Revert "kasan: drop skip_kasan_poison variable in free_pages_prepare"

This reverts commit 487a32ec24be819e747af8c2ab0d5c515508086a.

should_skip_kasan_poison() 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.

Link: https://lkml.kernel.org/r/20230310042914.3805818-2-pcc@google.com
Fixes: 487a32ec24be ("kasan: drop skip_kasan_poison variable in free_pages_prepare")
Link: https://linux-review.googlesource.com/id/Ic4f13affeebd20548758438bb9ed9ca40e312b79
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com> [arm64]
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: <stable@vger.kernel.org>	[6.1]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ac1fc986af44c46736d4891dea771473c3bee7f1..7136c36c5d01e456d32c1718c43491f9d5d59e6d 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1398,6 +1398,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
                        unsigned int order, bool check_free, 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);
@@ -1470,7 +1471,7 @@ 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. */