On Thu, Jan 12, 2017 at 03:02:13PM -0800, Linus Torvalds wrote: > splice(5, NULL, 1, NULL, 9223372036854775807, 0) = -1 EAGAIN > (Resource temporarily unavailable) > > and note that the commit in question introduces that -EAGAIN error code. > > The old code never returned EAGAIN at all (well, it could do so later, > if NONBLOCK was set, obviously, but that doesn't seem to be the case > here). > > So that commit seems to have introduced a new error case, and I > suspect systemd-nospawn simply doesn't handle it. It is expecting > splice_to_pipe() to actually block. > > Ergo: I think we need to do a wait_for_space() somewhere, getting rid > of the EAGAIN. > > Looking at the callers of "do_splice_to()", we already have the > wait_for_space() in do_splice(), but we do *not* have it in the > do_splice_from() case when both the input and output file descriptors > are pipes.
>From the look of his strace, the source is /dev/ptmx. Pipe-to-pipe splice goes into splice_pipe_to_pipe() anyway. do_splice_from() is pipe-to-non-pipe, and it doesn't go anywhere near default_file_splice_read()... do_splice_to() is the only thing that can call default_file_splice_read() and there are only two callers - do_splice() (with its wait_for_space()) and splice_direct_to_actor(), which has internal pipe for destination. That certainly shouldn't be calling wait_for_space() - there's no other thread that could possibly read from the destination getting more space in there. IOW, it's do_splice() -> do_splice_to() -> default_file_splice_read().
