linux-kernel - Re: [PATCH 9/9] iomap: Change calling convention for zeroing

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Message-ID: <20200825042711.GL12131@dread.disaster.area>
Date:   Tue, 25 Aug 2020 14:27:11 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 9/9] iomap: Change calling convention for zeroing

On Mon, Aug 24, 2020 at 09:35:59PM -0600, Andreas Dilger wrote:
> On Aug 24, 2020, at 9:26 PM, Matthew Wilcox <willy@...radead.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...

I think the problem here is that we have no guaranteed 64 bit size
type. when that was the case with off_t, we created loff_t to always
represent a 64 bit offset value. However, we never created one for
the count/size that is passed alongside loff_t in many places - it
was said that "syscalls are limited to 32 bit sizes" and
"size_t is 64 bit on 64 bit platforms" and so on and so we still
don't have a clean way to pass 64 bit sizes through the IO path.

We've been living with this shitty situation for a long time now, so
perhaps it's time for us to define lsize_t for 64 bit lengths and
start using that everywhere that needs a 64 bit clean path
through the code, regardless of whether the arch is 32 or 64 bit...

Thoughts?

-Dave.

-- 
Dave Chinner
david@...morbit.com