Re: [PATCH 01/11] vfs: copy_file_range source range over EOF should fail

From: bfields@fieldses.org (J. Bruce Fields)
To: Dave Chinner <david@fromorbit.com>
Cc: Olga Kornievskaia <olga.kornievskaia@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	Amir Goldstein <amir73il@gmail.com>,
	linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-nfs <linux-nfs@vger.kernel.org>,
	linux-unionfs@vger.kernel.org, ceph-devel@vger.kernel.org,
	linux-cifs@vger.kernel.org
Subject: Re: [PATCH 01/11] vfs: copy_file_range source range over EOF should fail
Date: Wed, 5 Dec 2018 11:51:51 -0500	[thread overview]
Message-ID: <20181205165151.GB5182@fieldses.org> (raw)
In-Reply-To: <20181204223102.GR6311@dastard>

On Wed, Dec 05, 2018 at 09:31:02AM +1100, Dave Chinner wrote:
> That's what the code does right now and *exactly what I'm trying to
> fix* because it EINVAL is ambiguous and not an indicator that we've
> reached the end of the source file. EINVAL can indicate several
> different errors, so it really has to be treated as a "copy failed"
> error by applications.
> 
> Have a look at read/pread() - they return 0 in this case to indicate
> a short read, and the value of zero is explicitly defined as meaning
> "read position is beyond EOF".  Applications know straight away that
> there is no more data to be read and there was no error, so can
> terminate on a successful short read.
> 
> We need to allow applications to terminate copy loops on a
> successful short copy.

I'm a little confused by your definition of "short copy" and "short
read".  Are you using that to mean a copy/read that returns zero?  I
usually see it used to mean any successful call that returned less than
the requested amount.  I'd expect a zero return to terminate a copy
loop, but not any positive return.

--b.

> IOWs, applications need to either:
> 
> 	- get an immediate error saying the range is invalid rather
> 	  than doing a short copy (as per the man page); or
> 	- have an explicit marker to say "no more data to be copied"
> 
> Applications need the "no more data to copy" case to be explicit and
> unambiguous so they can make sane decisions about whether a short
> copy was successful because the file was shorter than expected or
> whether a short copy was a result of a real error being encountered.
> The current behaviour is largely unusable for applications because
> they have to guess at the reason for EINVAL part way through a
> copy....
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com