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

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Message-Id: <E1KB7E3-0001Lf-K4@pomaz-ex.szeredi.hu>
Date:	Tue, 24 Jun 2008 14:02:19 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	johnpol@....mipt.ru
CC:	miklos@...redi.hu, jens.axboe@...cle.com,
	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

> > > basically like PageWriteback(), but for read-in.
> > 
> > OK it could be done, possibly at great pain.  But why is it important?
> 
> Maybe not that great if mark all readahead pages as, well, readahead,
> and do the same for readpage (essnetially it is the same).

It isn't that easy.  Readahead (->readpages()) is best effort, and is
allowed to not bring the page uptodate, since it will be retried with
->readpage().  I don't know whether any filesystems actually do that,
but it's allowed nonetheless.

> > What's the use case where it matters that splice-in should not block
> > on the read?
> 
> To be able to transfer what was already read?

That needs the consumer to be non-blocking...

Umm, one more reason why the ->confirm() stuff is currently busted:
pipe_read() will block on such a buffer even if pipe file is marked
O_NONBLOCK.  Fixing that would take a hell of a lot of added
complexity in pipe_poll(), etc...

Miklos
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