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Fri, 30 May 2025 09:48:39 -0700 (PDT) MIME-Version: 1.0 References: <20250530140446.2387131-1-ryan.roberts@arm.com> <20250530140446.2387131-2-ryan.roberts@arm.com> In-Reply-To: From: Jann Horn Date: Fri, 30 May 2025 18:48:03 +0200 X-Gm-Features: AX0GCFuAT1zEtvLEHv4Z2Un3xyWjY8-HX0RQ4IFigWsX-k02Th9HNwh8AhT-yZ8 Message-ID: Subject: Re: [RFC PATCH v1 1/6] fs/proc/task_mmu: Fix pte update and tlb maintenance ordering in pagemap_scan_pmd_entry() To: Ryan Roberts Cc: Catalin Marinas , Will Deacon , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Christophe Leroy , "David S. Miller" , Andreas Larsson , Juergen Gross , Ajay Kaher , Alexey Makhalov , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Boris Ostrovsky , "Aneesh Kumar K.V" , Andrew Morton , Peter Zijlstra , Arnd Bergmann , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Alexei Starovoitov , Andrey Ryabinin , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org, virtualization@lists.linux.dev, xen-devel@lists.xenproject.org, linux-mm@kvack.org, Andy Lutomirski Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 6984840005 X-Stat-Signature: xnkiggpoaqdtaotkky6o7yzru47bu6qp X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1748623721-666461 X-HE-Meta: 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 ZtKQb0pl JMkvq2zlXT4atl9QjF9m+j7PoFLvIy0IURa0/oExveunxOLzI1zwnl4RLCxiU8XOKUGDIDQ0LAzHbjUnGRibxYzaiTR5nqgua+pixlitvRhvFYJToRJg/DJ7ZDZ7JmQxfp811hB4qAeBqEXc5jaRBEtaY8l9Ys5lCGZMqnwRiL99Vo3qWqRljXaJ53bzxEIrP4xSaTEnsqsLrdahKWt4lJ13BXhG5hXC1iI6XLYRS/qB6IEpc9/61YwTmyKiCzMaKF+xHT+1bwzuuxj/0/Mss2k1EnJ9TzZIrQ9uAMa8YWL4VqK/ljoYjp0b77aEXTQCzHCAA++k4m5bytXfiZqOfRwUjtDLJe5+zmQymDn8tlUExTT8bcHs80LuN6w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, May 30, 2025 at 6:45=E2=80=AFPM Ryan Roberts = wrote: > On 30/05/2025 17:26, Jann Horn wrote: > > On Fri, May 30, 2025 at 4:04=E2=80=AFPM Ryan Roberts wrote: > >> pagemap_scan_pmd_entry() was previously modifying ptes while in lazy m= mu > >> mode, then performing tlb maintenance for the modified ptes, then > >> leaving lazy mmu mode. But any pte modifications during lazy mmu mode > >> may be deferred until arch_leave_lazy_mmu_mode(), inverting the requir= ed > >> ordering between pte modificaiton and tlb maintenance. > >> > >> Let's fix that by leaving mmu mode, forcing all the pte updates to be > >> actioned, before doing the tlb maintenance. > >> > >> This is a theorectical bug discovered during code review. > >> > >> Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and opt= ionally clear info about PTEs") > > > > Hmm... isn't lazy mmu mode supposed to also delay TLB flushes, and > > preserve the ordering of PTE modifications and TLB flushes? > > > > Looking at the existing implementations of lazy MMU: > > > > - In Xen PV implementation of lazy MMU, I see that TLB flush > > hypercalls are delayed as well (xen_flush_tlb(), > > xen_flush_tlb_one_user() and xen_flush_tlb_multi() all use > > xen_mc_issue(XEN_LAZY_MMU) which delays issuing if lazymmu is active). > > - The sparc version also seems to delay TLB flushes, and sparc's > > arch_leave_lazy_mmu_mode() seems to do TLB flushes via > > flush_tlb_pending() if necessary. > > - powerpc's arch_leave_lazy_mmu_mode() also seems to do TLB flushes. > > > > Am I missing something? > > I doubt it. I suspect this was just my misunderstanding then. I hadn't > appreciated that lazy mmu is also guarranteed to maintain flush ordering;= it's > chronically under-documented. Sorry for the noise here. On that basis, I = expect > the first 2 patches can definitely be dropped. Yeah looking at this code I agree that it could use significantly more verbose comments on the API contract.