Re: splice: move balance_dirty_pages_ratelimited() outside of splice actor

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 12 Jun 2007 20:15:41 +0200
Jens Axboe <jens.axboe@oracle.com> wrote:

> On Tue, Jun 12 2007, Jens Axboe wrote:
> > On Tue, Jun 12 2007, Andrew Morton wrote:
> > > On Tue, 12 Jun 2007 14:44:50 +0200 Jens Axboe <jens.axboe@oracle.com> wrote:
> > > 
> > > > splice
> > > 
> > > btw, I'm staring in profound mystification at this:
> > > 
> > > int generic_pipe_buf_steal(struct pipe_inode_info *pipe,
> > > 			   struct pipe_buffer *buf)
> > > {
> > > 	struct page *page = buf->page;
> > > 
> > > 	if (page_count(page) == 1) {
> > > 		lock_page(page);
> > > 		return 0;
> > > 	}
> > > 
> > > 	return 1;
> > > }
> > > 
> > > 
> > > afacit that `if page_count(page)' test could be replaced by
> > > `if today_is_tuesday()'.  But then I don't have the foggiest idea
> > > what it's trying to do.
> > > 
> > > It would be nice to get some comments in and around here.
> > > 
> > > Also, I was trying to work out the role and responsibility of the ->pin
> > > callback, and gave up.
> > > 
> > > There isn't a lot of point in explaining this over email - one should be
> > > able to gain an understanding of these things by reading the code.  I think
> > > the best way of tackling this would be to comprehensively document
> > > pipe_buf_operations and pipe_inode_info, please...
> > 
> > OK so I wont explain it in detail here, I'll write up a good set of
> > comments tonight.
> 
> I'll merge this into the #splice branch.

Great, thanks.

> +/**
> + * generic_pipe_buf_steal - attempt to take ownership of a @pipe_buffer
> + * @pipe:	the pipe that the buffer belongs to
> + * @buf:	the buffer to attempt to steal
> + *
> + * Description:
> + *	This function attempts to steal the @struct page attached to
> + *	@buf. If successful, this function returns 0 and returns with
> + *	the page locked. The caller may then reuse the page for whatever
> + *	he wishes, the typical use is insertion into a different file
> + *	page cache.
> + */
>  int generic_pipe_buf_steal(struct pipe_inode_info *pipe,
>  			   struct pipe_buffer *buf)
>  {
>  	struct page *page = buf->page;
>  
> +	/*
> +	 * A reference of one is golden, that means that the owner of this
> +	 * page is the only one holding a reference to it. lock the page
> +	 * and return OK.
> +	 */
>  	if (page_count(page) == 1) {
>  		lock_page(page);
>  		return 0;

I still don't get this code.  I guess I should have asked for pipe_buffer
docs too ;)

What sorts of pages can find themselves inside a pipe_buffer, and which of
these types of pages is the above test detecting?