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From:   Yang Shi <shy828301@gmail.com>
Date:   Wed, 24 May 2023 15:45:28 -0700
Message-ID: <CAHbLzkrkGx-+OVLexWAx0THeOsD1C1DHt2VjgnkYJe-MqeW3Zw@mail.gmail.com>
Subject: Re: [PATCH 25/31] mm/gup: remove FOLL_SPLIT_PMD use of pmd_trans_unstable()
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Tue, May 23, 2023 at 9:26=E2=80=AFPM Hugh Dickins <hughd@google.com> wro=
te:
>
> On Mon, 22 May 2023, Yang Shi wrote:
> > On Mon, May 22, 2023 at 7:26=E2=80=AFPM Yang Shi <shy828301@gmail.com> =
wrote:
> > > On Sun, May 21, 2023 at 10:22=E2=80=AFPM Hugh Dickins <hughd@google.c=
om> wrote:
> > > >
> > > > There is now no reason for follow_pmd_mask()'s FOLL_SPLIT_PMD block=
 to
> > > > distinguish huge_zero_page from a normal THP: follow_page_pte() han=
dles
> > > > any instability, and here it's a good idea to replace any pmd_none(=
*pmd)
> > > > by a page table a.s.a.p, in the huge_zero_page case as for a normal=
 THP.
> > > > (Hmm, couldn't the normal THP case have hit an unstably refaulted T=
HP
> > > > before?  But there are only two, exceptional, users of FOLL_SPLIT_P=
MD.)
> > > >
> > > > Signed-off-by: Hugh Dickins <hughd@google.com>
> > > > ---
> > > >  mm/gup.c | 19 ++++---------------
> > > >  1 file changed, 4 insertions(+), 15 deletions(-)
> > > >
> > > > diff --git a/mm/gup.c b/mm/gup.c
> > > > index bb67193c5460..4ad50a59897f 100644
> > > > --- a/mm/gup.c
> > > > +++ b/mm/gup.c
> > > > @@ -681,21 +681,10 @@ static struct page *follow_pmd_mask(struct vm=
_area_struct *vma,
> > > >                 return follow_page_pte(vma, address, pmd, flags, &c=
tx->pgmap);
> > > >         }
> > > >         if (flags & FOLL_SPLIT_PMD) {
> > > > -               int ret;
> > > > -               page =3D pmd_page(*pmd);
> > > > -               if (is_huge_zero_page(page)) {
> > > > -                       spin_unlock(ptl);
> > > > -                       ret =3D 0;
> > > > -                       split_huge_pmd(vma, pmd, address);
> > > > -                       if (pmd_trans_unstable(pmd))
> > > > -                               ret =3D -EBUSY;
> > >
> > > IIUC the pmd_trans_unstable() check was transferred to the implicit
> > > pmd_none() in pte_alloc(). But it will return -ENOMEM instead of
> > > -EBUSY. Won't it break some userspace? Or the pmd_trans_unstable() is
> > > never true? If so it seems worth mentioning in the commit log about
> > > this return value change.
>
> Thanks a lot for looking at these, but I disagree here.
>
> >
> > Oops, the above comment is not accurate. It will call
> > follow_page_pte() instead of returning -EBUSY if pmd is none.
>
> Yes.  Ignoring secondary races, if pmd is none, pte_alloc() will allocate
> an empty page table there, follow_page_pte() find !pte_present and return
> NULL; or if pmd is not none, follow_page_pte() will return no_page_table(=
)
> i.e. NULL.  And page NULL ends up with __get_user_pages() having another
> go round, instead of failing with -EBUSY.
>
> Which I'd say is better handling for such a transient case - remember,
> it's split_huge_pmd() (which should always succeed, but might be raced)
> in use there, not split_huge_page() (which might take years for pins to
> be removed before it can succeed).

It sounds like an improvement.

>
> > For other unstable cases, it will return -ENOMEM instead of -EBUSY.
>
> I don't think so: the possibly-failing __pte_alloc() only gets called
> in the pmd_none() case.

I mean what if pmd is not none for huge zero page. If it is not
pmd_none pte_alloc() just returns 0, then returns -ENOMEM instead of
-EBUSY. Or it is impossible that pmd end up being pmd_huge_trans or
!pmd_present? It should be very unlikely, for example, migration does
skip huge zero page, but I'm not sure whether there is any corner case
that I missed.

>
> Hugh
>
> >
> > >
> > > > -               } else {
> > > > -                       spin_unlock(ptl);
> > > > -                       split_huge_pmd(vma, pmd, address);
> > > > -                       ret =3D pte_alloc(mm, pmd) ? -ENOMEM : 0;
> > > > -               }
> > > > -
> > > > -               return ret ? ERR_PTR(ret) :
> > > > +               spin_unlock(ptl);
> > > > +               split_huge_pmd(vma, pmd, address);
> > > > +               /* If pmd was left empty, stuff a page table in the=
re quickly */
> > > > +               return pte_alloc(mm, pmd) ? ERR_PTR(-ENOMEM) :
> > > >                         follow_page_pte(vma, address, pmd, flags, &=
ctx->pgmap);
> > > >         }
> > > >         page =3D follow_trans_huge_pmd(vma, address, pmd, flags);
> > > > --
> > > > 2.35.3