Date: Tue, 19 Dec 2023 23:24:14 +0100
From: 
	Ahelenia Ziemiańska <nabijaczleweli@...ijaczleweli.xyz>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Andrew Morton <akpm@...ux-foudation.org>, 
	Li kunyu <kunyu@...china.com>, Zhao Lei <zhao_lei1@...erun.com>, 
	"Mike Rapoport (IBM)" <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	Zhang Zhengming <zhang.zhengming@....com>, linux-kernel@...r.kernel.org
Subject: [PATCH] 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 49d200deaa680501f19a247b1fffb29301e51d2b ("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.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@...ijaczleweli.xyz>
---
 kernel/relay.c | 162 -------------------------------------------------
 1 file changed, 162 deletions(-)

diff --git a/kernel/relay.c b/kernel/relay.c
index 83fe0325cde1..a8e90e98bf2c 100644
--- 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);
-- 
2.39.2

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