linux-kernel - Re: [PATCH] sendfile removal

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Date:	Sat, 2 Jun 2007 18:35:53 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	linux-kernel@...r.kernel.org, cotte@...ibm.com, hugh@...itas.com,
	neilb@...e.de, zanussi@...ibm.com, hch@...radead.org
Subject: Re: [PATCH] sendfile removal

On Sat, Jun 02 2007, Linus Torvalds wrote:
> 
> 
> On Sat, 2 Jun 2007, Jens Axboe wrote:
> >
> > splice() WILL return EAGAIN, but in that case it should have triggered
> > the read-ahead and thus started some IO.
> 
> That's not enough.
> 
> If the IO has already been started, splice needs to wait.

But splice doesn't know, page_cache_readahead() may not have started
anything.

> > For the from-file case, see __generic_file_splice_read(). splice does:
> > 
> >        if (!PageUptodate(page)) {
> >                /*
> >                 * If in nonblock mode then dont block on
> >                 * waiting
> >                 * for an in-flight io page
> >                 */
> >                if (flags & SPLICE_F_NONBLOCK) {
> >                        if (TestSetPageLocked(page))
> >                                break;
> >                } else
> >                        lock_page(page);
> 
> Yeah, that's just wrong.
> 
> Your suggested:
> 
> >       if ((flags & SPLICE_F_NONBLOCK) && spd.nr_pages) {
> >               if (TestSetPageLocked(page))
> >                       break;
> >       } else
> >               lock_page(page);
> > 
> > should do that - always block for the first page and potentially return
> > a partial results for the remaining pages that read-ahead kicked into
> > gear.
> 
> would work, but I suspect that for a server, returning EAGAIN once is 
> actually the best option - if it has a select() loop, and something else 
> is running, the "return EAGAIN once" actually makes tons of sense (it 
> would basically boil down to the kernel effectively saying "ok, try 
> anything else you might have pending in your queues first, if you get back 
> to me, I'll block then").

Well then the current code should work, _provided_ that we know
read-ahead kicked off the IO. Well almost, still needs a bit of
tweaking, with some knowledge of whether page_cache_readahead() actually
called into read_pages() or not.

-- 
Jens Axboe

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