From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id BE30C3CE48A; Thu, 19 Mar 2026 12:09:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773922149; cv=none; b=euszN183nJOgnMzDrYmHv2kDdGq4c02Wy+n1pqjKbifA2HXZgZ/sqTQHg4s3S1BrQSuGYYLmTBptR2rYWXLPQTrg9PbUfmtXugvLS6atS5fQEXI2S+vMJOWw81nijJUkFCGFSkDvXY0OO79jvnbgQU7jektkH00MmsKOb07jDjU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773922149; c=relaxed/simple; bh=TW5IgCCp8VHyytIuxKqF/1+2MrXUjyG2pTHEJSqB2Xk=; h=Message-ID:Date:MIME-Version:Subject:To:References:From: In-Reply-To:Content-Type; b=KxU3bFtaHCh95EN9/+jRRdX8s4oSDdhqRW3m1KQ5hrw59ouSt+FqLwNJAeXMt2U4nhUM3GB/Sq/q67LH4xO+AXM7NkHEWPYCYkI+faZEPpLC6HhaTyQ2v9M01TBAAIjI7OnJrSGIR2Mj/VsPB6da54QEue5fOQVZhKgzgBTM77c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id EA7B91A25; Thu, 19 Mar 2026 05:09:00 -0700 (PDT) Received: from [10.57.85.34] (unknown [10.57.85.34]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 185B93F778; Thu, 19 Mar 2026 05:09:02 -0700 (PDT) Message-ID: Date: Thu, 19 Mar 2026 12:09:01 +0000 Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/3] fork: skip MTE tagging for kernel stacks Content-Language: en-GB To: Muhammad Usama Anjum , Arnd Bergmann , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Kees Cook , Andrew Morton , David Hildenbrand , 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 , david.hildenbrand@arm.com References: <20260319114952.3241359-1-usama.anjum@arm.com> <20260319114952.3241359-3-usama.anjum@arm.com> From: Ryan Roberts In-Reply-To: <20260319114952.3241359-3-usama.anjum@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 19/03/2026 11:49, Muhammad Usama Anjum wrote: > The stack pointer always uses the match-all tag, so MTE never checks > tags on stack accesses. Tagging stack memory on every thread creation > is pure overhead. > > - Pass __GFP_SKIP_KASAN in gfp_mask for vmalloc-backed stacks so the > vmalloc path skips HW tag setup (see previous patch). > - For the cached VMAP reuse path, skip kasan_unpoison_range() when HW > tags are enabled since the memory will only be accessed through the > match-all tagged SP. > - For the normal page allocator path, pass __GFP_SKIP_KASAN directly > to the page allocator. > > Signed-off-by: Muhammad Usama Anjum > --- > kernel/fork.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/kernel/fork.c b/kernel/fork.c > index bb0c2613a5604..2baf4db39b5a4 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -345,7 +345,8 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node) > } > > /* Reset stack metadata. */ > - kasan_unpoison_range(vm_area->addr, THREAD_SIZE); > + if (!kasan_hw_tags_enabled()) > + kasan_unpoison_range(vm_area->addr, THREAD_SIZE); > > stack = kasan_reset_tag(vm_area->addr); > > @@ -358,7 +359,7 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node) > } > > stack = __vmalloc_node(THREAD_SIZE, THREAD_ALIGN, > - GFP_VMAP_STACK, > + GFP_VMAP_STACK | __GFP_SKIP_KASAN, Perhaps cleaner to include __GFP_SKIP_KASAN in GFP_VMAP_STACK ? > node, __builtin_return_address(0)); > if (!stack) > return -ENOMEM; > @@ -410,7 +411,8 @@ static void thread_stack_delayed_free(struct task_struct *tsk) > > static int alloc_thread_stack_node(struct task_struct *tsk, int node) > { > - struct page *page = alloc_pages_node(node, THREADINFO_GFP, > + struct page *page = alloc_pages_node(node, > + THREADINFO_GFP | __GFP_SKIP_KASAN, I think there are some other places that could benefit from __GFP_SKIP_KASAN; see arm64's arch_alloc_vmap_stack(), which allocates stacks for efi, irq and sdei. I think these are allocated at boot, so not really performance sensitive, but we might as well be consistent? You've also missed the alloc_thread_stack_node() implementation for !VMAP when PAGE_SIZE > STACK_SIZE. All of these sites use THREADINFO_GFP so perhaps it is better to just define THREADINFO_GFP to include __GFP_SKIP_KASAN ? Thanks, Ryan > THREAD_SIZE_ORDER); > > if (likely(page)) {