Re: [PATCH] atomic: Specify natural alignment for atomic_t

[Lance Yang <lance.yang@linux.dev>]

On 2025/8/25 20:07, David Laight wrote:
> On Mon, 25 Aug 2025 15:46:42 +0800
> Lance Yang <lance.yang@linux.dev> wrote:
> 
>> On 2025/8/25 14:17, Finn Thain wrote:
>>>
>>> On Mon, 25 Aug 2025, Lance Yang wrote:
>>>    
>>>>
>>>> What if we squash the runtime check fix into your patch?
>>>
>>> Did my patch not solve the problem?
>>
>> Hmm... it should solve the problem for natural alignment, which is a
>> critical fix.
>>
>> But it cannot solve the problem of forced misalignment from drivers using
>> #pragma pack(1). The runtime warning will still trigger in those cases.
>>
>> I built a simple test module on a kernel with your patch applied:
>>
>> ```
>> #include <linux/module.h>
>> #include <linux/init.h>
>>
>> struct __attribute__((packed)) test_container {
>>       char padding[49];
>>       struct mutex io_lock;
>> };
>>
>> static int __init alignment_init(void)
>> {
>>       struct test_container cont;
>>       pr_info("io_lock address offset mod 4: %lu\n", (unsigned long)&cont.io_lock % 4);
> 
> Doesn't that give a compilation warning from 'taking the address of a packed member'?
> Ignore that at your peril.

Hmm, I don't see that acctully ...

> 
> More problematic is that, IIRC, m68k kmalloc() allocates 16bit aligned memory.
> This has broken other things in the past.
> I doubt that increasing the alignment to 32bits would make much difference
> to the kernel memory footprint.

@Finn Given this new information, how about we just apply the runtime check
fix for now?

Since we plan to remove the entire pointer-encoding scheme later anyway, a
minimal and targeted change could be the logical choice. It's easy and safe
to backport, and it cleanly stops the warnings from all sources without
introducing new risks - exactly what we need for stable kernels.

Cheers,
Lance