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

[yuanlinyu <yuanlinyu@honor.com>]

> From: Enze Li <lienze@kylinos.cn>
> Sent: Saturday, December 20, 2025 1:44 PM
> To: Huacai Chen <chenhuacai@kernel.org>; yuanlinyu <yuanlinyu@honor.com>
> Cc: Alexander Potapenko <glider@google.com>; Marco Elver
> <elver@google.com>; Dmitry Vyukov <dvyukov@google.com>; Andrew Morton
> <akpm@linux-foundation.org>; WANG Xuerui <kernel@xen0n.name>;
> kasan-dev@googlegroups.com; linux-mm@kvack.org; loongarch@lists.linux.dev;
> linux-kernel@vger.kernel.org; enze.li@gmx.com
> Subject: Re: [PATCH v2 1/2] LoongArch: kfence: avoid use
> CONFIG_KFENCE_NUM_OBJECTS
> 
> 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

Hi Enze,

Sorry only test on arm64.

Could you help fix the compile issue and provide a correct change ?

Or I need sometime to resolve the issue.

> 
> Thanks,
> Enze
> 
> <...>