kasan: call clear_page with a match-all tag instead of changing page tag

Instead of changing the page's tag solely in order to obtain a pointer
with a match-all tag and then changing it back again, just convert the
pointer that we get from kmap_atomic() into one with a match-all tag
before passing it to clear_page().

On a certain microarchitecture, this has been observed to cause a
measurable improvement in microbenchmark performance, presumably as a
result of being able to avoid the atomic operations on the page tag.

Link: https://lkml.kernel.org/r/20230216195924.3287772-1-pcc@google.comSigned-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I0249822cc29097ca7a04ad48e8eb14871f80e711Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/highmem.h

@@ -243,12 +243,10 @@ static inline void clear_highpage(struct page *page)
 
 static inline void clear_highpage_kasan_tagged(struct page *page)
 {
-	u8 tag;
+	void *kaddr = kmap_local_page(page);
 
-	tag = page_kasan_tag(page);
-	page_kasan_tag_reset(page);
-	clear_highpage(page);
-	page_kasan_tag_set(page, tag);
+	clear_page(kasan_reset_tag(kaddr));
+	kunmap_local(kaddr);
 }
 
 #ifndef __HAVE_ARCH_TAG_CLEAR_HIGHPAGE