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b=USIKCcfyjFtFEaPVC8INwn/QcxQzBv9WvXILYl0bv3+zd6y8o59Z8T2xp/96MuKID WXVZwZyF+NQf8/5ZX6S/e+n6zPWca4NpWLPI6kRbuHsiBgX7ErEQi3YfKZ/LfimwFN mVvchsuZzrCGBWgSXJ2y5izJF1s1gFu90DK49uZ3OFMa8xoIZt0V992O1p0v/MCK94 26KJu+ManA0/aBVrzhxW3lk55nYrcFmC3azoj0lYcgBcvYhx2IZ6GjYdiziFM/fM+I fpzYoF0Q3TWWz2PGtN0woOOjuaJT8WQzmGty7LLIA7r9IfCK3fJJhcdRKQIr1iB0fd XXe5719IHqefw== Message-ID: <64d31de3-e838-4352-abef-cc742a5e5a56@kernel.org> Date: Tue, 3 Mar 2026 10:14:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] binfmt_elf: Align eligible read-only PT_LOAD segments to PMD_SIZE for THP To: hev , Matthew Wilcox Cc: Alexander Viro , Christian Brauner , Jan Kara , Kees Cook , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260302155046.286650-1-r@hev.cc> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 6FE541C0015 X-Stat-Signature: qkhoas1wd46ugcfhm598fu6e3zknt7du X-Rspam-User: X-HE-Tag: 1772529276-423745 X-HE-Meta: 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 87jujW6t cgt0ZJSNPEsaki1nkxxvA30StSPlOy+dqPzMqXRZqx/GTpJVLW4hz1pDLfuH2B08cf2w1mXOK2DouYFOjLvzzmRcb8Ea9fYDQV8+xQNOA+DHH79SJm+TuEz03OCPtSYkdxJ+ES+v9kExuJeRtGVHiBMdzuypQAG+iUE53EHUr2WNSt/KFdJ7Z619LCZjwX8RJKYKRZ61+e57Awc7H6i0e7uQqODDWDThewU5kCcZtZHSICmepT1KmXN8OVhLMWmXhsSSv5QJ4xuzVA6eVKf1RtfD6k+VyJxyQ/q2sI9Wkb6NNsEBRY+UUAh9eUzJaONG1TrvZiS3FwtPIlI7FukWsjha/rUsubeeqzihgtXl88aYJV5dd4/jOPbDoqL5Qn9p8yJIPUSj5yT8WcL4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/3/26 08:00, hev wrote: > On Tue, Mar 3, 2026 at 1:32 PM Matthew Wilcox wrote: >> >> On Tue, Mar 03, 2026 at 12:31:59PM +0800, hev wrote: >>> >>> This optimization is not entirely free. Increasing PT_LOAD alignment >>> can waste virtual address space, which is especially significant on >>> 32-bit systems, and it also reduces ASLR entropy by limiting the >>> number of possible load addresses. >>> >>> In addition, coarser alignment may have secondary microarchitectural >>> effects (eg. on indirect branch prediction), depending on the >>> workload. Because this change affects address space layout and >>> security-related properties, providing users with a way to opt out is >>> reasonable, rather than making it completely unconditional. This >>> behavior fits naturally under READ_ONLY_THP_FOR_FS. >> >> This isn't reasonable at all. You're asking distro maintainers to make >> a decision they have insufficient information to make. Almost none of >> our users compile their own kernels, and frankly those that do don't have >> enough information to make an informed decision about which way to choose. >> >> So if we're going to have a way to opt in/out, it needs to be something >> different. Maybe a heuristic based on size of text segment? Maybe an >> ELF flag? But then, if we're going to modify the binary, why not just >> set p_align and then we don't need this patch at all? > > I agree that a compile-time config is not a good fit here, and I’m > fine with dropping it in v2. > > Relying on ELF-side changes is problematic. Increasing p_align in the > linker inflates file size due to extra padding, and more importantly > it cannot help existing binaries. The loader is therefore the only > place where this can be done without ABI changes or file size > regressions. > > The logic here is deliberately strict rather than heuristic: the > segment must be read-only, at least PMD_SIZE in length, and PMD_SIZE > is capped at 32MB to avoid pathological address space waste. If these > conditions are not met, the layout is unchanged. > > I don’t see a reliable way to make a smarter decision at load time > without workload knowledge. With READ_ONLY_THP_FOR_FS already limiting > the scope and the THP policy applied at runtime, this keeps the > behavior constrained. A note that READ_ONLY_THP_FOR_FS will likely go away soon. -- Cheers, David