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From: Lance Yang <ioworker0@gmail.com>
Date: Wed, 5 Jun 2024 22:20:39 +0800
Message-ID: <CAK1f24kwf4gDwK=8X4z1bM9-H6_M9QKy6-ko9pTUZij-W=40wg@mail.gmail.com>
Subject: Re: [PATCH v6 2/3] mm/rmap: integrate PMD-mapped folio splitting into
 pagewalk loop
To: David Hildenbrand <david@redhat.com>
Cc: akpm@linux-foundation.org, willy@infradead.org, sj@kernel.org, 
	baolin.wang@linux.alibaba.com, maskray@google.com, ziy@nvidia.com, 
	ryan.roberts@arm.com, 21cnbao@gmail.com, mhocko@suse.com, 
	fengwei.yin@intel.com, zokeefe@google.com, shy828301@gmail.com, 
	xiehuan09@gmail.com, libang.li@antgroup.com, wangkefeng.wang@huawei.com, 
	songmuchun@bytedance.com, peterx@redhat.com, minchan@kernel.org, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
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Hi David,

On Wed, Jun 5, 2024 at 8:46=E2=80=AFPM David Hildenbrand <david@redhat.com>=
 wrote:
>
> On 21.05.24 06:02, Lance Yang wrote:
> > In preparation for supporting try_to_unmap_one() to unmap PMD-mapped
> > folios, start the pagewalk first, then call split_huge_pmd_address() to
> > split the folio.
> >
> > Since TTU_SPLIT_HUGE_PMD will no longer perform immediately, we might
> > encounter a PMD-mapped THP missing the mlock in the VM_LOCKED range dur=
ing
> > the page walk. It=E2=80=99s probably necessary to mlock this THP to pre=
vent it from
> > being picked up during page reclaim.
> >
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Suggested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>
> > ---
>
> [...] again, sorry for the late review.

No worries at all, thanks for taking time to review!

>
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index ddffa30c79fb..08a93347f283 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1640,9 +1640,6 @@ static bool try_to_unmap_one(struct folio *folio,=
 struct vm_area_struct *vma,
> >       if (flags & TTU_SYNC)
> >               pvmw.flags =3D PVMW_SYNC;
> >
> > -     if (flags & TTU_SPLIT_HUGE_PMD)
> > -             split_huge_pmd_address(vma, address, false, folio);
> > -
> >       /*
> >        * For THP, we have to assume the worse case ie pmd for invalidat=
ion.
> >        * For hugetlb, it could be much worse if we need to do pud
> > @@ -1668,20 +1665,35 @@ static bool try_to_unmap_one(struct folio *foli=
o, struct vm_area_struct *vma,
> >       mmu_notifier_invalidate_range_start(&range);
> >
> >       while (page_vma_mapped_walk(&pvmw)) {
> > -             /* Unexpected PMD-mapped THP? */
> > -             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
> > -
> >               /*
> >                * If the folio is in an mlock()d vma, we must not swap i=
t out.
> >                */
> >               if (!(flags & TTU_IGNORE_MLOCK) &&
> >                   (vma->vm_flags & VM_LOCKED)) {
> >                       /* Restore the mlock which got missed */
> > -                     if (!folio_test_large(folio))
> > +                     if (!folio_test_large(folio) ||
> > +                         (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
> >                               mlock_vma_folio(folio, vma);
>
> Can you elaborate why you think this would be required? If we would have
> performed the  split_huge_pmd_address() beforehand, we would still be
> left with a large folio, no?

Yep, there would still be a large folio, but it wouldn't be PMD-mapped.

After Weifeng's series[1], the kernel supports mlock for PTE-mapped large
folio, but there are a few scenarios where we don't mlock a large folio, su=
ch
as when it crosses a VM_LOCKed VMA boundary.

 -                     if (!folio_test_large(folio))
 +                     if (!folio_test_large(folio) ||
 +                         (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))

And this check is just future-proofing and likely unnecessary. If encounter=
ing a
PMD-mapped THP missing the mlock for some reason, we can mlock this
THP to prevent it from being picked up during page reclaim, since it is ful=
ly
mapped and doesn't cross the VMA boundary, IIUC.

What do you think?
I would appreciate any suggestions regarding this check ;)

[1] https://lore.kernel.org/all/20230918073318.1181104-3-fengwei.yin@intel.=
com/T/#mdab40248cf3705581d8bfb64e1ebf2d9cd81c095

>
> >                       goto walk_done_err;
> >               }
> >
> > +             if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > +                     /*
> > +                      * We temporarily have to drop the PTL and start =
once
> > +                      * again from that now-PTE-mapped page table.
> > +                      */
> > +                     split_huge_pmd_locked(vma, range.start, pvmw.pmd,=
 false,
> > +                                           folio);
>
> Using range.start here is a bit weird. Wouldn't this be pvmw.address?
> [did not check]

Hmm... we may adjust range.start before the page walk, but pvmw.address
does not.

At least for now, pvmw.address seems better. Will adjust as you suggested.

>
> > +                     pvmw.pmd =3D NULL;
> > +                     spin_unlock(pvmw.ptl);
> > +                     pvmw.ptl =3D NULL;
>
>
> Would we want a
>
> page_vma_mapped_walk_restart() that is exactly for that purpose?

Nice, let's add page_vma_mapped_walk_restart() for that purpose :)

Thanks again for your time!

Lance

>
> > +                     flags &=3D ~TTU_SPLIT_HUGE_PMD;
> > +                     continue;
> > +             }
> > +
> > +             /* Unexpected PMD-mapped THP? */
> > +             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
>
> --
> Cheers,
>
> David / dhildenb
>