[PATCH v4 1/2] Revert "kasan: drop skip_kasan_poison variable in free_pages_prepare"

[Peter Collingbourne]

This reverts commit 487a32ec24be819e747af8c2ab0d5c515508086a.

The should_skip_kasan_poison() function reads the PG_skip_kasan_poison
flag from page->flags. However, this line of code in free_pages_prepare():

page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;

clears most of page->flags, including PG_skip_kasan_poison, before calling
should_skip_kasan_poison(), which meant that it would never return true
as a result of the page flag being set. Therefore, fix the code to call
should_skip_kasan_poison() before clearing the flags, as we were doing
before the reverted patch.

This fixes a measurable performance regression introduced in the
reverted commit, where munmap() takes longer than intended if HW
tags KASAN is supported and enabled at runtime. Without this patch,
we see a single-digit percentage performance regression in a particular
mmap()-heavy benchmark when enabling HW tags KASAN, and with the patch,
there is no statistically significant performance impact when enabling
HW tags KASAN.

Signed-off-by: Peter Collingbourne <pcc at google.com>
Fixes: 487a32ec24be ("kasan: drop skip_kasan_poison variable in free_pages_prepare")
Cc: <stable at vger.kernel.org> # 6.1
Link: https://linux-review.googlesource.com/id/Ic4f13affeebd20548758438bb9ed9ca40e312b79
Reviewed-by: Andrey Konovalov <andreyknvl at gmail.com>
---
 mm/page_alloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1c54790c2d17..c58ebf21ce63 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1413,6 +1413,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
 			unsigned int order, fpi_t fpi_flags)
 {
 	int bad = 0;
+	bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
 	bool init = want_init_on_free();
 
 	VM_BUG_ON_PAGE(PageTail(page), page);
@@ -1489,7 +1490,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
 	 * With hardware tag-based KASAN, memory tags must be set before the
 	 * page becomes unavailable via debug_pagealloc or arch_free_page.
 	 */
-	if (!should_skip_kasan_poison(page, fpi_flags)) {
+	if (!skip_kasan_poison) {
 		kasan_poison_pages(page, order, init);
 
 		/* Memory is already initialized if KASAN did it internally. */
-- 
2.40.0.rc1.284.g88254d51c5-goog