Re: [PATCH] slab: Fix too strict alignment check in create_cache()

[Guenter Roeck <linux@roeck-us.net>]

On 11/20/24 07:03, Vlastimil Babka wrote:
> On 11/20/24 13:49, Geert Uytterhoeven wrote:
>> On m68k, where the minimum alignment of unsigned long is 2 bytes:
>>
>>      Kernel panic - not syncing: __kmem_cache_create_args: Failed to create slab 'io_kiocb'. Error -22
>>      CPU: 0 UID: 0 PID: 1 Comm: swapper Not tainted 6.12.0-atari-03776-g7eaa1f99261a #1783
>>      Stack from 0102fe5c:
>> 	    0102fe5c 00514a2b 00514a2b ffffff00 00000001 0051f5ed 00425e78 00514a2b
>> 	    0041eb74 ffffffea 00000310 0051f5ed ffffffea ffffffea 00601f60 00000044
>> 	    0102ff20 000e7a68 0051ab8e 004383b8 0051f5ed ffffffea 000000b8 00000007
>> 	    01020c00 00000000 000e77f0 0041e5f0 005f67c0 0051f5ed 000000b6 0102fef4
>> 	    00000310 0102fef4 00000000 00000016 005f676c 0060a34c 00000010 00000004
>> 	    00000038 0000009a 01000000 000000b8 005f668e 0102e000 00001372 0102ff88
>>      Call Trace: [<00425e78>] dump_stack+0xc/0x10
>>       [<0041eb74>] panic+0xd8/0x26c
>>       [<000e7a68>] __kmem_cache_create_args+0x278/0x2e8
>>       [<000e77f0>] __kmem_cache_create_args+0x0/0x2e8
>>       [<0041e5f0>] memset+0x0/0x8c
>>       [<005f67c0>] io_uring_init+0x54/0xd2
>>
>> The minimal alignment of an integral type may differ from its size,
>> hence is not safe to assume that an arbitrary freeptr_t (which is
>> basically an unsigned long) is always aligned to 4 or 8 bytes.
>>
>> As nothing seems to require the additional alignment, it is safe to fix
>> this by relaxing the check to the actual minimum alignment of freeptr_t.
>>
>> Fixes: aaa736b186239b7d ("io_uring: specify freeptr usage for SLAB_TYPESAFE_BY_RCU io_kiocb cache")
>> Fixes: d345bd2e9834e2da ("mm: add kmem_cache_create_rcu()")
>> Reported-by: Guenter Roeck <linux@roeck-us.net>
>> Closes: https://lore.kernel.org/37c588d4-2c32-4aad-a19e-642961f200d7@roeck-us.net
>> Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
> 
> Thanks, will add it to slab pull for 6.13.
> 
>> ---
>>   mm/slab_common.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 893d320599151845..f2f201d865c108bd 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -230,7 +230,7 @@ static struct kmem_cache *create_cache(const char *name,
>>   	if (args->use_freeptr_offset &&
>>   	    (args->freeptr_offset >= object_size ||
>>   	     !(flags & SLAB_TYPESAFE_BY_RCU) ||
>> -	     !IS_ALIGNED(args->freeptr_offset, sizeof(freeptr_t))))
>> +	     !IS_ALIGNED(args->freeptr_offset, __alignof(freeptr_t))))
> 
> Seems only bunch of places uses __alignof but many use __alignoff__ and this
> also is what seems to be documented?

__alignoff__ -> __alignof__

Guenter