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From: Lance Yang <ioworker0@gmail.com>
Date: Fri, 7 Jun 2024 09:50:49 +0800
Message-ID: <CAK1f24nT429ZdEcFUv0r0Sbihjum-Z8ghoYxML=tzaDpVWZk-w@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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On Thu, Jun 6, 2024 at 5:41=E2=80=AFPM David Hildenbrand <david@redhat.com>=
 wrote:
>
> On 06.06.24 11:38, Lance Yang wrote:
> > On Thu, Jun 6, 2024 at 4:06=E2=80=AFPM David Hildenbrand <david@redhat.=
com> wrote:
> >>
> >> On 06.06.24 10:01, David Hildenbrand wrote:
> >>> On 06.06.24 05:55, Lance Yang wrote:
> >>>> On Wed, Jun 5, 2024 at 10:28=E2=80=AFPM David Hildenbrand <david@red=
hat.com> 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@re=
dhat.com> wrote:
> >>>>>>>
> >>>>>>> On 21.05.24 06:02, Lance Yang wrote:
> >>>>>>>> In preparation for supporting try_to_unmap_one() to unmap PMD-ma=
pped
> >>>>>>>> folios, start the pagewalk first, then call split_huge_pmd_addre=
ss() 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 ra=
nge 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 =
*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 fo=
r invalidation.
> >>>>>>>>            * For hugetlb, it could be much worse if we need to d=
o pud
> >>>>>>>> @@ -1668,20 +1665,35 @@ static bool try_to_unmap_one(struct foli=
o *folio, 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 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);
> >
> > Should we still keep the '!pvmw.pte' here? Something like:
> >
> > if (!folio_test_large(folio) || !pvmw.pte)
> >      mlock_vma_folio(folio, vma);
>
> I was wondering the same the whole time ...
>
> >
> > We can mlock the THP to prevent it from being picked up during page rec=
laim.
> >
> > David, I=E2=80=99d like to hear your thoughts on this ;)
>
> but I think there is no need to for now, in the context of your patchset.=
 :)

Agreed. Let's drop it for now :)

Thanks a lot for your thoughts!
Lance

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