Re: iov_iter_pipe warning.

[Al Viro]

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;