On Sun, 24 Sep 2017 10:27:39 -0400, Al Viro wrote: > BTW, there's something fishy in bio_copy_user_iov(). If the area we'd asked for > had been too large for a single bio, we are going to create a bio and have > bio_add_pc_page() eventually fill it up to limit. Then we return into > __blk_rq_map_user_iov(), advance iter and call bio_copy_user_iov() again. > Fine, but... now we might have non-zero iter->iov_offset. And this > bmd->is_our_pages = map_data ? 0 : 1; > memcpy(bmd->iov, iter->iov, sizeof(struct iovec) * iter->nr_segs); > iov_iter_init(&bmd->iter, iter->type, bmd->iov, > iter->nr_segs, iter->count); > does not even look at iter->iov_offset. As the result, when it gets to > bio_uncopy_user(), we copy the data from each bio into the *beginning* of > the user area, overwriting that from the other bio. Yeah, something is wrong with bio_copy_user_iov. Our datapath hangs when IO flows through unmodified SG (it forces bio_copy if iov_count is set). I did not look at details, but same IO pattern and memory layout work well with bio_map (module refcount problem). > Anyway, I'd added the obvious fix to #work.iov_iter, reordered it and > force-pushed the result. I'll give it a try, thanks! -- wbr, Vitaly
