The patch titled Subject: Revert "kasan: drop skip_kasan_poison variable in free_pages_prepare" has been added to the -mm mm-hotfixes-unstable branch. Its filename is revert-kasan-drop-skip_kasan_poison-variable-in-free_pages_prepare.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Peter Collingbourne pcc@google.com Subject: Revert "kasan: drop skip_kasan_poison variable in free_pages_prepare" Date: Thu, 9 Mar 2023 20:29:13 -0800
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/Ic4f13affeebd20548758438bb9ed9ca40e... 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 ---
--- a/mm/page_alloc.c~revert-kasan-drop-skip_kasan_poison-variable-in-free_pages_prepare +++ a/mm/page_alloc.c @@ -1398,6 +1398,7 @@ static __always_inline bool free_pages_p 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_p * 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. */ _
Patches currently in -mm which might be from pcc@google.com are
revert-kasan-drop-skip_kasan_poison-variable-in-free_pages_prepare.patch kasan-call-clear_page-with-a-match-all-tag-instead-of-changing-page-tag.patch kasan-remove-pg_skip_kasan_poison-flag.patch