Received-SPF: pass (google.com: domain of linux-kernel+bounces-82881-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Precedence: bulk
MIME-Version: 1.0
References: <20240227070418.62292-1-ioworker0@gmail.com> <90471b2f-826e-4275-a9a3-ec673c3e6af8@redhat.com>
 <CAK1f24nr=giiYW2CVtbpuUxNshb9ksLhtY_v-SK1_AVvQPUEdQ@mail.gmail.com> <2aa8b0f0-4fd4-469a-ba72-82fe01d37f15@redhat.com>
In-Reply-To: <2aa8b0f0-4fd4-469a-ba72-82fe01d37f15@redhat.com>
From: Lance Yang <ioworker0@gmail.com>
Date: Tue, 27 Feb 2024 16:45:23 +0800
Message-ID: <CAK1f24mVT7ucbUQZVnFQqN4KrgwTYHQ0q2ySXjq1UYW3s1m9SQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm/memory: Fix boundary check for next PFN in folio_pte_batch()
To: David Hildenbrand <david@redhat.com>
Cc: akpm@linux-foundation.org, ryan.roberts@arm.com, 21cnbao@gmail.com, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Tue, Feb 27, 2024 at 4:33=E2=80=AFPM David Hildenbrand <david@redhat.com=
> wrote:
>
> On 27.02.24 09:23, Lance Yang wrote:
> > Hey David,
> >
> > Thanks for taking time to review!
> >
> > On Tue, Feb 27, 2024 at 3:30=E2=80=AFPM David Hildenbrand <david@redhat=
com> wrote:
> >>
> >> On 27.02.24 08:04, Lance Yang wrote:
> >>> Previously, in folio_pte_batch(), only the upper boundary of the
> >>> folio was checked using '>=3D' for comparison. This led to
> >>> incorrect behavior when the next PFN exceeded the lower boundary
> >>> of the folio, especially in corner cases where the next PFN might
> >>> fall into a different folio.
> >>
> >> Which commit does this fix?
> >>
> >> The introducing commit (f8d937761d65c87e9987b88ea7beb7bddc333a0e) is
> >> already in mm-stable, so we would need a Fixes: tag. Unless, Ryan's
> >> changes introduced a problem.
> >>
> >> BUT
> >>
> >> I don't see what is broken. :)
> >>
> >> Can you please give an example/reproducer?
> >
> > For example1:
> >
> > PTE0 is present for large folio1.
> > PTE1 is present for large folio1.
> > PTE2 is present for large folio1.
> > PTE3 is present for large folio1.
> >
> > folio_nr_pages(folio1) is 4.
> > folio_nr_pages(folio2) is 4.
> >
> > pte =3D *start_ptep =3D PTE0;
> > max_nr =3D folio_nr_pages(folio2);
> >
> > If folio_pfn(folio1) < folio_pfn(folio2),
> > the return value of folio_pte_batch(folio2, start_ptep, pte, max_nr)
> > will be 4(Actually it should be 0).
> >
> > For example2:
> >
> > PTE0 is present for large folio2.
> > PTE1 is present for large folio1.
> > PTE2 is present for large folio1.
> > PTE3 is present for large folio1.
> >
> > folio_nr_pages(folio1) is 4.
> > folio_nr_pages(folio2) is 4.
> >
> > pte =3D *start_ptep =3D PTE0;
> > max_nr =3D folio_nr_pages(folio1);
> >
>
> In both cases, start_ptep does not map the folio.
>
> It's a BUG in your caller unless I am missing something important.

Sorry, I understood.

Thanks for your clarification!
Lance

>
>
> > If max_nr=3D4, the return value of folio_pte_batch(folio1, start_ptep,
> > pte, max_nr)
> > will be 1(Actually it should be 0).
> >
> > folio_pte_batch() will soon be exported, and IMO, these corner cases ma=
y need
> > to be handled.
>
> No, you should fix your caller.
>
> The function cannot possibly do something reasonable if start_ptep does
> not map the folio.
>
> nr =3D pte_batch_hint(start_ptep, pte);
> ...
> ptep =3D start_ptep + nr; /* nr is >=3D 1 */
> ...
> return min(ptep - start_ptep, max_nr); /* will return something > 0 */
>
> Which would return > 0 for something that does not map that folio.
>
>
> I was trying to avoid official kernel docs for this internal helper,
> maybe we have to improve it now.
>
> --
> Cheers,
>
> David / dhildenb
>