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From:   James Houghton <jthoughton@google.com>
Date:   Wed, 6 Dec 2023 13:01:23 -0800
Message-ID: <CADrL8HUH8BoNRNuO1rGCjNMEdj-TDVvCg7GCpTPzNkHjuCsc1w@mail.gmail.com>
Subject: Re: [PATCH 0/2] arm64: hugetlb: Fix page fault loop for
 sw-dirty/hw-clean contiguous PTEs
To:     Ryan Roberts <ryan.roberts@arm.com>
Cc:     Steve Capper <steve.capper@arm.com>, Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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On Wed, Dec 6, 2023 at 2:24=E2=80=AFAM Ryan Roberts <ryan.roberts@arm.com> =
wrote:
>
> On 05/12/2023 17:54, James Houghton wrote:
> > On Tue, Dec 5, 2023 at 6:43=E2=80=AFAM Ryan Roberts <ryan.roberts@arm.c=
om> wrote:
> > Thanks for pointing this out. So (1) is definitely a bug. The second
> > patch in this series makes it impossible to create such a PTE via
> > pte_modify (by forcing sw-dirty PTEs to be hw-dirty as well).
>
> Yes; I think the second patch should be sufficient; I took a quick look a=
t the
> other helpers and I don't see anything else that could get the PTE to the
> invalid state.
>
> I have a series that starts using the contpte bit for (multi-size) THP
> opportunistically. This bug will affect that too I think. Your patch #2 w=
ill fix
> for both hugetlb and my series. I'd rather not apply an equivalent to you=
r patch
> #1 because its not quite as straightforward in my code path. But I'm pret=
ty
> confident that patch # is all that's needed here.

There is no need to apply a patch #1-equivalent for multi-size THPs.
:) If multi-size THP has the same problem as HugeTLB, patch #2 will
fix it too. I don't think multi-size THP has the equivalent problem --
in fact, I'm not sure how multi-size THP keeps the PTE_DIRTY,
PTE_WRITE (DBM), and the PTE_RDONLY bits in sync (they do need to be
in-sync with each other when the contiguous bit is being used,
right?).

I included patch #1 (with cc:stable) because it's a more direct fix
for HugeTLB that might be slightly easier to backport. If you think
that patch #1 should be dropped and patch #2 should be backported,
please let me know.

Thanks for the review!