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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 , Will Deacon , Ryan.Roberts@arm.com, david.hildenbrand@arm.com Subject: Re: [PATCH v2 1/3] vmalloc: add __GFP_SKIP_KASAN support Message-ID: References: <20260324132631.482520-1-usama.anjum@arm.com> <20260324132631.482520-2-usama.anjum@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260324132631.482520-2-usama.anjum@arm.com> X-Rspamd-Queue-Id: 2EB0718000C X-Stat-Signature: fo1htoa73oeurfy6dxpgdwdu9k57cbre X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1775844629-428861 X-HE-Meta: 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 SIti253l a/dnU8qiFUk2lvqpNL0K490aEmXAhyIpg8CRMm6EPT9j9atw+HGJ0BVPjKe5NDWL7l7LHmvp7rrVNWq5/pO09TDRK1793Tsn8T/xSB3uS1Yenqk3YcJ07SeM2keJSvqfYmKeMpCWzsIEZioyekQJgQ/eciyRBpic9eOar661F5ncAOiBjrE8VmR8kaZewCKZ1i53mU57VgMkiYeBRDd5Dcmd8O+Ec2SllbaZ7O8AoT5uAPLY0H2D+NKXHRYSP898J3j1wTnCO3LEMhlZd8vddnJsqxaMK4S363lq6ItqIEGoXDGAl30vvBllXAzAMJzzF1nowzWOKk9WfRFG6vnOxtd7GYxwXIZB2ECq62+i1sIE0dHs= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Mar 24, 2026 at 01:26:27PM +0000, 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 > + * > * 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; > } In the cover letter, you said that __GFP_SKIP_KASAN is only meant for KASAN_HW_TAGS. IIUC, here you skip passing KASAN_VMALLOC_PROT_NORMAL even for KASAN_SW_TAGS. The flag is used in mm/kasan/shadow.c. -- Catalin