[PATCH] mm: add ___GFP_NOINIT flag which disables zeroing on alloc

[Hyunsoon Kim]

This patch allows programmer to avoid zero initialization on page
allocation even when the kernel config "CONFIG_INIT_ON_ALLOC_DEFAULT"
is enabled. The configuration is made to prevent uninitialized
heap memory flaws, and Android has applied this for security and
deterministic execution times. Please refer to below.

https://android-review.googlesource.com/c/kernel/common/+/1235132

However, there is a case that the zeroing page memory is unnecessary
when the page is used on specific purpose and will be zeroed
automatically by hardware that accesses the memory through DMA.
For instance, page allocation used for IP packet reception from Exynos
modem is solely used for packet reception. Although the page will be
freed eventually and reused for some other purpose, initialization at
that moment of reuse will be sufficient to avoid uninitialized heap
memory flaws. To support this kind of control, this patch creates new
gfp type called ___GFP_NOINIT, that allows no zeroing at the moment
of page allocation, called by many related APIs such as page_frag_alloc,
alloc_pages, etc.

Signed-off-by: Hyunsoon Kim <h10.kim@xxxxxxxxxxx>
---
 include/linux/gfp.h | 2 ++
 include/linux/mm.h  | 4 +++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 8572a14..4ddd947 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -58,6 +58,8 @@ struct vm_area_struct;
 #else
 #define ___GFP_NOLOCKDEP	0
 #endif
+#define ___GFP_NOINIT		0x1000000u
+
 /* If the above are modified, __GFP_BITS_SHIFT may need updating */
 
 /*
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8ba4342..06a23bb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2907,7 +2907,9 @@ static inline void kernel_unpoison_pages(struct page *page, int numpages) { }
 DECLARE_STATIC_KEY_FALSE(init_on_alloc);
 static inline bool want_init_on_alloc(gfp_t flags)
 {
-	if (static_branch_unlikely(&init_on_alloc))
+	if (flags & ___GFP_NOINIT)
+		return false;
+	else if (static_branch_unlikely(&init_on_alloc))
 		return true;
 	return flags & __GFP_ZERO;
 }
-- 
2.7.4