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 D87833CD8CA; Wed, 22 Apr 2026 13:21:36 +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=1776864098; cv=none; b=MsDYjirRPA4eVeh9VdhQn7VkKD4BZpdTcJsf0UrLLP0346kBwSwx2NdCnxBwId8ZrS02vtgtlS6zkH1iJSvU08iq1O54au1J/9ZcSBR4c1HOEkAhjgpVnicSVI9jgJ59TPqEeAIsyzvb95SPRTo3aueRYb/BphuwL9JDEKY8Yzw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776864098; c=relaxed/simple; bh=SUhegrWOX2svV6mP7JpNfDMarrvi6LsYsdK7yk17+VU=; h=Message-ID:Date:MIME-Version:From:Subject:To:References: In-Reply-To:Content-Type; b=QJjX+63I+EAgbh5XwMPGk9mlb9Nq/57hQfX3Gs7hHALR9st/fVJd2AkzGlhCCalQyXQHMWANkgKvLChqqqkfGjnDcjKtzbtTT4KmRR5j435T+F8FILPQAXvHrE1xUiUUc5HWfLKiu0tahmzVAnas+kiOI3PzQoyr8VPuBOp1KkQ= 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; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=JYqwc4in; 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 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="JYqwc4in" 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 9B5DA22D9; Wed, 22 Apr 2026 06:21:30 -0700 (PDT) Received: from [10.57.89.34] (unknown [10.57.89.34]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4E8873FD46; Wed, 22 Apr 2026 06:21:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1776864096; bh=SUhegrWOX2svV6mP7JpNfDMarrvi6LsYsdK7yk17+VU=; h=Date:From:Subject:To:References:In-Reply-To:From; b=JYqwc4inV6mHPq9hN6YI3+EkaNg0zntDPZFli+fquHEtjJdrWF/oJ5zhx8l8amFZt 75lBDJU26hCuf55I0FuvZTs90Wt0Z76/DiSrzpQgsEu2PvtFoTrgN5ba58f1cvF76i /OPYInSuiknn1QL1DIDErtNh4sFAgF1KfVoDDCYk= Message-ID: <727df89e-2069-4a7d-b3c0-88f89cd3dcf8@arm.com> Date: Wed, 22 Apr 2026 14:21:29 +0100 Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Ryan Roberts Subject: Re: [PATCH v2 1/3] vmalloc: add __GFP_SKIP_KASAN support 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: <20260324132631.482520-1-usama.anjum@arm.com> <20260324132631.482520-2-usama.anjum@arm.com> Content-Language: en-GB In-Reply-To: <20260324132631.482520-2-usama.anjum@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 24/03/2026 13:26, Muhammad Usama Anjum wrote: > For allocations that will be accessed only with match-all pointers > (e.g., kernel stacks), setting tags is wasted work. If the caller > already set __GFP_SKIP_KASAN, don’t skip zeroing the pages and > don’t set KASAN_VMALLOC_PROT_NORMAL so kasan_unpoison_vmalloc() > returns early without tagging. > > Before this patch, __GFP_SKIP_KASAN wasn't being used with vmalloc > APIs. So it wasn't being checked. Now its being checked and acted > upon. Other KASAN modes are unchanged because __GFP_SKIP_KASAN isn't > defined there. > > This is a preparatory patch for optimizing kernel stack allocations. > > Signed-off-by: Muhammad Usama Anjum > --- > Changes since v1: > - Simplify skip conditions based on the fact that __GFP_SKIP_KASAN > is zero in non-hw-tags mode. > - Add __GFP_SKIP_KASAN to GFP_VMALLOC_SUPPORTED list of flags > --- > mm/vmalloc.c | 11 ++++++++--- > 1 file changed, 8 insertions(+), 3 deletions(-) > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > index c607307c657a6..69ae205effb46 100644 > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -3939,7 +3939,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask, > __GFP_NOFAIL | __GFP_ZERO |\ > __GFP_NORETRY | __GFP_RETRY_MAYFAIL |\ > GFP_NOFS | GFP_NOIO | GFP_KERNEL_ACCOUNT |\ > - GFP_USER | __GFP_NOLOCKDEP) > + GFP_USER | __GFP_NOLOCKDEP | __GFP_SKIP_KASAN) > > static gfp_t vmalloc_fix_flags(gfp_t flags) > { > @@ -3980,6 +3980,8 @@ static gfp_t vmalloc_fix_flags(gfp_t flags) > * > * %__GFP_NOWARN can be used to suppress failure messages. > * > + * %__GFP_SKIP_KASAN can be used to skip poisoning You mean skip *un*poisoning, I think? But you would only want this to apply to the actaul pages mapped by vmalloc. You wouldn't want to skip unpoisoning for any allocated meta data; I think that is currently possible since the gfp_flags that are passed into __vmalloc_node_range_noprof() are passed down to __get_vm_area_node() unmdified. You probably want to explicitly ensure __GFP_SKIP_KASAN is clear for that internal call? > + * > * Can not be called from interrupt nor NMI contexts. > * Return: the address of the area or %NULL on failure > */ > @@ -4041,7 +4043,9 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align, > * kasan_unpoison_vmalloc(). > */ > if (pgprot_val(prot) == pgprot_val(PAGE_KERNEL)) { > - if (kasan_hw_tags_enabled()) { > + bool skip_kasan = gfp_mask & __GFP_SKIP_KASAN; > + > + if (kasan_hw_tags_enabled() && !skip_kasan) { > /* > * Modify protection bits to allow tagging. > * This must be done before mapping. > @@ -4057,7 +4061,8 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align, > } > > /* Take note that the mapping is PAGE_KERNEL. */ > - kasan_flags |= KASAN_VMALLOC_PROT_NORMAL; > + if (!skip_kasan) > + kasan_flags |= KASAN_VMALLOC_PROT_NORMAL; It's pretty ugly to use the absence of this flag to rely on kasan_unpoison_vmalloc() not unpoisoning. Perhaps it is preferable to just not call kasan_unpoison_vmalloc() for the skip_kasan case? > } > > /* Allocate physical pages and map them into vmalloc space. */ Perhaps something like this would work: ---8<--- diff --git a/mm/vmalloc.c b/mm/vmalloc.c index c31a8615a8328..c340db141df57 100644 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c @@ -3979,6 +3979,8 @@ static gfp_t vmalloc_fix_flags(gfp_t flags) * under moderate memory pressure. * * %__GFP_NOWARN can be used to suppress failure messages. + + * %__GFP_SKIP_KASAN skip unpoisoning of mapped pages (when prot=PAGE_KERNEL). * * Can not be called from interrupt nor NMI contexts. * Return: the address of the area or %NULL on failure @@ -3993,6 +3995,9 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align, kasan_vmalloc_flags_t kasan_flags = KASAN_VMALLOC_NONE; unsigned long original_align = align; unsigned int shift = PAGE_SHIFT; + bool skip_kasan = gfp_mask & __GFP_SKIP_KASAN; + + gfp_mask &= ~__GFP_SKIP_KASAN; if (WARN_ON_ONCE(!size)) return NULL; @@ -4041,7 +4046,7 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align, * kasan_unpoison_vmalloc(). */ if (pgprot_val(prot) == pgprot_val(PAGE_KERNEL)) { - if (kasan_hw_tags_enabled()) { + if (kasan_hw_tags_enabled() && !skip_kasan) { /* * Modify protection bits to allow tagging. * This must be done before mapping. @@ -4054,6 +4059,12 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align, * poisoned and zeroed by kasan_unpoison_vmalloc(). */ gfp_mask |= __GFP_SKIP_KASAN | __GFP_SKIP_ZERO; + } else if (skip_kasan) { + /* + * Skip page_alloc unpoisoning physical pages backing + * VM_ALLOC mapping, as requested by caller. + */ + gfp_mask |= __GFP_SKIP_KASAN; } /* Take note that the mapping is PAGE_KERNEL. */ @@ -4078,7 +4089,8 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align, (gfp_mask & __GFP_SKIP_ZERO)) kasan_flags |= KASAN_VMALLOC_INIT; /* KASAN_VMALLOC_PROT_NORMAL already set if required. */ - area->addr = kasan_unpoison_vmalloc(area->addr, size, kasan_flags); + if (!skip_kasan) + area->addr = kasan_unpoison_vmalloc(area->addr, size, kasan_flags); /* * In this function, newly allocated vm_struct has VM_UNINITIALIZED ---8<--- Thanks, Ryan