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

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Message-ID: <20080624111913.GP20851@kernel.dk>
Date:	Tue, 24 Jun 2008 13:19:13 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	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

On Tue, Jun 24 2008, Miklos Szeredi wrote:
> > > 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?

It's an unfortunate side effect of the read-ahead, I'd much rather just
get rid of that. It _should_ behave like the non-ra case, when a page is
added it merely has IO started on it. So we want to have that be
something like

        if (!PageUptodate(page) && !PageInFlight(page))
                ...

basically like PageWriteback(), but for read-in.

-- 
Jens Axboe

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