Re: [PATCH] SUNRPC: Flush dcache only when receiving more seeking

[He Zhe]

On 9/19/20 12:23 AM, Chuck Lever wrote:
>
>> On Sep 18, 2020, at 8:50 AM, zhe.he@xxxxxxxxxxxxx wrote:
>>
>> From: He Zhe <zhe.he@xxxxxxxxxxxxx>
>>
>> commit ca07eda33e01 ("SUNRPC: Refactor svc_recvfrom()") introduces
>> svc_flush_bvec to after sock_recvmsg, but sometimes we receive less than we
>> seek, which triggers the following warning.
>>
>> WARNING: CPU: 0 PID: 18266 at include/linux/bvec.h:101 bvec_iter_advance+0x44/0xa8
>> Attempted to advance past end of bvec iter
>> Modules linked in: sch_fq_codel openvswitch nsh nf_conncount nf_nat
>> nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4
>> CPU: 1 PID: 18266 Comm: nfsd Not tainted 5.9.0-rc5 #1
>> Hardware name: Xilinx Zynq Platform
>> [<80112ec0>] (unwind_backtrace) from [<8010c3a8>] (show_stack+0x18/0x1c)
>> [<8010c3a8>] (show_stack) from [<80755214>] (dump_stack+0x9c/0xd0)
>> [<80755214>] (dump_stack) from [<80125e64>] (__warn+0xdc/0xf4)
>> [<80125e64>] (__warn) from [<80126244>] (warn_slowpath_fmt+0x84/0xac)
>> [<80126244>] (warn_slowpath_fmt) from [<80c88514>] (bvec_iter_advance+0x44/0xa8)
>> [<80c88514>] (bvec_iter_advance) from [<80c88940>] (svc_tcp_read_msg+0x10c/0x1bc)
>> [<80c88940>] (svc_tcp_read_msg) from [<80c895d4>] (svc_tcp_recvfrom+0x98/0x63c)
>> [<80c895d4>] (svc_tcp_recvfrom) from [<80c97bf4>] (svc_handle_xprt+0x48c/0x4f8)
>> [<80c97bf4>] (svc_handle_xprt) from [<80c98038>] (svc_recv+0x94/0x1e0)
>> [<80c98038>] (svc_recv) from [<804747cc>] (nfsd+0xf0/0x168)
>> [<804747cc>] (nfsd) from [<80148a0c>] (kthread+0x144/0x154)
>> [<80148a0c>] (kthread) from [<80100114>] (ret_from_fork+0x14/0x20)
>>
>> Fixes: ca07eda33e01 ("SUNRPC: Refactor svc_recvfrom()")
>> Cc: <stable@xxxxxxxxxxxxxxx> # 5.8+
>> Signed-off-by: He Zhe <zhe.he@xxxxxxxxxxxxx>
>> ---
>> net/sunrpc/svcsock.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
>> index d5805fa1d066..ea3bc9635448 100644
>> --- a/net/sunrpc/svcsock.c
>> +++ b/net/sunrpc/svcsock.c
>> @@ -277,7 +277,7 @@ static ssize_t svc_tcp_read_msg(struct svc_rqst *rqstp, size_t buflen,
>> 		buflen -= seek;
>> 	}
>> 	len = sock_recvmsg(svsk->sk_sock, &msg, MSG_DONTWAIT);
>> -	if (len > 0)
>> +	if (len > (seek & PAGE_MASK))
> I don't understand how this addresses the WARNING. Can you provide
> an example set of inputs that trigger the issue?

I was trying to meet the not warning condition in bvec_iter_advance to make
the flushing meaningful.

svc_flush_bvec
    bvec_iter_advance
        WARN_ONCE(bytes > iter->bi_size,...

Here are my steps:
mkdir /root/mount_point/
mount /dev/sda1 /root/mount_point/
systemctl restart nfs-server
exportfs
mount -vvv -t nfs 127.0.0.1:/root/mount_point/ /mnt
cp /bin/bash ./bash.tmp

>
> Also this change introduces a mixed-sign comparison, so NACK on
> this particular patch unless it can be demonstrated that the
> implicit type conversion here is benign (I don't think it is,
> but I haven't thought through it).

Thanks, I didn't notice the different types. What about this?
if (len > 0 && (size_t)len > (seek & PAGE_MASK))


Zhe

>
>
>> 		svc_flush_bvec(bvec, len, seek);
>>
>> 	/* If we read a full record, then assume there may be more
>> -- 
>> 2.17.1
>>
> --
> Chuck Lever
>
>
>