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[2a02:3100:ae9e:1d01:58d3:443d:4fa6:1cd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-482b14b802dsm6407476f8f.17.2026.08.19.11.55.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 19 Aug 2026 11:55:59 -0700 (PDT) Date: Wed, 19 Aug 2026 20:55:57 +0200 From: Karl Mehltretter To: Arnd Bergmann Cc: Catalin Marinas , Will Deacon , Mark Rutland , Ard Biesheuvel , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] arm64: compat: Keep alignment address arithmetic 32-bit Message-ID: References: <20260817000231.21311-1-kmehltretter@gmail.com> <4078f942-e29a-4f6f-b6fc-780e8c6d6874@app.fastmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4078f942-e29a-4f6f-b6fc-780e8c6d6874@app.fastmail.com> On Wed, Aug 19, 2026 at 01:19:20PM +0100, Arnd Bergmann wrote: > > I don't see yet why 64K_PAGES makes a difference. I had > expected TASK_SIZE to be 4GiB regardless of the page size, and > the top page (but not the bottom page) to be reachable from normal > userspace. There is a special case in arch/arm64/include/asm/processor.h: #if defined(CONFIG_ARM64_64K_PAGES) && defined(CONFIG_KUSER_HELPERS) #define TASK_SIZE_32 UL(0x100000000) #else #define TASK_SIZE_32 (UL(0x100000000) - PAGE_SIZE) #endif Only ARM64_64K_PAGES with KUSER_HELPERS gives compat tasks the full 4 GiB. The 64K kuser mapping then occupies the last page. I'll send a v2 soon. Karl