Re: [PATCH v2 1/2] LoongArch: kfence: avoid use CONFIG_KFENCE_NUM_OBJECTS

[Enze Li <lienze@kylinos.cn>]

On 2025/12/19 10:13, Huacai Chen wrote:
> Hi, Enze,
> 
> On Thu, Dec 18, 2025 at 2:39 PM yuan linyu <yuanlinyu@honor.com> wrote:
>>
>> use common kfence macro KFENCE_POOL_SIZE for KFENCE_AREA_SIZE definition
>>
>> Signed-off-by: yuan linyu <yuanlinyu@honor.com>
>> ---
>>  arch/loongarch/include/asm/pgtable.h | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
>> index f41a648a3d9e..e9966c9f844f 100644
>> --- a/arch/loongarch/include/asm/pgtable.h
>> +++ b/arch/loongarch/include/asm/pgtable.h
>> @@ -10,6 +10,7 @@
>>  #define _ASM_PGTABLE_H
>>
>>  #include <linux/compiler.h>
>> +#include <linux/kfence.h>
>>  #include <asm/addrspace.h>
>>  #include <asm/asm.h>
>>  #include <asm/page.h>
>> @@ -96,7 +97,7 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
>>  #define MODULES_END    (MODULES_VADDR + SZ_256M)
>>
>>  #ifdef CONFIG_KFENCE
>> -#define KFENCE_AREA_SIZE       (((CONFIG_KFENCE_NUM_OBJECTS + 1) * 2 + 2) * PAGE_SIZE)
>> +#define KFENCE_AREA_SIZE       (KFENCE_POOL_SIZE + (2 * PAGE_SIZE))
> Can you remember why you didn't use KFENCE_POOL_SIZE at the first place?

I don't recall the exact reason off the top of my head, but I believe it
was due to complex dependency issues with the header files where
KFENCE_POOL_SIZE is defined.  To avoid those complications, we likely
opted to use KFENCE_NUM_OBJECTS directly.

I checked out the code at commit
(6ad3df56bb199134800933df2afcd7df3b03ef33 "LoongArch: Add KFENCE (Kernel
Electric-Fence) support") and encountered the following errors when
compiling with this patch applied.

8<------------------------------------------------------
  CC      arch/loongarch/kernel/asm-offsets.s
In file included from ./arch/loongarch/include/asm/pgtable.h:13,
                 from ./include/linux/pgtable.h:6,
                 from ./include/linux/mm.h:29,
                 from arch/loongarch/kernel/asm-offsets.c:9:
./include/linux/kfence.h:93:35: warning: 'struct kmem_cache' declared
inside parameter list will n
ot be visible outside of this definition or declaration
   93 | void kfence_shutdown_cache(struct kmem_cache *s);
      |                                   ^~~~~~~~~~
./include/linux/kfence.h:99:29: warning: 'struct kmem_cache' declared
inside parameter list will n
ot be visible outside of this definition or declaration
   99 | void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t
flags);
      |                             ^~~~~~~~~~
./include/linux/kfence.h:117:50: warning: 'struct kmem_cache' declared
inside parameter list will
not be visible outside of this definition or declaration
  117 | static __always_inline void *kfence_alloc(struct kmem_cache *s,
size_t size, gfp_t flags)
      |                                                  ^~~~~~~~~~
./include/linux/kfence.h: In function 'kfence_alloc':
./include/linux/kfence.h:128:31: error: passing argument 1 of
'__kfence_alloc' from incompatible p
ointer type [-Wincompatible-pointer-types]
  128 |         return __kfence_alloc(s, size, flags);
      |                               ^
      |                               |
      |                               struct kmem_cache *
./include/linux/kfence.h:99:41: note: expected 'struct kmem_cache *' but
argument is of type 'stru
ct kmem_cache *'
   99 | void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t
flags);
      |                      ~~~~~~~~~~~~~~~~~~~^
------------------------------------------------------>8

Similarly, after applying this patch to the latest code
(dd9b004b7ff3289fb7bae35130c0a5c0537266af "Merge tag 'trace-v6.19-rc1'")
from the master branch of the Linux repository and enabling KFENCE, I
encountered the following compilation errors.

8<------------------------------------------------------
  CC      arch/loongarch/kernel/asm-offsets.s
In file included from ./arch/loongarch/include/asm/pgtable.h:13,
                 from ./include/linux/pgtable.h:6,
                 from ./include/linux/mm.h:31,
                 from arch/loongarch/kernel/asm-offsets.c:11:
./include/linux/kfence.h:97:35: warning: 'struct kmem_cache' declared
inside parameter list will n
ot be visible outside of this definition or declaration
   97 | void kfence_shutdown_cache(struct kmem_cache *s);
      |                                   ^~~~~~~~~~
./include/linux/kfence.h:103:29: warning: 'struct kmem_cache' declared
inside parameter list will
not be visible outside of this definition or declaration
  103 | void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t
flags);
      |                             ^~~~~~~~~~
./include/linux/kfence.h:121:50: warning: 'struct kmem_cache' declared
inside parameter list will
not be visible outside of this definition or declaration
  121 | static __always_inline void *kfence_alloc(struct kmem_cache *s,
size_t size, gfp_t flags)
      |                                                  ^~~~~~~~~~
./include/linux/kfence.h: In function 'kfence_alloc':
./include/linux/kfence.h:132:31: error: passing argument 1 of
'__kfence_alloc' from incompatible p
ointer type [-Wincompatible-pointer-types]
  132 |         return __kfence_alloc(s, size, flags);
      |                               ^
      |                               |
      |                               struct kmem_cache *
./include/linux/kfence.h:103:41: note: expected 'struct kmem_cache *'
but argument is of type 'str
uct kmem_cache *'
  103 | void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t
flags);
      |                      ~~~~~~~~~~~~~~~~~~~^
------------------------------------------------------>8

So, this patch currently runs into compilation issues.  linyu probably
didn't have KFENCE enabled when compiling locally, which is why this
error was missed.  You can enable it as follows:

  Kernel hacking
    Memory Debugging
      [*] KFENCE: low-overhead sampling-based memory safety

Thanks,
Enze

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