Re: [PATCH v6 1/4] mm: defer THP insertion to khugepaged

[Nico Pache <npache@redhat.com>]

On Sat, Jun 14, 2025 at 5:25 AM Klara Modin <klarasmodin@gmail.com> wrote:
>
> Hi,
>
> On 2025-05-14 21:38:54 -0600, Nico Pache wrote:
> > setting /transparent_hugepages/enabled=always allows applications
> > to benefit from THPs without having to madvise. However, the page fault
> > 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.
> >
> > Acked-by: Zi Yan <ziy@nvidia.com>
> > Co-developed-by: Rafael Aquini <raquini@redhat.com>
> > Signed-off-by: Rafael Aquini <raquini@redhat.com>
> > Signed-off-by: Nico Pache <npache@redhat.com>
>
> ...
>
> > @@ -315,13 +318,20 @@ static ssize_t enabled_store(struct kobject *kobj,
> >
> >       if (sysfs_streq(buf, "always")) {
> >               clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
> > +             clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG, &transparent_hugepage_flags);
> >               set_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
> > +     } else if (sysfs_streq(buf, "defer")) {
> > +             clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
> > +             clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
> > +             set_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG, &transparent_hugepage_flags);
> >       } else if (sysfs_streq(buf, "madvise")) {
> >               clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
> > +             clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG, &transparent_hugepage_flags);
> >               set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
> >       } else if (sysfs_streq(buf, "never")) {
> >               clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
> >               clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
> > +             clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG, &transparent_hugepage_flags);
> >       } else
> >               ret = -EINVAL;
> >
> > @@ -954,18 +964,31 @@ static int __init setup_transparent_hugepage(char *str)
> >                       &transparent_hugepage_flags);
> >               clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> >                         &transparent_hugepage_flags);
> > +             clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
> > +                       &transparent_hugepage_flags);
> >               ret = 1;
> > +     } else if (!strcmp(str, "defer")) {
> > +             clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
> > +                       &transparent_hugepage_flags);
> > +             clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> > +                       &transparent_hugepage_flags);
> > +             set_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
> > +                       &transparent_hugepage_flags);
>
> There should probably be a corresponding
>                 ret = 1;
> here. Otherwise the cannot parse message will displayed even if defer
> was set.
Thanks Klara-- I will make sure to add it on the next version!
>
> >       } else if (!strcmp(str, "madvise")) {
> >               clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
> >                         &transparent_hugepage_flags);
> > +             clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
> > +                       &transparent_hugepage_flags);
> >               set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> > -                     &transparent_hugepage_flags);
> > +                       &transparent_hugepage_flags);
> >               ret = 1;
> >       } else if (!strcmp(str, "never")) {
> >               clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
> >                         &transparent_hugepage_flags);
> >               clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> >                         &transparent_hugepage_flags);
> > +             clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
> > +                       &transparent_hugepage_flags);
> >               ret = 1;
> >       }
> >  out:
> > --
> > 2.49.0
> >
>
> Regards,
> Klara Modin
>