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[185.219.167.205]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48b0691d681sm5371173f8f.23.2026.10.01.05.32.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 01 Oct 2026 05:32:49 -0700 (PDT) Message-ID: <5e183c24-4acd-4b79-b601-bd1aa2f53dce@redhat.com> Date: Thu, 1 Oct 2026 14:32:46 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 4/7] maccess: Use a scoped guard for page faults To: "David Hildenbrand (Arm)" , Muhammad Usama Anjum Cc: Catalin Marinas , Will Deacon , Mark Rutland , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Jiri Olsa , Emil Tsalapatis , Ihor Solodrai , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" , open list , "open list:BPF [GENERAL] (Safe Dynamic Programs and Tools)" , "open list:MEMORY MANAGEMENT - CORE" References: <20260824160523.3907021-1-usama.anjum@arm.com> <20260824160523.3907021-5-usama.anjum@arm.com> <79daa73b-0d34-4e84-8450-56f90bc700d4@kernel.org> <98e9ac67-7df3-4c0a-8814-37e196d94020@arm.com> <7454d7a3-57c6-4883-a66b-26129e633c6e@kernel.org> From: Viktor Malik Content-Language: en-US In-Reply-To: <7454d7a3-57c6-4883-a66b-26129e633c6e@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/1/26 13:33, David Hildenbrand (Arm) wrote: > On 8/25/26 14:07, Muhammad Usama Anjum wrote: >> On 25/08/2026 11:36 am, David Hildenbrand (Arm) wrote: >>> On 8/24/26 18:04, Muhammad Usama Anjum wrote: >>>> Kernel nofault copy and string paths open-code page-fault disable and >>>> enable around label-based loops, duplicating cleanup on success and >>>> failure. >>>> >>>> Use a page-fault scope guard instead. Leaving the scope now re-enables >>>> page faults on both paths without separate cleanup at the fault label. >>>> >>>> No functional change. >>>> >>>> Signed-off-by: Muhammad Usama Anjum >>>> --- >>>> mm/maccess.c | 53 +++++++++++++++++++++++++--------------------------- >>>> 1 file changed, 25 insertions(+), 28 deletions(-) >>>> >>>> diff --git a/mm/maccess.c b/mm/maccess.c >>>> index c59a0e092d24a..f695ceefe6fcc 100644 >>>> --- a/mm/maccess.c >>>> +++ b/mm/maccess.c >>>> @@ -38,18 +38,17 @@ long copy_from_kernel_nofault(void *dst, const void *src, size_t size) >>>> if (!size) >>>> return 0; >>>> >>>> - pagefault_disable(); >>>> - if (!(align & 7)) >>>> - copy_from_kernel_nofault_loop(dst, src, size, u64, Efault); >>>> - if (!(align & 3)) >>>> - copy_from_kernel_nofault_loop(dst, src, size, u32, Efault); >>>> - if (!(align & 1)) >>>> - copy_from_kernel_nofault_loop(dst, src, size, u16, Efault); >>>> - copy_from_kernel_nofault_loop(dst, src, size, u8, Efault); >>>> - pagefault_enable(); >>>> + scoped_guard(pagefault) { >>>> + if (!(align & 7)) >>>> + copy_from_kernel_nofault_loop(dst, src, size, u64, Efault); >>>> + if (!(align & 3)) >>>> + copy_from_kernel_nofault_loop(dst, src, size, u32, Efault); >>>> + if (!(align & 1)) >>>> + copy_from_kernel_nofault_loop(dst, src, size, u16, Efault); >>>> + copy_from_kernel_nofault_loop(dst, src, size, u8, Efault); >>>> + } >>> >>> While I understand what pagefault_disable+pagefault_enable does, it is >>> completely unclear what scoped_guard(pagefault) does. >>> >>> Should this be scoped_guard(disabled_pagefaults) or sth like that? >>> >> >> The `pagefault` guard is already defined in `include/linux/uaccess.h`: > > Ah, sorry, I thought this was added in this series. > > It's completely confusing. > > guard(pagefault)(); > > meaning "disable pagefault", what?! :) > > CCin Viktor: I see that the patch that introduced that has no acks from MM. I > don't blame you, because the file is not actually listed under the MM section. You're right, looking at it back, it's a really unfortunate name. Renaming to nofault or disable_pagefault is certainly better. Thanks for pointing this out. Viktor > > I think this really needs to be cleaned up to clarify that we *disable* pagefaults. > > Usama, can you throw in a patch to clean that up? (calling it nofault, > disable_pagefaults or whatever) >