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vmalloc: add __GFP_SKIP_KASAN support

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Patch series "kasan: hw_tags: Disable tagging for stack and page-tables",
v4.

Stacks and page tables are always accessed with the match-all tag, so
assigning a new random tag every time at allocation and setting invalid
tag at deallocation time, just adds overhead without improving the
detection.

With __GFP_SKIP_KASAN the page keeps its poison tag and KASAN_TAG_KERNEL
(match-all tag) is stored in the page flags while keeping the poison tag
in the hardware.  The benefit of it is that 256 tag setting instruction
per 4 kB page aren't needed at allocation and deallocation time.

Thus match-all pointers still work, while non-match tags (other than
poison tag) still fault.

__GFP_SKIP_KASAN only skips for KASAN_HW_TAGS mode, so coverage is
unchanged.

Benchmark:
The benchmark has two modes. In thread mode, the child process forks
and creates N threads. In pgtable mode, the parent maps and faults a
specified memory size and then forks repeatedly with children exiting
immediately.

Thread benchmark:
2000 iterations, 2000 threads:	2.575 s → 2.229 s (~13.4% faster)

The pgtable samples:
- 2048 MB, 2000 iters		19.08 s → 17.62 s (~7.6% faster)


This patch (of 3):

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, skip tag setting of vmalloc pages.

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 is ignored for them in
the page allocator, and in vmalloc too we ignore this flag for them.

This is a preparatory patch for optimizing kernel stack allocations.

Link: https://lore.kernel.org/20260429102704.680174-1-dev.jain@arm.com
Link: https://lore.kernel.org/20260429102704.680174-2-dev.jain@arm.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Co-developed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ben Segall <bsegall@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Muhammad Usama Anjum 2026-04-29 15:57:02 +05:30 committed by Andrew Morton
parent 838376c60d
commit b56ca146a2
2 changed files with 12 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
include/linux/gfp_types.h
View File

@ -281,9 +281,9 @@ enum {
* *
* %__GFP_SKIP_KASAN makes KASAN skip unpoisoning on page allocation. * %__GFP_SKIP_KASAN makes KASAN skip unpoisoning on page allocation.
* Used for userspace and vmalloc pages; the latter are unpoisoned by * Used for userspace and vmalloc pages; the latter are unpoisoned by
* kasan_unpoison_vmalloc instead. For userspace pages, results in * kasan_unpoison_vmalloc instead. If passed to vmalloc, kasan_unpoison_vmalloc
* poisoning being skipped as well, see should_skip_kasan_poison for * is skipped too. For userspace pages, results in poisoning being skipped as
* details. Only effective in HW_TAGS mode. * well, see should_skip_kasan_poison for details. Only effective in HW_TAGS mode.
*/ */
#define __GFP_NOWARN ((__force gfp_t)___GFP_NOWARN) #define __GFP_NOWARN ((__force gfp_t)___GFP_NOWARN)
#define __GFP_COMP ((__force gfp_t)___GFP_COMP) #define __GFP_COMP ((__force gfp_t)___GFP_COMP)

13
mm/vmalloc.c
View File

@ -3933,7 +3933,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
__GFP_NOFAIL | __GFP_ZERO |\ __GFP_NOFAIL | __GFP_ZERO |\
__GFP_NORETRY | __GFP_RETRY_MAYFAIL |\ __GFP_NORETRY | __GFP_RETRY_MAYFAIL |\
GFP_NOFS | GFP_NOIO | GFP_KERNEL_ACCOUNT |\ 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) static gfp_t vmalloc_fix_flags(gfp_t flags)
{ {
@ -3974,6 +3974,9 @@ static gfp_t vmalloc_fix_flags(gfp_t flags)
* *
* %__GFP_NOWARN can be used to suppress failure messages. * %__GFP_NOWARN can be used to suppress failure messages.
* *
* %__GFP_SKIP_KASAN can be used to skip unpoisoning of mapped pages
* (when prot=%PAGE_KERNEL).
*
* Can not be called from interrupt nor NMI contexts. * Can not be called from interrupt nor NMI contexts.
* Return: the address of the area or %NULL on failure * Return: the address of the area or %NULL on failure
*/ */
@ -3987,6 +3990,7 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align,
kasan_vmalloc_flags_t kasan_flags = KASAN_VMALLOC_NONE; kasan_vmalloc_flags_t kasan_flags = KASAN_VMALLOC_NONE;
unsigned long original_align = align; unsigned long original_align = align;
unsigned int shift = PAGE_SHIFT; unsigned int shift = PAGE_SHIFT;
bool skip_vmalloc_kasan = kasan_hw_tags_enabled() && (gfp_mask & __GFP_SKIP_KASAN);
if (WARN_ON_ONCE(!size)) if (WARN_ON_ONCE(!size))
return NULL; return NULL;
@ -4017,7 +4021,7 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align,
again: again:
area = __get_vm_area_node(size, align, shift, VM_ALLOC | area = __get_vm_area_node(size, align, shift, VM_ALLOC |
VM_UNINITIALIZED | vm_flags, start, end, node, VM_UNINITIALIZED | vm_flags, start, end, node,
gfp_mask, caller); gfp_mask & ~__GFP_SKIP_KASAN, caller);
if (!area) { if (!area) {
bool nofail = gfp_mask & __GFP_NOFAIL; bool nofail = gfp_mask & __GFP_NOFAIL;
warn_alloc(gfp_mask, NULL, warn_alloc(gfp_mask, NULL,
@ -4035,7 +4039,7 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align,
* kasan_unpoison_vmalloc(). * kasan_unpoison_vmalloc().
*/ */
if (pgprot_val(prot) == pgprot_val(PAGE_KERNEL)) { if (pgprot_val(prot) == pgprot_val(PAGE_KERNEL)) {
if (kasan_hw_tags_enabled()) { if (kasan_hw_tags_enabled() && !skip_vmalloc_kasan) {
/* /*
* Modify protection bits to allow tagging. * Modify protection bits to allow tagging.
* This must be done before mapping. * This must be done before mapping.
@ -4072,7 +4076,8 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align,
(gfp_mask & __GFP_SKIP_ZERO)) (gfp_mask & __GFP_SKIP_ZERO))
kasan_flags |= KASAN_VMALLOC_INIT; kasan_flags |= KASAN_VMALLOC_INIT;
/* KASAN_VMALLOC_PROT_NORMAL already set if required. */ /* KASAN_VMALLOC_PROT_NORMAL already set if required. */
area->addr = kasan_unpoison_vmalloc(area->addr, size, kasan_flags); if (!skip_vmalloc_kasan)
area->addr = kasan_unpoison_vmalloc(area->addr, size, kasan_flags);
/* /*
* In this function, newly allocated vm_struct has VM_UNINITIALIZED * In this function, newly allocated vm_struct has VM_UNINITIALIZED