[PATCH v2 27/34] kasan, page_alloc: allow skipping memory init for HW_TAGS

[andrey.konovalov at linux.dev]

From: Andrey Konovalov <andreyknvl at google.com>

This patch adds a new GFP flag __GFP_SKIP_ZERO that allows to skip
memory initialization. The flag is only effective with HW_TAGS KASAN.

This flag will be used by vmalloc code for page_alloc allocations
backing vmalloc() mappings in a following patch. The reason to skip
memory initialization for these pages in page_alloc is because vmalloc
code will be initializing them instead.

With the current implementation, when __GFP_SKIP_ZERO is provided,
__GFP_ZEROTAGS is ignored. This doesn't matter, as these two flags are
never provided at the same time. However, if this is changed in the
future, this particular implementation detail can be changed as well.

Signed-off-by: Andrey Konovalov <andreyknvl at google.com>

---

Changes v1->v2:
- This is a new patch.
---
 include/linux/gfp.h | 16 +++++++++++-----
 mm/page_alloc.c     | 13 ++++++++++++-
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 8a3083d4cbbe..5dbde04e8e7b 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -54,10 +54,11 @@ struct vm_area_struct;
 #define ___GFP_THISNODE		0x200000u
 #define ___GFP_ACCOUNT		0x400000u
 #define ___GFP_ZEROTAGS		0x800000u
-#define ___GFP_SKIP_KASAN_UNPOISON	0x1000000u
-#define ___GFP_SKIP_KASAN_POISON	0x2000000u
+#define ___GFP_SKIP_ZERO	0x1000000u
+#define ___GFP_SKIP_KASAN_UNPOISON	0x2000000u
+#define ___GFP_SKIP_KASAN_POISON	0x4000000u
 #ifdef CONFIG_LOCKDEP
-#define ___GFP_NOLOCKDEP	0x4000000u
+#define ___GFP_NOLOCKDEP	0x8000000u
 #else
 #define ___GFP_NOLOCKDEP	0
 #endif
@@ -234,7 +235,11 @@ struct vm_area_struct;
  * %__GFP_ZERO returns a zeroed page on success.
  *
  * %__GFP_ZEROTAGS zeroes memory tags at allocation time if the memory itself
- * is being zeroed (either via __GFP_ZERO or via init_on_alloc).
+ * is being zeroed (either via __GFP_ZERO or via init_on_alloc, provided that
+ * __GFP_SKIP_ZERO is not set).
+ *
+ * %__GFP_SKIP_ZERO makes page_alloc skip zeroing memory.
+ * Only effective when HW_TAGS KASAN is enabled.
  *
  * %__GFP_SKIP_KASAN_UNPOISON makes KASAN skip unpoisoning on page allocation.
  * Only effective in HW_TAGS mode.
@@ -246,6 +251,7 @@ struct vm_area_struct;
 #define __GFP_COMP	((__force gfp_t)___GFP_COMP)
 #define __GFP_ZERO	((__force gfp_t)___GFP_ZERO)
 #define __GFP_ZEROTAGS	((__force gfp_t)___GFP_ZEROTAGS)
+#define __GFP_SKIP_ZERO ((__force gfp_t)___GFP_SKIP_ZERO)
 #define __GFP_SKIP_KASAN_UNPOISON ((__force gfp_t)___GFP_SKIP_KASAN_UNPOISON)
 #define __GFP_SKIP_KASAN_POISON   ((__force gfp_t)___GFP_SKIP_KASAN_POISON)
 
@@ -253,7 +259,7 @@ struct vm_area_struct;
 #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
 
 /* Room for N __GFP_FOO bits */
-#define __GFP_BITS_SHIFT (26 + IS_ENABLED(CONFIG_LOCKDEP))
+#define __GFP_BITS_SHIFT (27 + IS_ENABLED(CONFIG_LOCKDEP))
 #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
 
 /**
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7065d0e763e9..366b08b761ee 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2395,10 +2395,21 @@ static inline bool should_skip_kasan_unpoison(gfp_t flags, bool init_tags)
 	return init_tags || (flags & __GFP_SKIP_KASAN_UNPOISON);
 }
 
+static inline bool should_skip_init(gfp_t flags)
+{
+	/* Don't skip if a software KASAN mode is enabled. */
+	if (!IS_ENABLED(CONFIG_KASAN_HW_TAGS))
+		return false;
+
+	/* For hardware tag-based KASAN, skip if requested. */
+	return (flags & __GFP_SKIP_ZERO);
+}
+
 inline void post_alloc_hook(struct page *page, unsigned int order,
 				gfp_t gfp_flags)
 {
-	bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags);
+	bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags) &&
+			!should_skip_init(gfp_flags);
 	bool init_tags = init && (gfp_flags & __GFP_ZEROTAGS);
 
 	set_page_private(page, 0);
-- 
2.25.1