[PATCH RFC v2 04/25] stackdepot: reserve 5 extra bits in depot_stack_handle_t

[glider@google.com]

Some users (currently only KMSAN) may want to use spare bits in
depot_stack_handle_t. Let them do so and provide get_dsh_extra_bits()
and set_dsh_extra_bits() to access those bits.

Signed-off-by: Alexander Potapenko <glider@google.com>
To: Alexander Potapenko <glider@google.com>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: linux-mm@kvack.org
---

Change-Id: I23580dbde85908eeda0bdd8f83a8c3882ab3e012
---
 include/linux/stackdepot.h |  8 ++++++++
 lib/stackdepot.c           | 24 +++++++++++++++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
index 24d49c732341..ac1b5a78d7f6 100644
--- a/include/linux/stackdepot.h
+++ b/include/linux/stackdepot.h
@@ -12,6 +12,11 @@
 #define _LINUX_STACKDEPOT_H
 
 typedef u32 depot_stack_handle_t;
+/*
+ * Number of bits in the handle that stack depot doesn't use. Users may store
+ * information in them.
+ */
+#define STACK_DEPOT_EXTRA_BITS 5
 
 depot_stack_handle_t stack_depot_save(unsigned long *entries,
 				      unsigned int nr_entries, gfp_t gfp_flags);
@@ -20,5 +25,8 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 			       unsigned long **entries);
 
 unsigned int filter_irq_stacks(unsigned long *entries, unsigned int nr_entries);
+depot_stack_handle_t set_dsh_extra_bits(depot_stack_handle_t handle,
+					unsigned int bits);
+unsigned int get_dsh_extra_bits(depot_stack_handle_t handle);
 
 #endif
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index decb1e073b58..460efd3ef742 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -40,8 +40,10 @@
 #define STACK_ALLOC_ALIGN 4
 #define STACK_ALLOC_OFFSET_BITS (STACK_ALLOC_ORDER + PAGE_SHIFT - \
 					STACK_ALLOC_ALIGN)
+
 #define STACK_ALLOC_INDEX_BITS (DEPOT_STACK_BITS - \
-		STACK_ALLOC_NULL_PROTECTION_BITS - STACK_ALLOC_OFFSET_BITS)
+		STACK_ALLOC_NULL_PROTECTION_BITS - \
+		STACK_ALLOC_OFFSET_BITS - STACK_DEPOT_EXTRA_BITS)
 #define STACK_ALLOC_SLABS_CAP 8192
 #define STACK_ALLOC_MAX_SLABS \
 	(((1LL << (STACK_ALLOC_INDEX_BITS)) < STACK_ALLOC_SLABS_CAP) ? \
@@ -54,6 +56,7 @@ union handle_parts {
 		u32 slabindex : STACK_ALLOC_INDEX_BITS;
 		u32 offset : STACK_ALLOC_OFFSET_BITS;
 		u32 valid : STACK_ALLOC_NULL_PROTECTION_BITS;
+		u32 extra : STACK_DEPOT_EXTRA_BITS;
 	};
 };
 
@@ -72,6 +75,24 @@ static int next_slab_inited;
 static size_t depot_offset;
 static DEFINE_SPINLOCK(depot_lock);
 
+depot_stack_handle_t set_dsh_extra_bits(depot_stack_handle_t handle,
+					u32 bits)
+{
+	union handle_parts parts = { .handle = handle };
+
+	parts.extra = bits & ((1U << STACK_DEPOT_EXTRA_BITS) - 1);
+	return parts.handle;
+}
+EXPORT_SYMBOL_GPL(set_dsh_extra_bits);
+
+u32 get_dsh_extra_bits(depot_stack_handle_t handle)
+{
+	union handle_parts parts = { .handle = handle };
+
+	return parts.extra;
+}
+EXPORT_SYMBOL_GPL(get_dsh_extra_bits);
+
 static bool init_stack_slab(void **prealloc)
 {
 	if (!*prealloc)
@@ -132,6 +153,7 @@ static struct stack_record *depot_alloc_stack(unsigned long *entries, int size,
 	stack->handle.slabindex = depot_index;
 	stack->handle.offset = depot_offset >> STACK_ALLOC_ALIGN;
 	stack->handle.valid = 1;
+	stack->handle.extra = 0;
 	memcpy(stack->entries, entries, size * sizeof(unsigned long));
 	depot_offset += required_size;
 
-- 
2.24.0.rc0.303.g954a862665-goog