARGE_FAIL ||
> > +                             ret == SCAN_COPY_MC ||
> > +                             ret == SCAN_PAGE_LOCK ||
> > +                             ret == SCAN_PAGE_COUNT)
> > +                             goto next;
> > +                     else
> > +                             break;
>
> Better to merge this patch into patch 6, which can be helped to
> understand your logic.
Sounds good, it wasnt part of the original logic/RFCs so i separated
it out to get some review on it.
>