[PATCH v2 1/3] vmalloc: add __GFP_SKIP_KASAN support

[Muhammad Usama Anjum <usama.anjum@arm.com>]

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 <usama.anjum@arm.com>
---
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;
 	}
 
 	/* Allocate physical pages and map them into vmalloc space. */
-- 
2.47.3