linux-kernel - Re: [PATCH RFC] fs: cache-align lock_class_keys in struct file_system

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Message-ID: <o6cnjqy4ivjqaj4n5xphstfnk5jznufaygwmfkm2gyixqgfump@7fc6c6h6d5if>
Date: Tue, 30 Dec 2025 23:04:46 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Eric Sandeen <sandeen@...hat.com>
Cc: linux-fsdevel <linux-fsdevel@...r.kernel.org>, 
	linux-kernel@...r.kernel.org, Christian Brauner <brauner@...nel.org>, 
	David Howells <dhowells@...hat.com>, lkp@...el.com, oe-lkp@...ts.linux.dev, 
	Alexander Viro <aviro@...hat.com>
Subject: Re: [PATCH RFC] fs: cache-align lock_class_keys in struct
 file_system_type

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@...el.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@...hat.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.

Instead something depends on the old layout for correctness.

By any chance is this type-punned somewhere?

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.