linux-kernel - Re: [PATCH 2/3] splice: allow non-block splice only when in file is seekable

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Date:	Wed, 26 May 2010 14:23:37 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Changli Gao <xiaosuo@...il.com>
CC:	miklos@...redi.hu, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] splice: allow non-block splice only when in file is 
	seekable

On Wed, 26 May 2010, Changli Gao wrote:
> On Wed, May 26, 2010 at 6:29 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
> > On Wed, 26 May 2010, Changli Gao wrote:
> >> allow non-block splice only when in file is seekable.
> >>
> >> do_splice_to() is split to two parts: read data from in file to spd, and move
> >> data from spd to pipe. If there isn't much space in pipe for the data in spd,
> >> and the splice is called with non-block flag, the data dropped due to non
> >> enough space in pipe will be lost forever when in file isn't seekable.
> >
> > This is a gratuitous ABI change.  I don't think we should hold the
> > hand of application writers,
> 
> Yea, it is an ABI change. But without that, we'll fail user silently
> currently. Some fundamental change is needed to fix this issue fully,
> if you think it is a bug.

I don't think it's a bug.  The same thing will happen without
SPLICE_F_NONBLOCK if splice_to_pipe() is interrupted by a signal while
it blocks.

To prevent data loss the application needs to be aware of the fact
that doing a splice from a non-seekable fd needs to be done carefully,
making sure that it doesn't block on the pipe.

Miklos
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