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Mon, 02 Mar 2026 23:20:22 -0800 (PST) MIME-Version: 1.0 References: <20260303063751.2531716-1-pjaroszynski@nvidia.com> In-Reply-To: <20260303063751.2531716-1-pjaroszynski@nvidia.com> From: James Houghton Date: Mon, 2 Mar 2026 23:19:46 -0800 X-Gm-Features: AaiRm51Ep2_L29b_JroBYwNE4zf0aj9rqC2fV9F6QzDJPa3ta2uUEeGpii49xJM Message-ID: Subject: Re: [PATCH] arm64: contpte: fix set_access_flags() no-op check for SMMU/ATS faults To: Piotr Jaroszynski Cc: Ryan Roberts , Will Deacon , Catalin Marinas , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, Alistair Popple , Jason Gunthorpe , John Hubbard , Zi Yan , Breno Leitao , stable@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 10909100013 X-Stat-Signature: r1j1ejeizj78gwyb96kmt7sctbb4mmc7 X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1772522424-638204 X-HE-Meta: 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 GyS0QyOl ZWFmX4pX8qHfFHUxBzc/VAZQpDISOxtk60brlRnA8F3HwWRXIrvjhV2ZaMfeJa5YpNokBw0RDa90vfLjniKZ7dYNmlaQ3+bn9ZC+dA1S0vHbKahD83gX4d0G4A7gc7/Z1wVcHuoo48GucHOfEgBpGJbRU+yk3jckChYHLKfbA+HpIbX3CTlov5DMlAXNG+QOiGzaPLCIr7mY6O4/MvkE2bgJhkBi5Uulp7JD3Y1tM1ewl0DtAgNXu7xKm+XXaHDZbEdSh+TfuBn7iiP4tiWfbyeGmlUI2qI/d/h9GxqfZmG5jnPvTrEXdw7k5sKM89QhhtJo++8G7E+CYPMd3KzCvS/tW7wGVXZk+6GnK0ya436cG9rdx2FSZxnbUGQc83beic9Jb3GLIeDaOA9ok3OCki8BUvD830wXPw4hhskJVG2seYGxd21H/+YUCng== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Mar 2, 2026 at 10:38=E2=80=AFPM Piotr Jaroszynski wrote: > > contpte_ptep_set_access_flags() compared the gathered ptep_get() value > against the requested entry to detect no-ops. ptep_get() ORs AF/dirty > from all sub-PTEs in the CONT block, so a dirty sibling can make the > target appear already-dirty. When the gathered value matches entry, the > function returns 0 even though the target sub-PTE still has PTE_RDONLY > set in hardware. > > For CPU page-table walks this is benign: with FEAT_HAFDBS the hardware > may set AF/dirty on any sub-PTE and the CPU TLB treats the gathered > result as authoritative for the entire range. But an SMMU without HTTU > (or with HA/HD disabled in CD.TCR) evaluates each descriptor > individually and will keep raising F_PERMISSION on the unchanged target > sub-PTE, causing an infinite fault loop. > > Gathering can therefore cause false no-ops when only a sibling has been > updated: > - write faults: target still has PTE_RDONLY (needs PTE_RDONLY cleared) > - read faults: target still lacks PTE_AF > > Fix by checking all sub-PTEs' access flags individually (not via the > gathered view) before returning no-op, and use the raw target PTE for > the write-bit unfold decision. The access-flag mask matches the one > used by __ptep_set_access_flags(). > > Per Arm ARM (DDI 0487) D8.7.1 ("The Contiguous bit"), any sub-PTE in a CO= NT > range may become the effective cached translation and software must > maintain consistent attributes across the range. > > Fixes: 4602e5757bcc ("arm64/mm: wire up PTE_CONT for user mappings") > > Reviewed-by: Alistair Popple > Cc: Ryan Roberts > Cc: Catalin Marinas > Cc: Will Deacon > Cc: Jason Gunthorpe > Cc: John Hubbard > Cc: Zi Yan > Cc: Breno Leitao > Cc: stable@vger.kernel.org > Signed-off-by: Piotr Jaroszynski Thanks for the fix! This is similar (sort of) to a HugeTLB page fault loop I stumbled upon a while ago[1]. (I wonder if there have been more cases like this.) Feel free to add: Reviewed-by: James Houghton [1] https://lore.kernel.org/all/20231204172646.2541916-1-jthoughton@google.= com