[PATCH v2] fork: Honor task_struct's declared alignment

[PATCH v2] fork: Honor task_struct's declared alignment

[Karl Mehltretter]

Since commit cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant
size"), struct task_struct is declared __attribute__((aligned(64))) on
all architectures.

But fork_init() sets the task_struct slab cache's alignment to

	align = max(L1_CACHE_BYTES, ARCH_MIN_TASKALIGN)

which is smaller than 64 on architectures whose cache lines are below
64 bytes: e.g. 32 on ARMv5.

In practice plain SLUB happens to hand out 64-byte-aligned objects
anyway. With CONFIG_SLUB_DEBUG_ON the red-zone padding shifts objects
to the requested alignment.

With CONFIG_UBSAN_ALIGNMENT=y a boot on QEMU versatilepb (ARM926EJ-S,
v7.2-rc2, gcc 13.3) floods the console with reports like:

  UBSAN: misaligned-access in include/linux/sched.h:2087:9
  member access within misaligned address c295d7e0 for type 'struct task_struct'
  which requires 64 byte alignment
  CPU: 0 UID: 0 PID: 15 Comm: pr/ttyAMA-1 Not tainted 7.2.0-rc2 #1 VOLUNTARY

Set the slab alignment to at least the type's declared alignment.
Replace the hardcoded L1_CACHE_BYTES with SLAB_HWCACHE_ALIGN so the
allocator applies cache_line_size(). On x86, arm and arm64 that is the
cache line size of the booted CPU. Everywhere else it falls back to
L1_CACHE_BYTES and nothing changes. ARCH_MIN_TASKALIGN (e.g. 4096 with
x86 VSMP) still applies through the explicit align argument.

Fixes: cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant size")
Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---

Notes (format-patch):
    v2:
     - fold in Vlastimil's suggestion: let SLAB_HWCACHE_ALIGN replace the
       hardcoded L1_CACHE_BYTES
    v1: https://lore.kernel.org/all/20260710123957.31774-1-kmehltretter@gmail.com/

 kernel/fork.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index f0e2e131a9a5..3a0093efb29f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -857,14 +857,15 @@ void __init fork_init(void)
 #ifndef ARCH_MIN_TASKALIGN
 #define ARCH_MIN_TASKALIGN	0
 #endif
-	int align = max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN);
+	int align = max(ARCH_MIN_TASKALIGN,
+			__alignof__(struct task_struct));
 	unsigned long useroffset, usersize;
 
 	/* create a slab on which task_structs can be allocated */
 	task_struct_whitelist(&useroffset, &usersize);
 	task_struct_cachep = kmem_cache_create_usercopy("task_struct",
 			arch_task_struct_size, align,
-			SLAB_PANIC|SLAB_ACCOUNT,
+			SLAB_PANIC|SLAB_ACCOUNT|SLAB_HWCACHE_ALIGN,
 			useroffset, usersize, NULL);
 
 	/* do the arch specific task caches init */

base-commit: af5e34a41cd607c00ef752e00331736570992354
-- 
2.39.5 (Apple Git-154)

Re: [PATCH v2] fork: Honor task_struct's declared alignment

[Bradley Morgan]

On 4 August 2026 07:40:06 BST, Karl Mehltretter <kmehltretter@gmail.com>
wrote:
>Since commit cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant
>size"), struct task_struct is declared __attribute__((aligned(64))) on
>all architectures.
>
>But fork_init() sets the task_struct slab cache's alignment to
>
>	align = max(L1_CACHE_BYTES, ARCH_MIN_TASKALIGN)
>
>which is smaller than 64 on architectures whose cache lines are below
>64 bytes: e.g. 32 on ARMv5.
>
>In practice plain SLUB happens to hand out 64-byte-aligned objects
>anyway. With CONFIG_SLUB_DEBUG_ON the red-zone padding shifts objects
>to the requested alignment.
>
>With CONFIG_UBSAN_ALIGNMENT=y a boot on QEMU versatilepb (ARM926EJ-S,
>v7.2-rc2, gcc 13.3) floods the console with reports like:
>
>  UBSAN: misaligned-access in include/linux/sched.h:2087:9
>  member access within misaligned address c295d7e0 for type 'struct task_struct'
>  which requires 64 byte alignment
>  CPU: 0 UID: 0 PID: 15 Comm: pr/ttyAMA-1 Not tainted 7.2.0-rc2 #1 VOLUNTARY
>
>Set the slab alignment to at least the type's declared alignment.
>Replace the hardcoded L1_CACHE_BYTES with SLAB_HWCACHE_ALIGN so the
>allocator applies cache_line_size(). On x86, arm and arm64 that is the
>cache line size of the booted CPU. Everywhere else it falls back to
>L1_CACHE_BYTES and nothing changes. ARCH_MIN_TASKALIGN (e.g. 4096 with
>x86 VSMP) still applies through the explicit align argument.
>
>Fixes: cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant size")
>Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Reviewed-by: Bradley Morgan <include@grrlz.net>


>Assisted-by: Claude:claude-fable-5
>Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
>---
>
>Notes (format-patch):
>    v2:
>     - fold in Vlastimil's suggestion: let SLAB_HWCACHE_ALIGN replace the
>       hardcoded L1_CACHE_BYTES
>    v1: https://lore.kernel.org/all/20260710123957.31774-1-kmehltretter@gmail.com/
>
> kernel/fork.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
>diff --git a/kernel/fork.c b/kernel/fork.c
>index f0e2e131a9a5..3a0093efb29f 100644
>--- a/kernel/fork.c
>+++ b/kernel/fork.c
>@@ -857,14 +857,15 @@ void __init fork_init(void)
> #ifndef ARCH_MIN_TASKALIGN
> #define ARCH_MIN_TASKALIGN	0
> #endif
>-	int align = max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN);
>+	int align = max(ARCH_MIN_TASKALIGN,
>+			__alignof__(struct task_struct));
> 	unsigned long useroffset, usersize;
> 
> 	/* create a slab on which task_structs can be allocated */
> 	task_struct_whitelist(&useroffset, &usersize);
> 	task_struct_cachep = kmem_cache_create_usercopy("task_struct",
> 			arch_task_struct_size, align,
>-			SLAB_PANIC|SLAB_ACCOUNT,
>+			SLAB_PANIC|SLAB_ACCOUNT|SLAB_HWCACHE_ALIGN,
> 			useroffset, usersize, NULL);
> 
> 	/* do the arch specific task caches init */
>
>base-commit: af5e34a41cd607c00ef752e00331736570992354
>

Thanks!

Re: [PATCH v2] fork: Honor task_struct's declared alignment

[Vlastimil Babka (SUSE)]

On 8/4/26 08:40, Karl Mehltretter wrote:
> Since commit cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant
> size"), struct task_struct is declared __attribute__((aligned(64))) on
> all architectures.
> 
> But fork_init() sets the task_struct slab cache's alignment to
> 
> 	align = max(L1_CACHE_BYTES, ARCH_MIN_TASKALIGN)
> 
> which is smaller than 64 on architectures whose cache lines are below
> 64 bytes: e.g. 32 on ARMv5.
> 
> In practice plain SLUB happens to hand out 64-byte-aligned objects
> anyway. With CONFIG_SLUB_DEBUG_ON the red-zone padding shifts objects
> to the requested alignment.
> 
> With CONFIG_UBSAN_ALIGNMENT=y a boot on QEMU versatilepb (ARM926EJ-S,
> v7.2-rc2, gcc 13.3) floods the console with reports like:
> 
>   UBSAN: misaligned-access in include/linux/sched.h:2087:9
>   member access within misaligned address c295d7e0 for type 'struct task_struct'
>   which requires 64 byte alignment
>   CPU: 0 UID: 0 PID: 15 Comm: pr/ttyAMA-1 Not tainted 7.2.0-rc2 #1 VOLUNTARY
> 
> Set the slab alignment to at least the type's declared alignment.
> Replace the hardcoded L1_CACHE_BYTES with SLAB_HWCACHE_ALIGN so the
> allocator applies cache_line_size(). On x86, arm and arm64 that is the
> cache line size of the booted CPU. Everywhere else it falls back to
> L1_CACHE_BYTES and nothing changes. ARCH_MIN_TASKALIGN (e.g. 4096 with
> x86 VSMP) still applies through the explicit align argument.
> 
> Fixes: cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant size")
> Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Thanks but I suggested only SLAB_HWCACHE_ALIGN, not the whole thing :) Can
be removed when a maintainer applies this.

> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
> 
> Notes (format-patch):
>     v2:
>      - fold in Vlastimil's suggestion: let SLAB_HWCACHE_ALIGN replace the
>        hardcoded L1_CACHE_BYTES
>     v1: https://lore.kernel.org/all/20260710123957.31774-1-kmehltretter@gmail.com/
> 
>  kernel/fork.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index f0e2e131a9a5..3a0093efb29f 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -857,14 +857,15 @@ void __init fork_init(void)
>  #ifndef ARCH_MIN_TASKALIGN
>  #define ARCH_MIN_TASKALIGN	0
>  #endif
> -	int align = max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN);
> +	int align = max(ARCH_MIN_TASKALIGN,
> +			__alignof__(struct task_struct));
>  	unsigned long useroffset, usersize;
>  
>  	/* create a slab on which task_structs can be allocated */
>  	task_struct_whitelist(&useroffset, &usersize);
>  	task_struct_cachep = kmem_cache_create_usercopy("task_struct",
>  			arch_task_struct_size, align,
> -			SLAB_PANIC|SLAB_ACCOUNT,
> +			SLAB_PANIC|SLAB_ACCOUNT|SLAB_HWCACHE_ALIGN,
>  			useroffset, usersize, NULL);
>  
>  	/* do the arch specific task caches init */
> 
> base-commit: af5e34a41cd607c00ef752e00331736570992354

Re: [PATCH v2] fork: Honor task_struct's declared alignment

[Karl Mehltretter]

On Tue, Aug 04, 2026 at 09:45:31AM +0100, Vlastimil Babka (SUSE) wrote:
> > Fixes: cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant size")
> > Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> 
> Thanks but I suggested only SLAB_HWCACHE_ALIGN, not the whole thing :) Can
> be removed when a maintainer applies this.
> 

Ok, I believe Suggested-by doesn't necessarily mean the whole thing, but I
agree, here it maybe was too much. Fine to drop it.

Thanks, 
Karl