kernel: relay: remove relay_file_splice_read dead code, doesn't work

Documentation/filesystems/relay.rst says to use
	return debugfs_create_file(filename, mode, parent, buf,
	                           &relay_file_operations);
and this is the only way relay_file_operations is used.

Thus: debugfs_create_file(&relay_file_operations)
   -> __debugfs_create_file(&debugfs_full_proxy_file_operations,
                            &relay_file_operations)
   -> dentry{inode: {i_fop: &debugfs_full_proxy_file_operations},
             d_fsdata: &relay_file_operations
                       | DEBUGFS_FSDATA_IS_REAL_FOPS_BIT}

debugfs_full_proxy_file_operations.open is full_proxy_open, which extracts
the &relay_file_operations from the dentry, and allocates via
__full_proxy_fops_init() new fops, with trivial wrappers around release,
llseek, read, write, poll, and unlocked_ioctl, then replaces the fops on
the opened file therewith.

Naturally, all thusly-created debugfs files have .splice_read = NULL.
This was introduced in commit 49d200deaa68 ("debugfs: prevent access to
removed files' private data") from 2016-03-22.

AFAICT, relay_file_operations is the only struct file_operations used for
debugfs which defines a .splice_read callback.  Hooking it up with

>	diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
>	index 5063434be0fc..952fcf5b2afa 100644
>	--- a/fs/debugfs/file.c
>	+++ b/fs/debugfs/file.c
>	@@ -328,6 +328,11 @@ FULL_PROXY_FUNC(write, ssize_t, filp,
>	 			loff_t *ppos),
>	 		ARGS(filp, buf, size, ppos));
>
>	+FULL_PROXY_FUNC(splice_read, long, in,
>	+		PROTO(struct file *in, loff_t *ppos, struct pipe_inode_info *pipe,
>	+			size_t len, unsigned int flags),
>	+		ARGS(in, ppos, pipe, len, flags));
>	+
>	 FULL_PROXY_FUNC(unlocked_ioctl, long, filp,
>	 		PROTO(struct file *filp, unsigned int cmd, unsigned long arg),
>	 		ARGS(filp, cmd, arg));
>	@@ -382,6 +387,8 @@ static void __full_proxy_fops_init(struct file_operations *proxy_fops,
>	 		proxy_fops->write = full_proxy_write;
>	 	if (real_fops->poll)
>	 		proxy_fops->poll = full_proxy_poll;
>	+	if (real_fops->splice_read)
>	+		proxy_fops->splice_read = full_proxy_splice_read;
>	 	if (real_fops->unlocked_ioctl)
>	 		proxy_fops->unlocked_ioctl = full_proxy_unlocked_ioctl;
>	 }

shows it just doesn't work, and splicing always instantly returns empty
(subsequent reads actually return the contents).

No-one noticed it became dead code in 2016, who knows if it worked back
then. Clearly no-one cares; just delete it.

Link: https://lkml.kernel.org/r/dtexwpw6zcdx7dkx3xj5gyjp5syxmyretdcbcdtvrnukd4vvuh@tarta.nabijaczleweli.xyz
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Li kunyu <kunyu@nfschina.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zhang Zhengming <zhang.zhengming@h3c.com>
Cc: Zhao Lei <zhao_lei1@hoperun.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/relay.c b/kernel/relay.c
index 83fe0325cde14d68d75f0b35318a034323692c18..a8e90e98bf2c136f5febe9d911186256de8d873d 100644 (file)
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -1073,167 +1073,6 @@ static ssize_t relay_file_read(struct file *filp,
        return written;
 }
 
-static void relay_consume_bytes(struct rchan_buf *rbuf, int bytes_consumed)
-{
-       rbuf->bytes_consumed += bytes_consumed;
-
-       if (rbuf->bytes_consumed >= rbuf->chan->subbuf_size) {
-               relay_subbufs_consumed(rbuf->chan, rbuf->cpu, 1);
-               rbuf->bytes_consumed %= rbuf->chan->subbuf_size;
-       }
-}
-
-static void relay_pipe_buf_release(struct pipe_inode_info *pipe,
-                                  struct pipe_buffer *buf)
-{
-       struct rchan_buf *rbuf;
-
-       rbuf = (struct rchan_buf *)page_private(buf->page);
-       relay_consume_bytes(rbuf, buf->private);
-}
-
-static const struct pipe_buf_operations relay_pipe_buf_ops = {
-       .release        = relay_pipe_buf_release,
-       .try_steal      = generic_pipe_buf_try_steal,
-       .get            = generic_pipe_buf_get,
-};
-
-static void relay_page_release(struct splice_pipe_desc *spd, unsigned int i)
-{
-}
-
-/*
- *     subbuf_splice_actor - splice up to one subbuf's worth of data
- */
-static ssize_t subbuf_splice_actor(struct file *in,
-                              loff_t *ppos,
-                              struct pipe_inode_info *pipe,
-                              size_t len,
-                              unsigned int flags,
-                              int *nonpad_ret)
-{
-       unsigned int pidx, poff, total_len, subbuf_pages, nr_pages;
-       struct rchan_buf *rbuf = in->private_data;
-       unsigned int subbuf_size = rbuf->chan->subbuf_size;
-       uint64_t pos = (uint64_t) *ppos;
-       uint32_t alloc_size = (uint32_t) rbuf->chan->alloc_size;
-       size_t read_start = (size_t) do_div(pos, alloc_size);
-       size_t read_subbuf = read_start / subbuf_size;
-       size_t padding = rbuf->padding[read_subbuf];
-       size_t nonpad_end = read_subbuf * subbuf_size + subbuf_size - padding;
-       struct page *pages[PIPE_DEF_BUFFERS];
-       struct partial_page partial[PIPE_DEF_BUFFERS];
-       struct splice_pipe_desc spd = {
-               .pages = pages,
-               .nr_pages = 0,
-               .nr_pages_max = PIPE_DEF_BUFFERS,
-               .partial = partial,
-               .ops = &relay_pipe_buf_ops,
-               .spd_release = relay_page_release,
-       };
-       ssize_t ret;
-
-       if (rbuf->subbufs_produced == rbuf->subbufs_consumed)
-               return 0;
-       if (splice_grow_spd(pipe, &spd))
-               return -ENOMEM;
-
-       /*
-        * Adjust read len, if longer than what is available
-        */
-       if (len > (subbuf_size - read_start % subbuf_size))
-               len = subbuf_size - read_start % subbuf_size;
-
-       subbuf_pages = rbuf->chan->alloc_size >> PAGE_SHIFT;
-       pidx = (read_start / PAGE_SIZE) % subbuf_pages;
-       poff = read_start & ~PAGE_MASK;
-       nr_pages = min_t(unsigned int, subbuf_pages, spd.nr_pages_max);
-
-       for (total_len = 0; spd.nr_pages < nr_pages; spd.nr_pages++) {
-               unsigned int this_len, this_end, private;
-               unsigned int cur_pos = read_start + total_len;
-
-               if (!len)
-                       break;
-
-               this_len = min_t(unsigned long, len, PAGE_SIZE - poff);
-               private = this_len;
-
-               spd.pages[spd.nr_pages] = rbuf->page_array[pidx];
-               spd.partial[spd.nr_pages].offset = poff;
-
-               this_end = cur_pos + this_len;
-               if (this_end >= nonpad_end) {
-                       this_len = nonpad_end - cur_pos;
-                       private = this_len + padding;
-               }
-               spd.partial[spd.nr_pages].len = this_len;
-               spd.partial[spd.nr_pages].private = private;
-
-               len -= this_len;
-               total_len += this_len;
-               poff = 0;
-               pidx = (pidx + 1) % subbuf_pages;
-
-               if (this_end >= nonpad_end) {
-                       spd.nr_pages++;
-                       break;
-               }
-       }
-
-       ret = 0;
-       if (!spd.nr_pages)
-               goto out;
-
-       ret = *nonpad_ret = splice_to_pipe(pipe, &spd);
-       if (ret < 0 || ret < total_len)
-               goto out;
-
-        if (read_start + ret == nonpad_end)
-                ret += padding;
-
-out:
-       splice_shrink_spd(&spd);
-       return ret;
-}
-
-static ssize_t relay_file_splice_read(struct file *in,
-                                     loff_t *ppos,
-                                     struct pipe_inode_info *pipe,
-                                     size_t len,
-                                     unsigned int flags)
-{
-       ssize_t spliced;
-       int ret;
-       int nonpad_ret = 0;
-
-       ret = 0;
-       spliced = 0;
-
-       while (len && !spliced) {
-               ret = subbuf_splice_actor(in, ppos, pipe, len, flags, &nonpad_ret);
-               if (ret < 0)
-                       break;
-               else if (!ret) {
-                       if (flags & SPLICE_F_NONBLOCK)
-                               ret = -EAGAIN;
-                       break;
-               }
-
-               *ppos += ret;
-               if (ret > len)
-                       len = 0;
-               else
-                       len -= ret;
-               spliced += nonpad_ret;
-               nonpad_ret = 0;
-       }
-
-       if (spliced)
-               return spliced;
-
-       return ret;
-}
 
 const struct file_operations relay_file_operations = {
        .open           = relay_file_open,
@@ -1242,6 +1081,5 @@ const struct file_operations relay_file_operations = {
        .read           = relay_file_read,
        .llseek         = no_llseek,
        .release        = relay_file_release,
-       .splice_read    = relay_file_splice_read,
 };
 EXPORT_SYMBOL_GPL(relay_file_operations);