On Fri, Aug 07, 2020 at 01:27:27PM +0100, Al Viro wrote: > On Fri, Aug 07, 2020 at 07:35:08PM +0900, Tetsuo Handa wrote: > > syzbot is reporting hung task at pipe_release() [1], for for_each_bvec() from > > iterate_bvec() from iterate_all_kinds() from iov_iter_alignment() from > > ext4_unaligned_io() from ext4_dio_write_iter() from ext4_file_write_iter() from > > call_write_iter() from do_iter_readv_writev() from do_iter_write() from > > vfs_iter_write() from iter_file_splice_write() falls into infinite busy loop > > with pipe->mutex held. > > > > The reason of falling into infinite busy loop is that iter_file_splice_write() > > for some reason generates "struct bio_vec" entry with .bv_len=0 and .bv_offset=0 > > while for_each_bvec() cannot handle .bv_len == 0. > > broken in 1bdc76aea115 "iov_iter: use bvec iterator to implement iterate_bvec()", > unless I'm misreading it... > > Zero-length segments are not disallowed; it's not all that hard to filter them > out in iter_file_splice_write(), but the intent had always been to have > iterate_all_kinds() et.al. able to cope with those. > > How are these pipe_buffers with ->len == 0 generated in that reproducer, BTW? > There might be something else fishy going on... FWIW, my preference would be to have for_each_bvec() advance past zero-length segments; I'll need to go through its uses elsewhere in the tree first, though (after I grab some sleep),
