Re: [PATCH v18 41/42] SUNRPC: relocate struct rcu_head to the first field of struct rpc_xprt

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2025-12-05 at 16:18 +0900, Byungchul Park wrote:
> While compiling Linux kernel with DEPT on, the following error was
> observed:
> 
>    ./include/linux/rcupdate.h:1084:17: note: in expansion of macro
>    ‘BUILD_BUG_ON’
>    1084 | BUILD_BUG_ON(offsetof(typeof(*(ptr)), rhf) >= 4096);	\
>         | ^~~~~~~~~~~~
>    ./include/linux/rcupdate.h:1047:29: note: in expansion of macro
>    'kvfree_rcu_arg_2'
>    1047 | #define kfree_rcu(ptr, rhf) kvfree_rcu_arg_2(ptr, rhf)
>         |                             ^~~~~~~~~~~~~~~~
>    net/sunrpc/xprt.c:1856:9: note: in expansion of macro 'kfree_rcu'
>    1856 | kfree_rcu(xprt, rcu);
>         | ^~~~~~~~~
>     CC net/kcm/kcmproc.o
>    make[4]: *** [scripts/Makefile.build:203: net/sunrpc/xprt.o] Error 1
> 
> Since kfree_rcu() assumes 'offset of struct rcu_head in a rcu-managed
> struct < 4096', the offest of struct rcu_head in struct rpc_xprt should
> not exceed 4096 but does, due to the debug information added by DEPT.
> 
> Relocate struct rcu_head to the first field of struct rpc_xprt from an
> arbitrary location to avoid the issue and meet the assumption.
> 
> Reported-by: Yunseong Kim <ysk@kzalloc.com>
> Signed-off-by: Byungchul Park <byungchul@sk.com>
> ---
>  include/linux/sunrpc/xprt.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
> index f46d1fb8f71a..666e42a17a31 100644
> --- a/include/linux/sunrpc/xprt.h
> +++ b/include/linux/sunrpc/xprt.h
> @@ -211,6 +211,14 @@ enum xprt_transports {
>  
>  struct rpc_sysfs_xprt;
>  struct rpc_xprt {
> +	/*
> +	 * Place struct rcu_head within the first 4096 bytes of struct
> +	 * rpc_xprt if sizeof(struct rpc_xprt) > 4096, so that
> +	 * kfree_rcu() can simply work assuming that.  See the comment
> +	 * in kfree_rcu().
> +	 */
> +	struct rcu_head		rcu;
> +
>  	struct kref		kref;		/* Reference count */
>  	const struct rpc_xprt_ops *ops;		/* transport methods */
>  	unsigned int		id;		/* transport id */
> @@ -317,7 +325,6 @@ struct rpc_xprt {
>  #if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
>  	struct dentry		*debugfs;		/* debugfs directory */
>  #endif
> -	struct rcu_head		rcu;
>  	const struct xprt_class	*xprt_class;
>  	struct rpc_sysfs_xprt	*xprt_sysfs;
>  	bool			main; /*mark if this is the 1st transport */

Seems fine to me.

Reviewed-by: Jeff Layton <jlayton@kernel.org>