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(a)infradead.org&gt; 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?