warning for EOPNOTSUPP vfs_copy_file_range

From: He Zhe <zhe.he@windriver.com>
To: dchinner@redhat.com, amir73il@gmail.com,
	"viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Cc: He Zhe <Zhe.He@windriver.com>
Subject: warning for EOPNOTSUPP vfs_copy_file_range
Date: Thu, 19 May 2022 16:22:04 +0800	[thread overview]
Message-ID: <20f17f64-88cb-4e80-07c1-85cb96c83619@windriver.com> (raw)

Hi,

We are experiencing the following warning from
"WARN_ON_ONCE(ret == -EOPNOTSUPP);" in vfs_copy_file_range, from
64bf5ff58dff ("vfs: no fallback for ->copy_file_range")

# cat /sys/class/net/can0/phys_switch_id

WARNING: CPU: 7 PID: 673 at fs/read_write.c:1516 vfs_copy_file_range+0x380/0x440
Modules linked in: llce_can llce_logger llce_mailbox llce_core sch_fq_codel
openvswitch nsh nf_conncount nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4
CPU: 7 PID: 673 Comm: cat Not tainted 5.15.38-yocto-standard #1
Hardware name: Freescale S32G399A (DT)
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : vfs_copy_file_range+0x380/0x440
lr : vfs_copy_file_range+0x16c/0x440
sp : ffffffc00e0f3ce0
x29: ffffffc00e0f3ce0 x28: ffffff88157b5a40 x27: 0000000000000000
x26: ffffff8816ac3230 x25: ffffff881c060008 x24: 0000000000001000
x23: 0000000000000000 x22: 0000000000000000 x21: ffffff881cc99540
x20: ffffff881cc9a340 x19: ffffffffffffffa1 x18: ffffffffffffffff
x17: 0000000000000001 x16: 0000adfbb5178cde x15: ffffffc08e0f3647
x14: 0000000000000000 x13: 34613178302f3061 x12: 3178302b636e7973
x11: 0000000000058395 x10: 00000000fd1c5755 x9 : ffffffc008361950
x8 : ffffffc00a7d4d58 x7 : 0000000000000000 x6 : 0000000000000001
x5 : ffffffc009e81000 x4 : ffffffc009e817f8 x3 : 0000000000000000
x2 : 0000000000000000 x1 : ffffff88157b5a40 x0 : ffffffffffffffa1
Call trace:
 vfs_copy_file_range+0x380/0x440
 __do_sys_copy_file_range+0x178/0x3a4
 __arm64_sys_copy_file_range+0x34/0x4c
 invoke_syscall+0x5c/0x130
 el0_svc_common.constprop.0+0x68/0x124
 do_el0_svc+0x50/0xbc
 el0_svc+0x54/0x130
 el0t_64_sync_handler+0xa4/0x130
 el0t_64_sync+0x1a0/0x1a4
cat: /sys/class/net/can0/phys_switch_id: Operation not supported

And we found this is triggered by the following stack. Specifically, all
netdev_ops in CAN drivers we can find now do not have ndo_get_port_parent_id and
ndo_get_devlink_port, which makes phys_switch_id_show return -EOPNOTSUPP all the
way back to vfs_copy_file_range.

phys_switch_id_show+0xf4/0x11c
dev_attr_show+0x2c/0x6c
sysfs_kf_seq_show+0xb8/0x150
kernfs_seq_show+0x38/0x44
seq_read_iter+0x1c4/0x4c0
kernfs_fop_read_iter+0x44/0x50
generic_file_splice_read+0xdc/0x190
do_splice_to+0xa0/0xfc
splice_direct_to_actor+0xc4/0x250
do_splice_direct+0x94/0xe0
vfs_copy_file_range+0x16c/0x440
__do_sys_copy_file_range+0x178/0x3a4
__arm64_sys_copy_file_range+0x34/0x4c
invoke_syscall+0x5c/0x130
el0_svc_common.constprop.0+0x68/0x124
do_el0_svc+0x50/0xbc
el0_svc+0x54/0x130
el0t_64_sync_handler+0xa4/0x130
el0t_64_sync+0x1a0/0x1a4

According to the original commit log, this warning is for operational validity
checks to generic_copy_file_range(). The reading will eventually return as
not supported as printed above. But is this warning still necessary? If so we
might want to remove it to have a cleaner dmesg.

Thanks,
Zhe