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

[Oliver Sang <oliver.sang@intel.com>]

hi, Eric Sandeen,

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>

just FYI. we tried this patch, and think it can fix the issue we observed.

we applied it on top of 51a146e0595c6, so:

* a474b9ba68671 fs: cache-align lock_class_keys in struct
* 51a146e0595c6 fs: Remove internal old mount API code
*   d5bc4e31f2a3f Merge patch series "statmount: accept fd as a parameter"

results:

=========================================================================================
compiler/kconfig/rootfs/sleep/tbox_group/testcase:
  gcc-14/i386-randconfig-2006-20250804/debian-11.1-i386-20220923.cgz/1/vm-snb-i386/boot

d5bc4e31f2a3f301 51a146e0595c638c58097a1660f a474b9ba68671230bd0a712a0f9
---------------- --------------------------- ---------------------------
       fail:runs  %reproduction    fail:runs  %reproduction    fail:runs
           |             |             |             |             |
           :200        100%         200:200          0%            :200   dmesg.BUG:kernel_hang_in_boot_stage
           :200        100%         200:200          0%            :200   last_state.booting
           :200        100%         200:200          0%            :200   last_state.is_incomplete_run

> ---
> 
> 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];
> 
>