Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Jan 13, 2017 at 08:47:31PM +0000, Al Viro wrote:
> On Fri, Jan 13, 2017 at 12:32:37PM -0800, Linus Torvalds wrote:
> 
> > Ugh. I still think your patch is butt-ugly, and the index comparisons
> > make me nervous, but..
> 
> No arguments here - 6am on 20-odd hours of uptime is _not_ a good time
> for writing, especially since the data structure needs better documentation
> and probably a couple of inlined helpers.  I'll try to massage the damn
> thing into more readable form.

FWIW, I think it will be more readable if we separate the "advance" and
"truncate" parts like this (warning: not even build-tested).  Comments?

static inline void pipe_truncate(struct iov_iter *i)
{
        struct pipe_inode_info *pipe = i->pipe;
        if (pipe->nrbufs) {
                size_t off = i->iov_offset;
                int idx = i->idx;
                int n;

		n = (pipe->curbuf + pipe->nrbufs - idx) & (pipe->buffers - 1);
                if (off) {
                        pipe->bufs[idx].len = off - pipe->bufs[idx].offset;
                        /* free all after idx; n can't be 0 */
                        idx = next_idx(idx, pipe);
                        n--;
                } else {
                        /* free all _starting_ at idx.
                         * n is 0 when we have nothing to do
                         * *or* when we are truncating full pipe to empty.
                         */
                        if (pipe->nrbufs == pipe->buffers && !n)
                                n = pipe->buffers;
                }
                while (n--) {
                        pipe_buf_release(pipe, &pipe->bufs[idx]);
                        idx = next_idx(idx, pipe);
                        pipe->nrbufs--;
                }
        }
}

static void pipe_advance(struct iov_iter *i, size_t size)
{
        struct pipe_inode_info *pipe = i->pipe;
        if (unlikely(i->count < size))
                size = i->count;
        if (size) {
                struct pipe_buffer *buf;
                size_t off = i->iov_offset, left = size;
                int idx = i->idx;
                if (off) /* make it relative to the beginning of buffer */
                        left += off - pipe->bufs[idx].offset;
                while (1) {
                        buf = &pipe->bufs[idx];
                        if (left <= buf->len)
                                break;
                        left -= buf->len;
                        idx = next_idx(idx, pipe);
                }
                i->idx = idx;
                i->iov_offset = buf->offset + left;
		i->count -= size;
        }
        /* ... and discard everything past that point */
        pipe_truncate(i);
}