Re: iov_iter_pipe warning.

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Apr 11, 2017 at 08:58:53PM -0400, Dave Jones wrote:
> [  462.696557] in->f_op = ffffffffa02df980, ->splice_write = ffffffff812b2b20
> $ grep ffffffffa02df980 /proc/kallsyms 
> ffffffffa02df980 r nfs4_file_operations	[nfsv4]
> $ grep ffffffff812b2b20 /proc/kallsyms 
> ffffffff812b2b20 T iter_file_splice_write

Let's try to figure out whether it's read or write side going wrong.

diff --git a/fs/splice.c b/fs/splice.c
index 006ba50f4ece..0a7c0bd3e164 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -970,10 +970,26 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
 	while (len) {
 		size_t read_len;
 		loff_t pos = sd->pos, prev_pos = pos;
+		if (WARN_ON(pipe->nrbufs)) {
+			printk(KERN_ERR "in->f_op = %p, ->splice_write = %p\n",
+				in->f_op,
+				sd->u.file->f_op->splice_write);
+		}
 
 		ret = do_splice_to(in, &pos, pipe, len, flags);
 		if (unlikely(ret <= 0))
 			goto out_release;
+		{
+			int idx = pipe->curbuf;
+			int n = pipe->nrbufs;
+			size_t size = 0;
+			while (n--) {
+				size += pipe->bufs[idx++].len;
+				if (idx == pipe->buffers)
+					idx = 0;
+			}
+			WARN_ON(size != ret);
+		}
 
 		read_len = ret;
 		sd->total_len = read_len;