From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 EB882423E9F for ; Tue, 4 Aug 2026 07:45:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785829537; cv=none; b=rU3qD1XUwvVGBKPaAvdna74bsxYjI4xELa+Wf3hD6xYDriGqyVYei9dUXs8E6JLfQxVLpgrJ72xprGDpdAy0O1eu9eYCPb053AT2+ROxEXw9gN1OtOdjxXdTEgCmtseR/W+u18Y4PIrwJYTtupIYdIX8tYzykXDDjTwTvGlmKEU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785829537; c=relaxed/simple; bh=aZkgCqD0kzlOPmGRC3yEoApOP8F4Mi4CVHGuGs7Lskw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=lLxq3T5l2BuVycNf83dl92FfgxH1m454Mq79lnw3Lj3NneWdNAlXhSN3frmkB+FdEKmYFq1CptOjPwjqMQ0TWkGDcur5dwWG1gcgYdWyyZPqu7KRONik7IASvd+oGVW2/QrNosJ7BqPWFfljMOtUQrPYqjGNXQ8ovTPn/OVh9/w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=d/98Lr4d; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="d/98Lr4d" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 01D2F1F000E9; Tue, 4 Aug 2026 07:45:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785829535; bh=4Ka+jl8gf05RP97qyqt/sUNR4hY0647pDsAbdaoC8Ic=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=d/98Lr4dSJNut1FfWPcs4jMI3FJmCyMVwfkYex5tcTa1uI6vxFZMMPb5NJxNzVaHD HLpDBBr1UpF5VFGeirh9PkpmekaYs2Utwq9xj7lUC0mILEy1sGy+gratdQiwbllsZv ZBqW/zzAvTTX8w3/oWwzfW3bwynM3gR13Gdb9Y+y46z+wUKXNYhxUoDOUlje9QjPiE bhVJB1Vf7c34A7nI06DHV7duYp/HW4435PvFKbJRzU2wfW4vudzFzNz84QAGuOoLWo tnpyWjbm7qFuPdb0M67QvAf3+JPMAE9znLbq3fo4guQgXkdS9ZB4P/oZgolWhVjfER YG9FMfikYZ0sg== Message-ID: <3a8cc92d-3e4c-4976-8b01-b303cab3061b@kernel.org> Date: Tue, 4 Aug 2026 09:45:31 +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 v2] fork: Honor task_struct's declared alignment To: Karl Mehltretter , Ingo Molnar , Peter Zijlstra , Andrew Morton Cc: Kees Cook , Bradley Morgan , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260804064006.93930-1-kmehltretter@gmail.com> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/4/26 08:40, Karl Mehltretter wrote: > Since commit cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant > size"), struct task_struct is declared __attribute__((aligned(64))) on > all architectures. > > But fork_init() sets the task_struct slab cache's alignment to > > align = max(L1_CACHE_BYTES, ARCH_MIN_TASKALIGN) > > which is smaller than 64 on architectures whose cache lines are below > 64 bytes: e.g. 32 on ARMv5. > > In practice plain SLUB happens to hand out 64-byte-aligned objects > anyway. With CONFIG_SLUB_DEBUG_ON the red-zone padding shifts objects > to the requested alignment. > > With CONFIG_UBSAN_ALIGNMENT=y a boot on QEMU versatilepb (ARM926EJ-S, > v7.2-rc2, gcc 13.3) floods the console with reports like: > > UBSAN: misaligned-access in include/linux/sched.h:2087:9 > member access within misaligned address c295d7e0 for type 'struct task_struct' > which requires 64 byte alignment > CPU: 0 UID: 0 PID: 15 Comm: pr/ttyAMA-1 Not tainted 7.2.0-rc2 #1 VOLUNTARY > > Set the slab alignment to at least the type's declared alignment. > Replace the hardcoded L1_CACHE_BYTES with SLAB_HWCACHE_ALIGN so the > allocator applies cache_line_size(). On x86, arm and arm64 that is the > cache line size of the booted CPU. Everywhere else it falls back to > L1_CACHE_BYTES and nothing changes. ARCH_MIN_TASKALIGN (e.g. 4096 with > x86 VSMP) still applies through the explicit align argument. > > Fixes: cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant size") > Suggested-by: Vlastimil Babka (SUSE) Thanks but I suggested only SLAB_HWCACHE_ALIGN, not the whole thing :) Can be removed when a maintainer applies this. > Assisted-by: Claude:claude-fable-5 > Signed-off-by: Karl Mehltretter Reviewed-by: Vlastimil Babka (SUSE) > --- > > Notes (format-patch): > v2: > - fold in Vlastimil's suggestion: let SLAB_HWCACHE_ALIGN replace the > hardcoded L1_CACHE_BYTES > v1: https://lore.kernel.org/all/20260710123957.31774-1-kmehltretter@gmail.com/ > > kernel/fork.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/kernel/fork.c b/kernel/fork.c > index f0e2e131a9a5..3a0093efb29f 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -857,14 +857,15 @@ void __init fork_init(void) > #ifndef ARCH_MIN_TASKALIGN > #define ARCH_MIN_TASKALIGN 0 > #endif > - int align = max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN); > + int align = max(ARCH_MIN_TASKALIGN, > + __alignof__(struct task_struct)); > unsigned long useroffset, usersize; > > /* create a slab on which task_structs can be allocated */ > task_struct_whitelist(&useroffset, &usersize); > task_struct_cachep = kmem_cache_create_usercopy("task_struct", > arch_task_struct_size, align, > - SLAB_PANIC|SLAB_ACCOUNT, > + SLAB_PANIC|SLAB_ACCOUNT|SLAB_HWCACHE_ALIGN, > useroffset, usersize, NULL); > > /* do the arch specific task caches init */ > > base-commit: af5e34a41cd607c00ef752e00331736570992354