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Mon, 02 Nov 2026 08:09:16 -0800 (PST) Received: from [192.168.1.22] ([83.231.69.9]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e71ba2415sm21300844f8f.50.2026.11.02.08.09.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 02 Nov 2026 08:09:15 -0800 (PST) Message-ID: <936b8e7c-37e4-42fc-8ec7-67280aa93333@gmail.com> Date: Mon, 2 Nov 2026 17:09:14 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/mm: Define the zapped-entry checks only with CONFIG_DEBUG_VM To: "Kiryl Shutsemau (Meta)" , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Rick Edgecombe Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , Peter Xu , Andrew Morton , "David Hildenbrand (Arm)" , "Mike Rapoport (Microsoft)" , Pasha Tatashin , Andrew Donnellan , Rohan McLure , Baolin Wang , Tejun Heo , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20261102144001.342837-1-kas@kernel.org> Content-Language: en-US From: "Jose A. Perez de Azpillaga" In-Reply-To: <20261102144001.342837-1-kas@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/2/26 3:39 PM, Kiryl Shutsemau (Meta) wrote: > Zapping a range calls arch_check_zapped_pte() for every batch of PTEs, > which is every page for small folios, and arch_check_zapped_pmd() or > arch_check_zapped_pud() for every huge leaf. On x86 these are out-of-line > functions whose only content is a VM_WARN_ON_ONCE(), so without > CONFIG_DEBUG_VM they are empty, but the call remains. With the SRSO "Safe > RET" mitigation the return of every such call mispredicts. > > Declare and define the checks only with CONFIG_DEBUG_VM. Otherwise the > empty inlines from apply, as on every other > architecture. They cannot be inlines in : VM_SHADOW_STACK > is defined in after it includes . > > In a KVM guest on Zen 4 with Safe RET, zapping 4K pages takes one branch > miss per page fewer. MADV_DONTNEED of 1 GiB gets 6% faster on anonymous > memory and 8% on shmem, munmap() 5%. THP is unchanged. > > Fixes: e5136e876581 ("mm: Warn on shadow stack memory in wrong vma") > Signed-off-by: Kiryl Shutsemau (Meta) LGTM: Reviewed-by: Jose A. Perez de Azpillaga -- cheers, jose a. p-a