linux-kernel - Re: [PATCH 1/2] btrfs: fix sparse address space warnings

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Date:	Tue, 30 Sep 2014 12:27:43 -0700
From:	Zach Brown <zab@...bo.net>
To:	Omar Sandoval <osandov@...ndov.com>
Cc:	Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	orenl@...lrox.com
Subject: Re: [PATCH 1/2] btrfs: fix sparse address space warnings

On Sun, Sep 28, 2014 at 03:26:04PM -0700, Omar Sandoval wrote:
> On Sun, Sep 28, 2014 at 01:48:11AM -0700, Omar Sandoval wrote:
> > diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
> > index 6528aa6..e0be577 100644
> > --- a/fs/btrfs/send.c
> > +++ b/fs/btrfs/send.c
> > @@ -515,7 +515,8 @@ static int write_buf(struct file *filp, const void *buf, u32 len, loff_t *off)
> >  	set_fs(KERNEL_DS);
> >  
> >  	while (pos < len) {
> > -		ret = vfs_write(filp, (char *)buf + pos, len - pos, off);
> > +		ret = vfs_write(filp, (__force const char __user *)buf + pos,
> > +				len - pos, off);
> >  		/* TODO handle that correctly */
> >  		/*if (ret == -ERESTARTSYS) {
> >  			continue;
> 
> Actually, looking at this now, it looks like this is just an open-coded
> kernel_write. I think this could be made a bit cleaner by using that instead;

Agreed, but notice that you'll want to be careful to update
write_buf()'s *off because passing a dereferenced copy to kernel_write()
will lose the pos update that vfs_write() is currently taking care of.

A carefully placed "*off += ret" in write_buf() will be fine.  (As fine
as having a magical private file position in the send context ever was.)

> the tradeoff is that each call to kernel_write will do the address
> space flip-flop, so if the write gets split up into many calls,
> there'd be some slight overhead.  That's probably a microoptimization,
> but 

Yeah, I don't think that overhead is going to be significant given all
of the work that's going on.

> I think it's worth looking
> into making kernel_read and kernel_write handle the retry logic.

I disagree.  I wouldn't broaden the scope to add retrying on behalf of
all kernel_write() callers and write methods (it's exported to modules,
too).  I'd leave the looping in btrfs and just call kernel_write() to
get rid of the segment juggling.

- z
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