From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DA51F274FE3; Thu, 26 Mar 2026 13:40:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774532421; cv=none; b=bC5LPOFj0dNCx3/lVo28j7AaEz9XpQULlQ/p72/SoGKrruBmbkGebEZo+2CkZubbfpneqTviXISQRqTJxj48HJ0tiNCDklf8xZGg9SOdSUQYUUtik1EhFcXahxydDwQNxNW+SkqvAv11JpmtG9MHPNahiZtmc04US1MZ0NC2dh8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774532421; c=relaxed/simple; bh=3HDVDzgBT6pBYb1dZp8e3KS9x5vaj16RyMrBU2w15JM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=agDeJij8pTjCwxLIuPaJGbi0AsmvgJzUEjOeJ57F/dv5as8qbK0/SP1lGmMmFjZsb75IQJKRIwgaxIgwzoM95Zoe9K3GjvGcAD4MdD+OHIinBs60DNKgU14qfrOMTaSmEszJnI5IBzhxBnFCtEDK6N7/ZUPdlZR0diZhN16d4i0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=T32UXXR1; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="T32UXXR1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 32B8DC116C6; Thu, 26 Mar 2026 13:40:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774532421; bh=3HDVDzgBT6pBYb1dZp8e3KS9x5vaj16RyMrBU2w15JM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=T32UXXR1tQTCoVlkvZWie5KvjD1ljqxmYFHsj8Z4NKFR0Ab3ZJzYYbaeT9/9rYcFn bcl7Sv6NyyjjAgHQWZ43wSGQ6LwhCXKF/O6Z1IQQ9hQkACUepRecG8TaNJMiwJjGcW 3XR31oPpvnJoJOfnwJayk7O+Ii1XctcYZ4HcIXGhSmdtwW/v3W57Tpg1rVJko2o1P1 OwH9tCyHSoYXXMsGmcGjJ4lnNF92WAqR+BQNMGCQq3IJk/pCjkwhmhc/EJW3Mno1xj 5R5VKqso4COmS/3kjV3jIqTtR6CLB0arm6zDK2vrLRs9t+3SQwO6foTw8Wk4DuAP8u hSFnBdvXMvuRA== Message-ID: <37c1e8d5-55a8-45ae-9845-80317dcea888@kernel.org> Date: Thu, 26 Mar 2026 14:40:09 +0100 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 0/3] KASAN: HW_TAGS: Disable tagging for stack and page-tables To: Muhammad Usama Anjum , "David Hildenbrand (Arm)" Cc: Arnd Bergmann , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Kees Cook , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Uladzislau Rezki , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrey Konovalov , Marco Elver , Vincenzo Frascino , Peter Collingbourne , Catalin Marinas , Will Deacon , Ryan.Roberts@arm.com References: <20260319114952.3241359-1-usama.anjum@arm.com> <1e71acc8-8dde-4fd5-8428-e76e8c86121b@kernel.org> <111b71af-a175-422d-a7d6-a204ce6f679f@arm.com> 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: <111b71af-a175-422d-a7d6-a204ce6f679f@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 3/23/26 16:06, Muhammad Usama Anjum wrote: > On 20/03/2026 8:53 am, David Hildenbrand (Arm) wrote: >> On 3/19/26 12:49, Muhammad Usama Anjum wrote: >>> Stacks and page tables are always accessed with the match‑all tag, >>> so assigning a new random tag every time at allocation and setting >>> invalid tag at deallocation time, just adds overhead without improving >>> the detection. >>> >>> With __GFP_SKIP_KASAN the page keeps its poison tag and KASAN_TAG_KERNEL >>> (match-all tag) is stored in the page flags while keeping the poison tag >>> in the hardware. The benefit of it is that 256 tag setting instruction >>> per 4 kB page aren't needed at allocation and deallocation time. >>> >>> Thus match‑all pointers still work, while non‑match tags (other than >>> poison tag) still fault. >>> >>> __GFP_SKIP_KASAN only skips for KASAN_HW_TAGS mode, so coverage is >>> unchanged. >>> >>> Benchmark: >>> The benchmark has two modes. In thread mode, the child process forks >>> and creates N threads. In pgtable mode, the parent maps and faults a >>> specified memory size and then forks repeatedly with children exiting >>> immediately. >>> >>> Thread benchmark: >>> 2000 iterations, 2000 threads: 2.575 s → 2.229 s (~13.4% faster) >>> >>> The pgtable samples: >>> - 2048 MB, 2000 iters 19.08 s → 17.62 s (~7.6% faster) >> >> As discussed offline, I think we should look into finding a better name >> for __GFP_SKIP_KASAN now that we are using it more broadly. It's confusing. > Agreed that its confusing and the name doesn't show its under-the-hood usage. > And I think I finally realized that __GFP_SKIP_KASAN is used for two independent use cases, something that really must be sorted out. >> >> The semantics are: >> * Only applies to HW KASAN right now. Otherwise it's ignored. So it >> doesn't give any guarantees. >> * Will currently leave memory tagged with some tag (poisoned), but >> tag checks will be disabled by using the match-all pointer. >> >> After pondering about that for a while, I realized that today, all >> memory is tagged by default, and __GFP_SKIP_KASAN is our mechanism to >> request memory that will not be tag-checked (close to if it would be not >> tagged). > KASAN uses the poisoning and un-poisoning terminologies. It depends upon > the type of KASAN enabled that how poisoning/unpoisoning is done. And that's an implementation detail. A random memory allocation shouldn't have to know what KASAN or POISONING is. :) > >> >> Is there a real difference to getting untagged memory, if supported by >> the architecture. >> >> So I was wondering if >> >> __GFP_UNTAGGED: if possible, return memory that is either >> untagged or that is tagged but has tag checks >> disabled when accessed through page_address(). >> Using this flag can speed up page allocation >> and freeing, and can reduce runtime overhead >> by not performing page checking. For now, >> only considered with HW-tag based KASAN. > Its again confusing as __GFP_UNTAGGED will not return untagged memory > in case of KASAN_SW_TAGS. > > As __GFP_SKIP_KASAN skips only for HW_TAGS mode, the more appropriate name > may be: > __GFP_SKIP_HW_POSION Also not really the right fit I think. > > No matter the final name, it may be worth the effort to rename / do better > handling of this in the code. Let's keep it a separate from this series. Well, the point I am making is that (1) you are adding more users of __GFP_SKIP_KASAN (2) __GFP_SKIP_KASAN is a mess I'll try to sort that out, but be prepared that the flag name might change underneath your feet :) -- Cheers, David