io_uring: remove punt of short reads to async context

We currently punt any short read on a regular file to async context,
but this fails if the short read is due to running into EOF. This is
especially problematic since we only do the single prep for commands
now, as we don't reset kiocb->ki_pos. This can result in a 4k read on
a 1k file returning zero, as we detect the short read and then retry
from async context. At the time of retry, the position is now 1k, and
we end up reading nothing, and hence return 0.

Instead of trying to patch around the fact that short reads can be
legitimate and won't succeed in case of retry, remove the logic to punt
a short read to async context. Simply return it.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1863,18 +1863,6 @@ static int io_read(struct io_kiocb *req, struct io_kiocb **nxt,
 		else
 			ret2 = loop_rw_iter(READ, req->file, kiocb, &iter);
 
-		/*
-		 * In case of a short read, punt to async. This can happen
-		 * if we have data partially cached. Alternatively we can
-		 * return the short read, in which case the application will
-		 * need to issue another SQE and wait for it. That SQE will
-		 * need async punt anyway, so it's more efficient to do it
-		 * here.
-		 */
-		if (force_nonblock && !(req->flags & REQ_F_NOWAIT) &&
-		    (req->flags & REQ_F_ISREG) &&
-		    ret2 > 0 && ret2 < io_size)
-			ret2 = -EAGAIN;
 		/* Catch -EAGAIN return for forced non-blocking submission */
 		if (!force_nonblock || ret2 != -EAGAIN) {
 			kiocb_done(kiocb, ret2, nxt, req->in_async);