io_uring: internally retry short reads

We've had a few application cases of not handling short reads properly,
and it is understandable as short reads aren't really expected if the
application isn't doing non-blocking IO.

Now that we retain the iov_iter over retries, we can implement internal
retry pretty trivially. This ensures that we don't return a short read,
even for buffered reads on page cache conflicts.

Cleanup the deep nesting and hard to read nature of io_read() as well,
it's much more straight forward now to read and understand. Added a
few comments explaining the logic as well.

Signed-off-by: Jens Axboe <[e-mail]>

fs/io_uring.c

···
 	struct iovec			fast_iov[UIO_FASTIOV];
 	const struct iovec		*free_iovec;
 	struct iov_iter			iter;
+	size_t				bytes_done;
 	struct wait_page_queue		wpq;
 };
 
···
 			       bool needs_lock);
 static int io_setup_async_rw(struct io_kiocb *req, const struct iovec *iovec,
 			     const struct iovec *fast_iov,
-			     struct iov_iter *iter);
+			     struct iov_iter *iter, bool force);
 
 static struct kmem_cache *req_cachep;
 
···
 	ret = io_import_iovec(rw, req, &iovec, &iter, false);
 	if (ret < 0)
 		goto end_req;
-	ret = io_setup_async_rw(req, iovec, inline_vecs, &iter);
+	ret = io_setup_async_rw(req, iovec, inline_vecs, &iter, false);
 	if (!ret)
 		return true;
 	kfree(iovec);
···
 		       struct io_comp_state *cs)
 {
 	struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw.kiocb);
+
+	/* add previously done IO, if any */
+	if (req->io && req->io->rw.bytes_done > 0) {
+		if (ret < 0)
+			ret = req->io->rw.bytes_done;
+		else
+			ret += req->io->rw.bytes_done;
+	}
 
 	if (req->flags & REQ_F_CUR_POS)
 		req->file->f_pos = kiocb->ki_pos;
···
 
 	memcpy(&rw->iter, iter, sizeof(*iter));
 	rw->free_iovec = NULL;
+	rw->bytes_done = 0;
 	/* can only be fixed buffers, no need to do anything */
 	if (iter->type == ITER_BVEC)
 		return;
···
 
 static int io_setup_async_rw(struct io_kiocb *req, const struct iovec *iovec,
 			     const struct iovec *fast_iov,
-			     struct iov_iter *iter)
+			     struct iov_iter *iter, bool force)
 {
-	if (!io_op_defs[req->opcode].async_ctx)
+	if (!force && !io_op_defs[req->opcode].async_ctx)
 		return 0;
 	if (!req->io) {
 		if (__io_alloc_async_ctx(req))
···
  * succeed, or in rare cases where it fails, we then fall back to using the
  * async worker threads for a blocking retry.
  */
-static bool io_rw_should_retry(struct io_kiocb *req, struct iovec *iovec,
-			       struct iovec *fast_iov, struct iov_iter *iter)
+static bool io_rw_should_retry(struct io_kiocb *req)
 {
 	struct kiocb *kiocb = &req->rw.kiocb;
 	int ret;
···
 	if (req->flags & REQ_F_NOWAIT)
 		return false;
 
-	/* already tried, or we're doing O_DIRECT */
-	if (kiocb->ki_flags & (IOCB_DIRECT | IOCB_WAITQ))
+	/* Only for buffered IO */
+	if (kiocb->ki_flags & IOCB_DIRECT)
 		return false;
 	/*
 	 * just use poll if we can, and don't attempt if the fs doesn't
···
 	 */
 	if (file_can_poll(req->file) || !(req->file->f_mode & FMODE_BUF_RASYNC))
 		return false;
-
-	/*
-	 * If request type doesn't require req->io to defer in general,
-	 * we need to allocate it here
-	 */
-	if (!req->io) {
-		if (__io_alloc_async_ctx(req))
-			return false;
-		io_req_map_rw(req, iovec, fast_iov, iter);
-	}
 
 	ret = kiocb_wait_page_queue_init(kiocb, &req->io->rw.wpq,
 						io_async_buf_func, req);
···
 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
 	struct kiocb *kiocb = &req->rw.kiocb;
 	struct iov_iter __iter, *iter = &__iter;
+	ssize_t io_size, ret, ret2;
 	size_t iov_count;
-	ssize_t io_size, ret, ret2 = 0;
 
 	if (req->io)
 		iter = &req->io->rw.iter;
···
 		return ret;
 	io_size = ret;
 	req->result = io_size;
+	ret = 0;
 
 	/* Ensure we clear previously set non-block flag */
 	if (!force_nonblock)
···
 	if (unlikely(ret))
 		goto out_free;
 
-	ret2 = io_iter_do_read(req, iter);
+	ret = io_iter_do_read(req, iter);
 
-	/* Catch -EAGAIN return for forced non-blocking submission */
-	if (!force_nonblock || (ret2 != -EAGAIN && ret2 != -EIO)) {
-		kiocb_done(kiocb, ret2, cs);
-	} else {
-copy_iov:
-		ret = io_setup_async_rw(req, iovec, inline_vecs, iter);
+	if (!ret) {
+		goto done;
+	} else if (ret == -EIOCBQUEUED) {
+		ret = 0;
+		goto out_free;
+	} else if (ret == -EAGAIN) {
+		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
 		if (ret)
 			goto out_free;
-		/* it's copied and will be cleaned with ->io */
-		iovec = NULL;
-		/* if we can retry, do so with the callbacks armed */
-		if (io_rw_should_retry(req, iovec, inline_vecs, iter)) {
-			ret2 = io_iter_do_read(req, iter);
-			if (ret2 == -EIOCBQUEUED) {
-				goto out_free;
-			} else if (ret2 != -EAGAIN) {
-				kiocb_done(kiocb, ret2, cs);
-				goto out_free;
-			}
-		}
+		return -EAGAIN;
+	} else if (ret < 0) {
+		goto out_free;
+	}
+
+	/* read it all, or we did blocking attempt. no retry. */
+	if (!iov_iter_count(iter) || !force_nonblock)
+		goto done;
+
+	io_size -= ret;
+copy_iov:
+	ret2 = io_setup_async_rw(req, iovec, inline_vecs, iter, true);
+	if (ret2) {
+		ret = ret2;
+		goto out_free;
+	}
+	/* it's copied and will be cleaned with ->io */
+	iovec = NULL;
+	/* now use our persistent iterator, if we aren't already */
+	iter = &req->io->rw.iter;
+retry:
+	req->io->rw.bytes_done += ret;
+	/* if we can retry, do so with the callbacks armed */
+	if (!io_rw_should_retry(req)) {
 		kiocb->ki_flags &= ~IOCB_WAITQ;
 		return -EAGAIN;
 	}
+
+	/*
+	 * Now retry read with the IOCB_WAITQ parts set in the iocb. If we
+	 * get -EIOCBQUEUED, then we'll get a notification when the desired
+	 * page gets unlocked. We can also get a partial read here, and if we
+	 * do, then just retry at the new offset.
+	 */
+	ret = io_iter_do_read(req, iter);
+	if (ret == -EIOCBQUEUED) {
+		ret = 0;
+		goto out_free;
+	} else if (ret > 0 && ret < io_size) {
+		/* we got some bytes, but not all. retry. */
+		goto retry;
+	}
+done:
+	kiocb_done(kiocb, ret, cs);
+	ret = 0;
 out_free:
 	if (iovec)
 		kfree(iovec);
···
 		kiocb_done(kiocb, ret2, cs);
 	} else {
 copy_iov:
-		ret = io_setup_async_rw(req, iovec, inline_vecs, iter);
+		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
 		if (!ret)
 			return -EAGAIN;
 	}