linux-kernel - Re: [RFC v2 1/5] mm: defer THP insertion to khugepaged

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Message-ID: <CAA1CXcCg1EjY5fP=uR9EUpQtwXy1VJJEho8PA1VmG4E03tvBdg@mail.gmail.com>
Date: Mon, 17 Feb 2025 12:24:50 -0700
From: Nico Pache <npache@...hat.com>
To: Usama Arif <usamaarif642@...il.com>
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Subject: Re: [RFC v2 1/5] mm: defer THP insertion to khugepaged

On Mon, Feb 17, 2025 at 7:59 AM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 11/02/2025 00:40, Nico Pache wrote:
> > setting /transparent_hugepages/enabled=always allows applications
> > to benefit from THPs without having to madvise. However, the pf handler
> > takes very few considerations to decide weather or not to actually use a
> > THP. This can lead to a lot of wasted memory. khugepaged only operates
> > on memory that was either allocated with enabled=always or MADV_HUGEPAGE.
> >
> > Introduce the ability to set enabled=defer, which will prevent THPs from
> > being allocated by the page fault handler unless madvise is set,
> > leaving it up to khugepaged to decide which allocations will collapse to a
> > THP. This should allow applications to benefits from THPs, while curbing
> > some of the memory waste.
> >
> > Signed-off-by: Nico Pache <npache@...hat.com>
> > ---
> >  include/linux/huge_mm.h | 15 +++++++++++++--
> >  mm/huge_memory.c        | 31 +++++++++++++++++++++++++++----
> >  2 files changed, 40 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 93e509b6c00e..fb381ca720ea 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -44,6 +44,7 @@ enum transparent_hugepage_flag {
> >       TRANSPARENT_HUGEPAGE_UNSUPPORTED,
> >       TRANSPARENT_HUGEPAGE_FLAG,
> >       TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> > +     TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,
>
> No strong preference, but maybe just TRANSPARENT_HUGEPAGE_DEFER_FLAG might be better?

Not a bad idea, TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG is pretty long!
>