linux-kernel - Re: [rfc patch 3/4] splice: remove confirm from pipe_buf

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Date:	Tue, 24 Jun 2008 10:54:51 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	jens.axboe@...cle.com
CC:	miklos@...redi.hu, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	torvalds@...ux-foundation.org
Subject: Re: [rfc patch 3/4] splice: remove confirm from pipe_buf_operations

> > The 'confirm' operation was only used for splicing from page cache, to
> > wait for read on a page to finish.  But generic_file_splice_read()
> > already blocks on readahead reads, so it seems logical to block on the
> > rare and slow single page reads too.
> > 
> > So wait for readpage to finish inside __generic_file_splice_read() and
> > remove the 'confirm' method.
> > 
> > This also fixes short return counts when the filesystem (e.g. fuse)
> > invalidates the page between insertation and removal.
> 
> One of the basic goals of splice is to allow the pipe buffer to only be
> consisten when a consumer asks for it, otherwise the filling will always
> be sync. There should be no blocking on reads in the splice-in path,
> only on consumption for splice-out.

What you are ignoring (and I've mentioned in the changelog) is that it
is *already* sync.  Look at the code: this starts I/O:

		page_cache_sync_readahead(mapping, &in->f_ra, in,
				index, req_pages - spd.nr_pages);

And this waits for it to finish:

		if (!PageUptodate(page)) {
			...
				lock_page(page);

The only way it will be async, is if there's no readahead.  But do we
want to optmize that case?

Miklos
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