On Tue, Sep 01, 2020 at 02:09:28AM +0100, Al Viro wrote: > 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). I used a new debug patch but not sure how to capture without printk_ratelimited() because the call sites are large, diff --git a/lib/iov_iter.c b/lib/iov_iter.c index d812eef23a32..214b93c3d8a8 100644 --- 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, iov_offset = %zu, iov_base = %px, iov_len = %lu\n", left, n, i->iov_offset, __p->iov_base, __p->iov_len); \ left = (STEP); \ __v.iov_len -= left; \ skip = __v.iov_len; \ diff --git a/mm/filemap.c b/mm/filemap.c index 1aaea26556cc..202b0ab28adf 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3408,6 +3408,7 @@ ssize_t generic_perform_write(struct file *file, if (mapping_writably_mapped(mapping)) flush_dcache_page(page); + printk_ratelimited("pos = %lld, offset = %ld, bytes = %ld\n", pos, offset, bytes); copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes); flush_dcache_page(page); Al, does it help? If not, can you cook a debug patch instead? [ 587.765400][T21348] pos = 468152746240, offset = 256, bytes = 3840 [ 587.765424][T21348] pos = 468152750080, offset = 0, bytes = 4096 [ 587.765439][T21348] pos = 468152754176, offset = 0, bytes = 4096 [ 587.765455][T21348] pos = 468152758272, offset = 0, bytes = 4096 [ 591.235409][T22038] ITER_IOVEC left = 0, n = 22476968, iov_offset = 0, iov_base = 00007ff12570c000, iov_len = 20 [ 591.313456][T22038] ITER_IOVEC left = 0, n = 22476948, iov_offset = 0, iov_base = 00007ff123250000, iov_len = 20 [ 591.363679][T22038] ITER_IOVEC left = 0, n = 22476928, iov_offset = 0, iov_base = 00007ff123650000, iov_len = 6370 [ 591.413217][T22038] ITER_IOVEC left = 0, n = 22470558, iov_offset = 0, iov_base = 00007ff123850000, iov_len = 20 [ 591.459754][T22038] ITER_IOVEC left = 0, n = 22470538, iov_offset = 0, iov_base = 00007ff12570c000, iov_len = 376 [ 591.507748][T22038] ITER_IOVEC left = 0, n = 22470162, iov_offset = 0, iov_base = 00007ff12570c000, iov_len = 3473 [ 591.557395][T22038] ITER_IOVEC left = 0, n = 22466689, iov_offset = 0, iov_base = 00007ff123250000, iov_len = 20 [ 591.605295][T22038] ITER_IOVEC left = 0, n = 22466669, iov_offset = 0, iov_base = 00007ff12570a000, iov_len = 3392 [ 591.653045][T22038] ITER_IOVEC left = 0, n = 22463277, iov_offset = 0, iov_base = 00007ff123250000, iov_len = 1329038 [ 591.705324][T22038] ITER_IOVEC left = 0, n = 21134239, iov_offset = 0, iov_base = 00007ff123850000, iov_len = 20
