Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Andreas Dilger <adilger@dilger.ca>
Message-Id: <E47B2C68-43F2-496F-AA91-A83EB3D91F28@dilger.ca>
Content-Type: multipart/signed;
 boundary="Apple-Mail=_23D8BB5C-5128-4552-921A-F0AEEE76FFB1";
 protocol="application/pgp-signature"; micalg=pgp-sha256
Mime-Version: 1.0 (Mac OS X Mail 10.3 \(3273\))
Subject: Re: [PATCH 9/9] iomap: Change calling convention for zeroing
Date:   Mon, 24 Aug 2020 21:35:59 -0600
In-Reply-To: <20200825032603.GL17456@casper.infradead.org>
Cc:     Dave Chinner <david@fromorbit.com>, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org
To:     Matthew Wilcox <willy@infradead.org>
References: <20200824145511.10500-1-willy@infradead.org>
 <20200824145511.10500-10-willy@infradead.org>
 <20200825002735.GI12131@dread.disaster.area>
 <20200825032603.GL17456@casper.infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


--Apple-Mail=_23D8BB5C-5128-4552-921A-F0AEEE76FFB1
Content-Transfer-Encoding: 7bit
Content-Type: text/plain;
	charset=us-ascii

On Aug 24, 2020, at 9:26 PM, Matthew Wilcox <willy@infradead.org> wrote:
> 
> On Tue, Aug 25, 2020 at 10:27:35AM +1000, Dave Chinner wrote:
>>> 	do {
>>> -		unsigned offset, bytes;
>>> -
>>> -		offset = offset_in_page(pos);
>>> -		bytes = min_t(loff_t, PAGE_SIZE - offset, count);
>>> +		loff_t bytes;
>>> 
>>> 		if (IS_DAX(inode))
>>> -			status = dax_iomap_zero(pos, offset, bytes, iomap);
>>> +			bytes = dax_iomap_zero(pos, length, iomap);
>> 
>> Hmmm. everything is loff_t here, but the callers are defining length
>> as u64, not loff_t. Is there a potential sign conversion problem
>> here? (sure 64 bit is way beyond anything we'll pass here, but...)
> 
> I've gone back and forth on the correct type for 'length' a few times.
> size_t is too small (not for zeroing, but for seek()).  An unsigned type
> seems right -- a length can't be negative, and we don't want to give
> the impression that it can.  But the return value from these functions
> definitely needs to be signed so we can represent an error.  So a u64
> length with an loff_t return type feels like the best solution.  And
> the upper layers have to promise not to pass in a length that's more
> than 2^63-1.

The problem with allowing a u64 as the length is that it leads to the
possibility of an argument value that cannot be returned.  Checking
length < 0 is not worse than checking length > 0x7ffffffffffffff,
and has the benefit of consistency with the other argument types and
signs...

Cheers, Andreas


--Apple-Mail=_23D8BB5C-5128-4552-921A-F0AEEE76FFB1
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment;
	filename=signature.asc
Content-Type: application/pgp-signature;
	name=signature.asc
Content-Description: Message signed with OpenPGP

-----BEGIN PGP SIGNATURE-----
Comment: GPGTools - http://gpgtools.org
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=dr3G
-----END PGP SIGNATURE-----

--Apple-Mail=_23D8BB5C-5128-4552-921A-F0AEEE76FFB1--