linux-kernel - Re: [PATCH 1/1] Btrfs: fix sparse warning

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Message-ID: <20140804183158.GA18659@lenny.home.zabbo.net>
Date:	Mon, 4 Aug 2014 11:31:58 -0700
From:	Zach Brown <zab@...bo.net>
To:	Fabian Frederick <fabf@...net.be>
Cc:	Josef Bacik <jbacik@...com>, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, Chris Mason <clm@...com>
Subject: Re: [PATCH 1/1] Btrfs: fix sparse warning

On Sat, Aug 02, 2014 at 02:24:49PM +0200, Fabian Frederick wrote:
> On Thu, 17 Jul 2014 12:01:52 -0700
> Zach Brown <zab@...bo.net> wrote:
> 
> > > > > @@ -515,7 +515,8 @@ static int write_buf(struct file *filp, const void *buf,
> > > > > u32 len, loff_t *off)
> > > >
> > > > Though this probably wants to be rewritten in terms of kernel_write().
> > > > That'd give an opportunity to get rid of the sctx->send_off and have it
> > > > use f_pos in the filp.
> > > 
> > > Do you mean directly call kernel_write from send_cmd/send_header ?
> > > I guess that loop around vfs_write in write_buf is there for something ...
> > 
> > write_buf() could still exist to iterate over the buffer in the case of
> > partial writes but it doesn't need to muck around with set_fs() and
> > forcing casts.
> > 
> > - z
> 
> Hello Zach,
> 
> 	Here's an untested patch which

Try testing it.  It's easy with virtualization and xfstests.

You'll find that sending to a file fails because each individual file
write call that makes up a send starts at offset 0 -- at the start of
the file.

Getting this right means getting the semantics around updating the send
descriptors f_pos right.  It requires having a bit of a think about send
semantics and f_pos update locking.

- z
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