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Mon, 26 Oct 2026 19:02:59 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface X-Archived-By: MIME-Version: 1.0 X-ThreadId: AmKN4IVPaSCN Date: Tue, 27 Oct 2026 00:02:38 +0100 From: "Arnd Bergmann" To: "James Houghton" , "Andrew Morton" , "Alexander Viro" Cc: "Thomas Gleixner" , "Peter Zijlstra" , "Kees Cook" , "David Hildenbrand (Red Hat)" , "Lorenzo Stoakes" , "Liam R. Howlett" , "Vlastimil Babka (SUSE)" , "Mike Rapoport" , "Suren Baghdasaryan" , "Michal Hocko" , "Sean Christopherson" , "Paolo Bonzini" , "Jim Mattson" , "Jens Axboe" , "Pavel Begunkov" , "James E . J . Bottomley" , "Martin K. Petersen" , "David Sterba" , "Chris Mason" , "Alexei Starovoitov" , "Daniel Borkmann" , "John Fastabend" , "Josh Poimboeuf" , "Steven Rostedt" , "Masami Hiramatsu" , "Mathieu Desnoyers" , "Ingo Molnar" , "Arnaldo Carvalho de Melo" , "Namhyung Kim" , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org, io-uring@vger.kernel.org, linux-scsi@vger.kernel.org, linux-btrfs@vger.kernel.org, bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org Message-Id: <3b0a8fc4-d140-4a90-b810-cbc1283fc083@app.fastmail.com> In-Reply-To: <20261026215226.3713941-11-jthoughton@google.com> References: <20261026215226.3713941-1-jthoughton@google.com> <20261026215226.3713941-11-jthoughton@google.com> Subject: Re: [RFC PATCH 10/10] uaccess: Move the NULL mm warning out of line Content-Type: text/plain Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: B67A62000A X-Stat-Signature: wm3kgtwymtqkxbnkjbioobffo35fosg5 X-Rspam-User: X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1793055804-259838 X-HE-Meta: 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 qKvICYSV ZToEG5tgfjEjk8t3qjRdKd9v4Muf+y5a25+CgXrmB5BETByAYcJQdcQgXBmIg7MwweyemScgBPw9EqQkPxDnpWRBvGzOeUOz+47+MR5hVfE0td2acgMHz67fnVKoK8TYe6QxLly1Hay8x/QytjJlWUYOj/EdRgtahrd5Gcfw7MzQNztDEN9J/fsN0EIk/HEE5AyvOsEeMllps8pM/oaAMEPwgcj52JDBQYKE1VnjaHU7J4bbodB3dzRlm2qkjj7bsqZrRQjEU0SXc3YnA47v2lRKJVxVNIdMFqfBA5WvR2r1l97CFv+H23JYLgdGVSRd+QnutgbOgNe17kfCo/tNbWHuOLUg0lXgaP8JfL3rvrz0qatEc5KYAKm/KsY/g8WMxJwqOhRrD0kxZsqA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Mon, Oct 26, 2026, at 22:52, James Houghton wrote: > uaccess_without_mm() is inlined into every user copy, including every > copy_{to,from}_user() call site on architectures that define > INLINE_COPY_USER, such as arm64. Each inlined WARN_ON_ONCE() adds its > own __bug_table entry. > > Keep the NULL current->mm check inline, but move the warning into a > single out-of-line, __cold function, uaccess_warn_without_mm(). > > The trade-off is that only the first offending access in the system > warns, rather than the first one from each call site. The stack trace > still identifies the caller, and the access is refused either way. > > Compared to the base of this series, with defconfig + KVM: > > inline WARN_ON_ONCE() out-of-line warning > x86_64 total +5,884 B (+0.014%) +9,714 B (+0.023%) > arm64 total +40,948 B (+0.10%) +24,609 B (+0.06%) > > On arm64, this removes the ~16 KB of __bug_table growth, while text > size is unchanged, as a call is the same size as the inline brk. On > x86_64, the call is larger than the inline ud2 and causes more .cold > splitting, so this is a slight regression. > > Signed-off-by: James Houghton > --- > NOTE: This patch is included for reference only, to show the cost of the > inline warning. I would suspect that the growth is larger on architectures that don't implement __WARN_FLAGS() but rely on the generic WARN_ON(). This would mainly affect 32-bit arm and mips machines, some of which are very memory limited, most of the 64-bit architectures have it. Arnd