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

[Andreas Dilger]

On Aug 24, 2020, at 9:26 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> 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

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