Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time

Andrew Morton
2021-05-13T02:52:38Z

On Wed, 12 May 2021 02:34:48 +0900 Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:

> currently when size is not supported by kmalloc_index, compiler will
> generate a run-time BUG() while compile-time error is also possible,
> and better. so changed BUG to BUILD_BUG_ON_MSG to make compile-time
> check possible.
>
> also removed code that allocates more than 32MB because current
> implementation supports only up to 32MB.
>

This explodes in mysterious ways. The patch as I have it is appended,
for reference.

gcc-10.3.0 allmodconfig.

mm/kfence/kfence_test.c: In function 'test_free_bulk':
mm/kfence/kfence_test.c:519:16: warning: unused variable 'size' [-Wunused-variable]
519 | const size_t size = setup_test_cache(test, 8 + prandom_u32_max(300), 0,
| ^~~~
In file included from <command-line>:
In function 'kmalloc_index',
inlined from 'test_alloc' at mm/kfence/kfence_test.c:270:82:
././include/linux/compiler_types.h:328:38: error: call to '__compiletime_assert_922' declared with attribute error: unexpected size in kmalloc_index()
328 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
| ^
././include/linux/compiler_types.h:309:4: note: in definition of macro '__compiletime_assert'
309 | prefix ## suffix(); \
| ^~~~~~
././include/linux/compiler_types.h:328:2: note: in expansion of macro '_compiletime_assert'
328 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
| ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
| ^~~~~~~~~~~~~~~~~~
./include/linux/slab.h:389:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
389 | BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
| ^~~~~~~~~~~~~~~~
make[2]: *** [mm/kfence/kfence_test.o] Error 1
make[1]: *** [mm/kfence] Error 2
make: *** [mm] Error 2

This patch suppresses the error:

--- a/mm/kfence/kfence_test.c~a
+++ a/mm/kfence/kfence_test.c
@@ -318,13 +318,13 @@ static void test_out_of_bounds_read(stru

/* Test both sides. */

- buf = test_alloc(test, size, GFP_KERNEL, ALLOCATE_LEFT);
+ buf = test_alloc(test, 32, GFP_KERNEL, ALLOCATE_LEFT);
expect.addr = buf - 1;
READ_ONCE(*expect.addr);
KUNIT_EXPECT_TRUE(test, report_matches(&expect));
test_free(buf);

- buf = test_alloc(test, size, GFP_KERNEL, ALLOCATE_RIGHT);
+ buf = test_alloc(test, 32, GFP_KERNEL, ALLOCATE_RIGHT);
expect.addr = buf + size;
READ_ONCE(*expect.addr);
KUNIT_EXPECT_TRUE(test, report_matches(&expect));
@@ -519,11 +519,11 @@ static void test_free_bulk(struct kunit
const size_t size = setup_test_cache(test, 8 + prandom_u32_max(300), 0,
(iter & 1) ? ctor_set_x : NULL);
void *objects[] = {
- test_alloc(test, size, GFP_KERNEL, ALLOCATE_RIGHT),
- test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
- test_alloc(test, size, GFP_KERNEL, ALLOCATE_LEFT),
- test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
- test_alloc(test, size, GFP_KERNEL, ALLOCATE_NONE),
+ test_alloc(test, 32, GFP_KERNEL, ALLOCATE_RIGHT),
+ test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
+ test_alloc(test, 32, GFP_KERNEL, ALLOCATE_LEFT),
+ test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
+ test_alloc(test, 32, GFP_KERNEL, ALLOCATE_NONE),
};

kmem_cache_free_bulk(test_cache, ARRAY_SIZE(objects), objects);

Is gcc-10.3.0 simply confused? test_out_of_bounds_read() is clearly
calling kmalloc_index(32) which is OK.

Anyway, I'll drop this patch for now so I can compile a kernel!

From: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Subject: mm, slub: change run-time assertion in kmalloc_index() to compile-time

Currently when size is not supported by kmalloc_index, compiler will
generate a run-time BUG() while compile-time error is also possible, and
better. So change BUG to BUILD_BUG_ON_MSG to make compile-time check
possible.

Also remove code that allocates more than 32MB because current
implementation supports only up to 32MB.

Link: https://lkml.kernel.org/r/20210511173448.GA54466@hyeyoo
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

include/linux/slab.h | 7 +++++--
mm/slab_common.c | 7 +++----
2 files changed, 8 insertions(+), 6 deletions(-)

--- a/include/linux/slab.h~mm-slub-change-run-time-assertion-in-kmalloc_index-to-compile-time
+++ a/include/linux/slab.h
@@ -346,6 +346,9 @@ static __always_inline enum kmalloc_cach
* 1 = 65 .. 96 bytes
* 2 = 129 .. 192 bytes
* n = 2^(n-1)+1 .. 2^n
+ *
+ * Note: there's no need to optimize kmalloc_index because it's evaluated
+ * in compile-time.
*/
static __always_inline unsigned int kmalloc_index(size_t size)
{
@@ -382,8 +385,8 @@ static __always_inline unsigned int kmal
if (size <= 8 * 1024 * 1024) return 23;
if (size <= 16 * 1024 * 1024) return 24;
if (size <= 32 * 1024 * 1024) return 25;
- if (size <= 64 * 1024 * 1024) return 26;
- BUG();
+
+ BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");

/* Will never be reached. Needed because the compiler may complain */
return -1;
--- a/mm/slab_common.c~mm-slub-change-run-time-assertion-in-kmalloc_index-to-compile-time
+++ a/mm/slab_common.c
@@ -755,8 +755,8 @@ struct kmem_cache *kmalloc_slab(size_t s

/*
* kmalloc_info[] is to make slub_debug=,kmalloc-xx option work at boot time.
- * kmalloc_index() supports up to 2^26=64MB, so the final entry of the table is
- * kmalloc-67108864.
+ * kmalloc_index() supports up to 2^25=32MB, so the final entry of the table is
+ * kmalloc-32M.
*/
const struct kmalloc_info_struct kmalloc_info[] __initconst = {
INIT_KMALLOC_INFO(0, 0),
@@ -784,8 +784,7 @@ const struct kmalloc_info_struct kmalloc
INIT_KMALLOC_INFO(4194304, 4M),
INIT_KMALLOC_INFO(8388608, 8M),
INIT_KMALLOC_INFO(16777216, 16M),
- INIT_KMALLOC_INFO(33554432, 32M),
- INIT_KMALLOC_INFO(67108864, 64M)
+ INIT_KMALLOC_INFO(33554432, 32M)
};

/*
_