[PATCH v2] kasan: call clear_page with a match-all tag instead of changing page tag

[Peter Collingbourne <pcc@google.com>]

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.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I0249822cc29097ca7a04ad48e8eb14871f80e711
---
v2:
- switched to kmap_local_page()

 include/linux/highmem.h | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 44242268f53b..212fd081b227 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -245,12 +245,10 @@ 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
-- 
2.39.2.637.g21b0678d19-goog