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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 , 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 X-Rspamd-Server: rspam10 X-Stat-Signature: x86w4hbw8sy3o9hnyjcppps4o8orkjt4 X-Rspam-User: X-Rspamd-Queue-Id: 6B68410000A X-HE-Tag: 1776864097-543312 X-HE-Meta: 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 YVndV3d7 q5Rmeb2xfFxCb3x2h8srpc6YQRxXgtCHlB1NwgnVImLXNdNWGSQ15Tv7Xak+TKoahk75LMraivHlXSp3OEUpF4o0LXz6tUHznH2m7TPYgXJgsk2EY19i/KZmvUN7nk2xj8JSrjaN5QskvqeoswlNR+S6LealkyZUigxneyNoJ4x9dnO3XHScqN51nL3iLJVfma1kRiqNjBcr0/JifJPt15O9vAnOO58oZuNHtKR48pVEaWW1pgZt9Te+fGjrnvIQ3hGZilHzwlfz4JMkPyGqW0KFtw3nt5yH7E7LnwPeynq94yfhAf4WLkCAQyGVhFvQTISPiRyO0nzCXgbqIYusKkz9zveMTkjU0J9QqeKXCLb90hzc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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