So I'm testing bcachefs with the loopback driver in dio mode, and noticing _significant_ brokenness in the bio_iov_iter_get_pages() path and elsewhere. 1) We don't check that we're not asking for more pages than we're in the original bio Noticed this because of another bug: 2) the loopback driver appears to never look at the underlying filesystem's block size, meaning if the filesystem advertises a block size of 4k the loopback device's blocksize will still be 512, and we'll end up issuing IOs the DIO path shouldn't allow due to alignment. 3) iov_iter_bvec_advance() looks like utter nonsense. We're synthesizing a fake bvec_iter and never using or even looking at one from the original bio, looking at the construction in iov_iter_bvec(). This is broken; you're assuming you're never going to see bios with partially completed bvec_iters, or things are going to explode. Try putting a md raid0 on top of two loopback devices with a sub page block size, things are just going to explode. iov_iter_bvec() needs to be changed to take a bio, not a bvec array, and iov_iter_bvec_advance() should probably just call bio_advance() - and bio_iov_bvec_set() needs to be changed to just copy bi_iter from the original bio into the dest bio. You guys made this way more complicated than it needed to be.
