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

[Bradley Morgan]

On 4 August 2026 07:40:06 BST, Karl Mehltretter <kmehltretter@xxxxxxxxx>
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@xxxxxxxxxx>

Reviewed-by: Bradley Morgan <include@xxxxxxxxx>


>Assisted-by: Claude:claude-fable-5
>Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
>---
>
>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@xxxxxxxxx/
>
> 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!