linux-kernel - Re: [PATCH v2 4/4] splice: fix updating sd->pos wrongly

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Message-Id: <E1OHxTV-0002u1-Vo@pomaz-ex.szeredi.hu>
Date:	Fri, 28 May 2010 13:11:37 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Changli Gao <xiaosuo@...il.com>
CC:	miklos@...redi.hu, axboe@...nel.dk, viro@...iv.linux.org.uk,
	mszeredi@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] splice: fix updating sd->pos wrongly

On Fri, 28 May 2010, Changli Gao wrote:
> On Fri, May 28, 2010 at 6:13 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
> > On Fri, 28 May 2010, Changli Gao wrote:
> >> On Fri, May 28, 2010 at 5:42 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
> >> > On Wed, 26 May 2010, Changli Gao wrote:
> >> >> fix updating sd->pos wrongly.
> >> >>
> >> >> In error path, we don't need to updating sd->pos, if the file isn't seekable.
> >> >
> >> > This patch is nonsense.  Why should we handle sd->pos != 0 case
> >> > differently?
> >> >
> >>
> >> If the in file isn't seekable, its splice_read won't update *ppos, so
> >> in the error path, we'd better not change it too. Otherwise, some
> >> assumption will go wrong.
> >
> > That may be true, but the patch is still nonsense.
> >
> > Look, your patch is updating/not updating sd->pos based on whether it
> > is zero or not.  It will prevent updating the position for sockets,
> > but it will also prevent updating the position for regular files if
> > the position is zero, which is really not what we want.
> 
> I think you misread my patch.

Right :)

>  Before checking the sd->pos, sd->pos
> already is updated with the value returned by splice_read(), so if in
> file is seekabble, sd->pos is non-zero when I checking it.

I see the logic now.  But it's really confusing, at least some
comments would be needed.

But I think it would be better not to try reusing sd->pos for reading,
but supplying an "loff_t *read_ppos" to splice_direct_to_actor and
leaving sd->pos alone (for the write side to use and update as
necessary).

Thanks,
Miklos
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