Message-ID: <20080929162743.GA15472@Krystal>
Date:	Mon, 29 Sep 2008 12:27:43 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Tom Zanussi <zanussi@...cast.net>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	prasad@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>, od@...e.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, hch@....de,
	David Wilder <dwilder@...ibm.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC PATCH 7/11] relay - Remove padding-related code from
	relay_read()/relay_splice_read() et al.

* Tom Zanussi (zanussi@...cast.net) wrote:
> Remove padding-related code from relay_read()/relay_splice_read() et al.
> 
> Because we no longer write padding, we no longer have to read it or
> account for it anywhere else, greatly simplifying the related code.
> 
> Signed-off-by: Tom Zanussi <zanussi@...cast.net>
> 

Hi Tom,

This question might sound a bit dumb, but I'll ask anyway : why do you
implement a splice_read rather than a splice_write in relay ?

splice_read allows reading information from a file or from a socket to a
pipe, while splice_write does the opposite.

So if you implement a relay splice_read, you probably consider the
relay buffers to be a "file", so you really have to send the information
to a pipe, and then you have to use this pipe to send the data
elsewhere.

My first reaction when looking at the splice implementation is that what
we would really want is a splice_write which would take the data from a
pipe (actually, we would have to write an actor which would make the
relay buffer behave like a pipe) and write it either to disk or to a
socket.

Is there something I am misunderstanding here ?

Thanks,

Mathieu

> ---
>  kernel/relay.c |  149
> ++++++++------------------------------------------------
>  1 files changed, 20 insertions(+), 129 deletions(-)
> 
> diff --git a/kernel/relay.c b/kernel/relay.c
> index d382528..b55466d 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -965,72 +965,13 @@ static void relay_file_read_consume(struct
> rchan_buf *buf,
>  				    size_t bytes_consumed)
>  {
>  	size_t subbuf_size = buf->chan->subbuf_size;
> -	size_t n_subbufs = buf->chan->n_subbufs;
> -	size_t read_subbuf;
> -
> -	if (buf->subbufs_produced == buf->subbufs_consumed &&
> -	    buf->offset == buf->bytes_consumed)
> -		return;
> -
> -	if (buf->bytes_consumed + bytes_consumed > subbuf_size) {
> -		relay_subbufs_consumed(buf->chan, buf->cpu, 1);
> -		buf->bytes_consumed = 0;
> -	}
>  
>  	buf->bytes_consumed += bytes_consumed;
> -	if (!read_pos)
> -		read_subbuf = buf->subbufs_consumed % n_subbufs;
> -	else
> -		read_subbuf = read_pos / buf->chan->subbuf_size;
> -	if (buf->bytes_consumed + buf->padding[read_subbuf] == subbuf_size) {
> -		if ((read_subbuf == buf->subbufs_produced % n_subbufs) &&
> -		    (buf->offset == subbuf_size))
> -			return;
> -		relay_subbufs_consumed(buf->chan, buf->cpu, 1);
> -		buf->bytes_consumed = 0;
> -	}
> -}
>  
> -/*
> - *	relay_file_read_avail - boolean, are there unconsumed bytes
> available?
> - */
> -static int relay_file_read_avail(struct rchan_buf *buf, size_t
> read_pos)
> -{
> -	size_t subbuf_size = buf->chan->subbuf_size;
> -	size_t n_subbufs = buf->chan->n_subbufs;
> -	size_t produced = buf->subbufs_produced;
> -	size_t consumed = buf->subbufs_consumed;
> -
> -	relay_file_read_consume(buf, read_pos, 0);
> -
> -	consumed = buf->subbufs_consumed;
> -
> -	if (unlikely(buf->offset > subbuf_size)) {
> -		if (produced == consumed)
> -			return 0;
> -		return 1;
> -	}
> -
> -	if (unlikely(produced - consumed >= n_subbufs)) {
> -		consumed = produced - n_subbufs + 1;
> -		buf->subbufs_consumed = consumed;
> +	if (buf->bytes_consumed == subbuf_size) {
> +		relay_subbufs_consumed(buf->chan, buf->cpu, 1);
>  		buf->bytes_consumed = 0;
>  	}
> -
> -	produced = (produced % n_subbufs) * subbuf_size + buf->offset;
> -	consumed = (consumed % n_subbufs) * subbuf_size + buf->bytes_consumed;
> -
> -	if (consumed > produced)
> -		produced += n_subbufs * subbuf_size;
> -
> -	if (consumed == produced) {
> -		if (buf->offset == subbuf_size &&
> -		    buf->subbufs_produced > buf->subbufs_consumed)
> -			return 1;
> -		return 0;
> -	}
> -
> -	return 1;
>  }
>  
>  /**
> @@ -1041,21 +982,19 @@ static int relay_file_read_avail(struct rchan_buf
> *buf, size_t read_pos)
>  static size_t relay_file_read_subbuf_avail(size_t read_pos,
>  					   struct rchan_buf *buf)
>  {
> -	size_t padding, avail = 0;
> +	size_t avail;
>  	size_t read_subbuf, read_offset, write_subbuf, write_offset;
>  	size_t subbuf_size = buf->chan->subbuf_size;
>  
>  	write_subbuf = (buf->data - buf->start) / subbuf_size;
> -	write_offset = buf->offset > subbuf_size ? subbuf_size : buf->offset;
> +	write_offset = buf->offset;
>  	read_subbuf = read_pos / subbuf_size;
>  	read_offset = read_pos % subbuf_size;
> -	padding = buf->padding[read_subbuf];
>  
> -	if (read_subbuf == write_subbuf) {
> -		if (read_offset + padding < write_offset)
> -			avail = write_offset - (read_offset + padding);
> -	} else
> -		avail = (subbuf_size - padding) - read_offset;
> +	avail = subbuf_size - read_offset;
> +
> +	if (read_subbuf == write_subbuf && read_offset < write_offset)
> +		avail = write_offset - read_offset;
>  
>  	return avail;
>  }
> @@ -1065,28 +1004,17 @@ static size_t
> relay_file_read_subbuf_avail(size_t read_pos,
>   *	@read_pos: file read position
>   *	@buf: relay channel buffer
>   *
> - *	If the @read_pos is in the middle of padding, return the
> - *	position of the first actually available byte, otherwise
> - *	return the original value.
> + *	If the @read_pos is 0, return the position of the first
> + *	unconsumed byte, otherwise return the original value.
>   */
>  static size_t relay_file_read_start_pos(size_t read_pos,
>  					struct rchan_buf *buf)
>  {
> -	size_t read_subbuf, padding, padding_start, padding_end;
>  	size_t subbuf_size = buf->chan->subbuf_size;
> -	size_t n_subbufs = buf->chan->n_subbufs;
> -	size_t consumed = buf->subbufs_consumed % n_subbufs;
> +	size_t consumed = buf->subbufs_consumed % buf->chan->n_subbufs;
>  
>  	if (!read_pos)
>  		read_pos = consumed * subbuf_size + buf->bytes_consumed;
> -	read_subbuf = read_pos / subbuf_size;
> -	padding = buf->padding[read_subbuf];
> -	padding_start = (read_subbuf + 1) * subbuf_size - padding;
> -	padding_end = (read_subbuf + 1) * subbuf_size;
> -	if (read_pos >= padding_start && read_pos < padding_end) {
> -		read_subbuf = (read_subbuf + 1) % n_subbufs;
> -		read_pos = read_subbuf * subbuf_size;
> -	}
>  
>  	return read_pos;
>  }
> @@ -1101,17 +1029,9 @@ static size_t relay_file_read_end_pos(struct
> rchan_buf *buf,
>  				      size_t read_pos,
>  				      size_t count)
>  {
> -	size_t read_subbuf, padding, end_pos;
> -	size_t subbuf_size = buf->chan->subbuf_size;
> -	size_t n_subbufs = buf->chan->n_subbufs;
> +	size_t end_pos = read_pos + count;
>  
> -	read_subbuf = read_pos / subbuf_size;
> -	padding = buf->padding[read_subbuf];
> -	if (read_pos % subbuf_size + count + padding == subbuf_size)
> -		end_pos = (read_subbuf + 1) * subbuf_size;
> -	else
> -		end_pos = read_pos + count;
> -	if (end_pos >= subbuf_size * n_subbufs)
> +	if (end_pos >= buf->chan->subbuf_size * buf->chan->n_subbufs)
>  		end_pos = 0;
>  
>  	return end_pos;
> @@ -1165,9 +1085,6 @@ static ssize_t relay_file_read_subbufs(struct file
> *filp, loff_t *ppos,
>  
>  	mutex_lock(&filp->f_path.dentry->d_inode->i_mutex);
>  	do {
> -		if (!relay_file_read_avail(buf, *ppos))
> -			break;
> -
>  		read_start = relay_file_read_start_pos(*ppos, buf);
>  		avail = relay_file_read_subbuf_avail(read_start, buf);
>  		if (!avail)
> @@ -1242,8 +1159,7 @@ static int 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 flags)
>  {
>  	unsigned int pidx, poff, total_len, subbuf_pages, nr_pages, ret;
>  	struct rchan_buf *rbuf = in->private_data;
> @@ -1251,9 +1167,6 @@ static int subbuf_splice_actor(struct file *in,
>  	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_BUFFERS];
>  	struct partial_page partial[PIPE_BUFFERS];
>  	struct splice_pipe_desc spd = {
> @@ -1265,7 +1178,8 @@ static int subbuf_splice_actor(struct file *in,
>  		.spd_release = relay_page_release,
>  	};
>  
> -	if (rbuf->subbufs_produced == rbuf->subbufs_consumed)
> +	if (rbuf->subbufs_produced == rbuf->subbufs_consumed &&
> +	    rbuf->offset == rbuf->bytes_consumed)
>  		return 0;
>  
>  	/*
> @@ -1280,46 +1194,25 @@ static int subbuf_splice_actor(struct file *in,
>  	nr_pages = min_t(unsigned int, subbuf_pages, PIPE_BUFFERS);
>  
>  	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;
> +		unsigned int this_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;
> -		}
>  	}
>  
> -	if (!spd.nr_pages)
> -		return 0;
> -
> -	ret = *nonpad_ret = splice_to_pipe(pipe, &spd);
> -	if (ret < 0 || ret < total_len)
> -		return ret;
> -
> -        if (read_start + ret == nonpad_end)
> -                ret += padding;
> +	ret = splice_to_pipe(pipe, &spd);
>  
>          return ret;
>  }
> @@ -1332,13 +1225,12 @@ static ssize_t relay_file_splice_read(struct
> file *in,
>  {
>  	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);
> +		ret = subbuf_splice_actor(in, ppos, pipe, len, flags);
>  		if (ret < 0)
>  			break;
>  		else if (!ret) {
> @@ -1355,8 +1247,7 @@ static ssize_t relay_file_splice_read(struct file
> *in,
>  			len = 0;
>  		else
>  			len -= ret;
> -		spliced += nonpad_ret;
> -		nonpad_ret = 0;
> +		spliced += ret;
>  	}
>  
>  	if (spliced)
> -- 
> 1.5.3.5
> 
> 
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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