Re: [PATCH v7 mm-new 04/10] mm: thp: enable THP allocation exclusively through khugepaged

[Yafang Shao <laoar.shao@gmail.com>]

On Thu, Sep 11, 2025 at 11:54 PM Lance Yang <lance.yang@linux.dev> wrote:
>
> On Wed, Sep 10, 2025 at 11:00 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > Currently, THP allocation cannot be restricted to khugepaged alone while
> > being disabled in the page fault path. This limitation exists because
> > disabling THP allocation during page faults also prevents the execution of
> > khugepaged_enter_vma() in that path.
> >
> > With the introduction of BPF, we can now implement THP policies based on
> > different TVA types. This patch adjusts the logic to support this new
> > capability.
> >
> > While we could also extend prtcl() to utilize this new policy, such a
> > change would require a uAPI modification.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > ---
> >  mm/huge_memory.c |  1 -
> >  mm/memory.c      | 13 ++++++++-----
> >  2 files changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 523153d21a41..1e9e7b32e2cf 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1346,7 +1346,6 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> >         ret = vmf_anon_prepare(vmf);
> >         if (ret)
> >                 return ret;
> > -       khugepaged_enter_vma(vma, vma->vm_flags);
> >
> >         if (!(vmf->flags & FAULT_FLAG_WRITE) &&
> >                         !mm_forbids_zeropage(vma->vm_mm) &&
> > diff --git a/mm/memory.c b/mm/memory.c
> > index d8819cac7930..d0609dc1e371 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -6289,11 +6289,14 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
> >         if (pud_trans_unstable(vmf.pud))
> >                 goto retry_pud;
> >
> > -       if (pmd_none(*vmf.pmd) &&
> > -           thp_vma_allowable_order(vma, vm_flags, TVA_PAGEFAULT, PMD_ORDER)) {
> > -               ret = create_huge_pmd(&vmf);
> > -               if (!(ret & VM_FAULT_FALLBACK))
> > -                       return ret;
> > +       if (pmd_none(*vmf.pmd)) {
> > +               if (vma_is_anonymous(vma))
> > +                       khugepaged_enter_vma(vma, vm_flags);
>
> Hmm... I'm a bit confused about the different conditions for calling
> khugepaged_enter_vma(). It's sometimes called for anonymous VMAs, other
> times ONLY for non-anonymous, and sometimes unconditionally ;)

Right, it is really confusing.

>
> Anyway, this isn't a blocker, just something I noticed. I might try to
> simplify that down the road.

please do it when you have a moment.

>
> Acked-by: Lance Yang <lance.yang@linux.dev>

Thanks for the review.

-- 
Regards
Yafang