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From: Lance Yang <ioworker0@gmail.com>
Date: Wed, 5 Jun 2024 22:57:01 +0800
Message-ID: <CAK1f24=7=QqDFoh=joC+o1eZfyJ8onyc9ELBZBzn1CFr4uPSwQ@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: Yin Fengwei <fengwei.yin@intel.com>, 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, 
	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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On Wed, Jun 5, 2024 at 10:39=E2=80=AFPM David Hildenbrand <david@redhat.com=
> wrote:
>
> On 05.06.24 16:28, David Hildenbrand wrote:
> > On 05.06.24 16:20, Lance Yang wrote:
> >> 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 migh=
t
> >>>> encounter a PMD-mapped THP missing the mlock in the VM_LOCKED range =
during
> >>>> the page walk. It=E2=80=99s probably necessary to mlock this THP to =
prevent 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 *fol=
io, 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 inva=
lidation.
> >>>>          * 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 *f=
olio, 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 s=
wap it 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 h=
ave
> >>> 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 la=
rge
> >> folio, but there are a few scenarios where we don't mlock a large foli=
o, such
> >> 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 enco=
untering 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 i=
s fully
> >> mapped and doesn't cross the VMA boundary, IIUC.
> >>
> >> What do you think?
> >> I would appreciate any suggestions regarding this check ;)
> >
> > Reading this patch only, I wonder if this change makes sense in the
> > context here.
> >
> > Before this patch, we would have PTE-mapped the PMD-mapped THP before
> > reaching this call and skipped it due to "!folio_test_large(folio)".
> >
> > After this patch, we either
> >
> > a) PTE-remap the THP after this check, but retry and end-up here again,
> > whereby we would skip it due to "!folio_test_large(folio)".
> >
> > b) Discard the PMD-mapped THP due to lazyfree directly. Can that
> > co-exist with mlock and what would be the problem here with mlock?
> >
> >

Thanks a lot for clarifying!

> > So if the check is required in this patch, we really have to understand
> > why. If not, we should better drop it from this patch.
> >
> > At least my opinion, still struggling to understand why it would be
> > required (I have 0 knowledge about mlock interaction with large folios =
:) ).
> >
>
> Looking at that series, in folio_references_one(), we do
>
>                         if (!folio_test_large(folio) || !pvmw.pte) {
>                                 /* Restore the mlock which got missed */
>                                 mlock_vma_folio(folio, vma);
>                                 page_vma_mapped_walk_done(&pvmw);
>                                 pra->vm_flags |=3D VM_LOCKED;
>                                 return false; /* To break the loop */
>                         }
>
> I wonder if we want that here as well now: in case of lazyfree we
> would not back off, right?
>
> But I'm not sure if lazyfree in mlocked areas are even possible.
>
> Adding the "!pvmw.pte" would be much clearer to me than the flag check.

Hmm... How about we drop it from this patch for now, and add it back if nee=
ded
in the future?

Thanks,
Lance

>
> --
> Cheers,
>
> David / dhildenb
>