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

[Byungchul Park <byungchul@sk.com>]

On Fri, Dec 05, 2025 at 04:27:52AM -0500, Jeff Layton wrote:
> 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>

Thank you, Jeff.

	Byungchul