On Mon, Aug 31, 2020 at 08:51:32PM -0400, Qian Cai wrote: > On Fri, Aug 07, 2020 at 09:34:08PM +0900, Tetsuo Handa wrote: > > On 2020/08/07 21:27, 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... > > I have been chasing something similar for a while as in, > > https://lore.kernel.org/linux-fsdevel/89F418A9-EB20-48CB-9AE0-52C700E6BB74@xxxxxx/ > > In my case, it seems the endless loop happens in iterate_iovec() instead where > I put a debug patch here, > > --- a/lib/iov_iter.c > +++ b/lib/iov_iter.c > @@ -33,6 +33,7 @@ > if (unlikely(!__v.iov_len)) \ > continue; \ > __v.iov_base = __p->iov_base; \ > + printk_ratelimited("ITER_IOVEC left = %zu, n = %zu\n", left, n); \ > left = (STEP); \ > __v.iov_len -= left; \ > skip = __v.iov_len; \ > > and end up seeing overflows ("n" supposes to be less than PAGE_SIZE) before the > soft-lockups and a dead system, > > [ 4300.249180][T470195] ITER_IOVEC left = 0, n = 48566423 > > Thoughts? Er... Where does that size come from? If that's generic_perform_write(), I'd like to see pos, offset and bytes at the time of call... ->iov_offset would also be interesting to see (along with the entire iovec array, really).
