On Aug 24, 2020, at 9:26 PM, Matthew Wilcox <wi...@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
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