Re: [PATCH RFC] fs: cache-align lock_class_keys in struct file_system_type

[Eric Sandeen <sandeen@redhat.com>]

On 12/30/25 4:04 PM, Mateusz Guzik wrote:
> On Tue, Dec 30, 2025 at 03:07:10PM -0600, Eric Sandeen wrote:
>> LKP reported that one of their tests was failing to even boot with my
>> "old mount API code" removal patch. The test was booting an i386 kernel
>> under QEMU, with lockdep enabled. Rather than a functional failure, it
>> seemed to have been slowed to a crawl and eventually timed out.
>>
>> I narrowed the problem down to the removal of the ->mount op from
>> file_system_type, which changed structure alignment and seems to have
>> caused cacheline issues with this structure. Annotating the alignment
>> fixes the problem for me.
>>
>> Reported-by: kernel test robot <oliver.sang@intel.com>
>> Closes: https://lore.kernel.org/oe-lkp/202512230315.1717476b-lkp@intel.com
>> Fixes: 51a146e05 ("fs: Remove internal old mount API code")
>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>> ---
>>
>> RFC because I honestly don't understand why this should be so critical,
>> especially the structure was not explicitly (or even very well) aligned
>> before. I would welcome insights from folks who are smarter than me!
>>
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 9949d253e5aa..b3d8cad15de1 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -2279,7 +2279,7 @@ struct file_system_type {
>>  	struct file_system_type * next;
>>  	struct hlist_head fs_supers;
>>  
>> -	struct lock_class_key s_lock_key;
>> +	struct lock_class_key s_lock_key ____cacheline_aligned;
>>  	struct lock_class_key s_umount_key;
>>  	struct lock_class_key s_vfs_rename_key;
>>  	struct lock_class_key s_writers_key[SB_FREEZE_LEVELS];
>>
> 
> There is no way is about cacheline bouncing. According to the linked
> thread the test vm has only 2 vcpus:
>> test machine: qemu-system-i386 -enable-kvm -cpu SandyBridge -smp 2 -m 4G
> 
> Even if the vcpu count was in hundreds and the ping pong was a problem it
> still would not have prevented bootup.

Fair enough, it really didn't make sense to me.

> Instead something depends on the old layout for correctness.

If I add ->mount back to the /end/ of the structure, it boots fine again,
so I guess nothing is expecting specific offsets within the structure
at least.

(If I turn off lockdep in the kernel config, it boots fine again too,
FWIW.)

> By any chance is this type-punned somewhere?

I don't think so... 

> While I can't be bothered to investigate, I *suspect* the way to catch
> this would patch out all of the lock_class_key vars & uses and boot with
> KMSAN (or was it KASAN?). Or whatever mechanism which can tell the
> access is oob.

Can't do KASAN or KMSAN on i386, AFAIK. :(

I suppose I could try such things on x86_64 just in case something shows
up...

Thanks!

-Eric