[PATCH] lib: bump stackdepot capacity from 16MB to 128MB

[Dmitry Vyukov]

KASAN uses stackdepot to memorize stacks for all kmalloc/kfree calls.
Current stackdepot capacity is 16MB (1024 top level entries x
4 pages on second level). Size of each stack is (num_frames + 3) *
sizeof(long). Which gives us ~84K stacks. This capacity was chosen
empirically and it is enough to run kernel normally. However,
when lots of configs are enabled and a fuzzer tries to maximize
code coverage, it easily hits the limit within tens of minutes.
I've tested for long a time with number of top level entries bumped 4x
(4096). And I think I've seen overflow only once. But I don't have
all configs enabled and code coverage has not reached maximum yet.
So bump it 8x to 8192. Since we have two-level table, memory cost
of this is very moderate -- currently the top-level table is 8KB,
with this patch it is 64KB, which is negligible under KASAN.

Here is some approx math.
128MB allows us to memorize ~670K stacks (assuming stack is ~200b).
I've grepped kernel for kmalloc|kfree|kmem_cache_alloc|kmem_cache_free|
kzalloc|kstrdup|kstrndup|kmemdup and it gives ~60K matches.
Most of alloc/free call sites are reachable with only one stack.
But some utility functions can have large fanout. Assuming average
fanout is 5x, total number of alloc/free stacks is ~300K.

Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: kasan-dev@xxxxxxxxxxxxxxxx
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: sploving1@xxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 lib/stackdepot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 60f77f1..4d830e2 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -50,7 +50,7 @@
 					STACK_ALLOC_ALIGN)
 #define STACK_ALLOC_INDEX_BITS (DEPOT_STACK_BITS - \
 		STACK_ALLOC_NULL_PROTECTION_BITS - STACK_ALLOC_OFFSET_BITS)
-#define STACK_ALLOC_SLABS_CAP 1024
+#define STACK_ALLOC_SLABS_CAP 8192
 #define STACK_ALLOC_MAX_SLABS \
 	(((1LL << (STACK_ALLOC_INDEX_BITS)) < STACK_ALLOC_SLABS_CAP) ? \
 	 (1LL << (STACK_ALLOC_INDEX_BITS)) : STACK_ALLOC_SLABS_CAP)
-- 
2.8.0.rc3.226.g39d4020