io_uring: add comments on how the async buffered read retry works

The retry based logic here isn't easy to follow unless you're already
familiar with how io_uring does task_work based retries. Add some
comments explaining the flow a little better.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2952,6 +2952,16 @@ static int io_read_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 	return io_rw_prep_async(req, READ, force_nonblock);
 }
 
+/*
+ * This is our waitqueue callback handler, registered through lock_page_async()
+ * when we initially tried to do the IO with the iocb armed our waitqueue.
+ * This gets called when the page is unlocked, and we generally expect that to
+ * happen when the page IO is completed and the page is now uptodate. This will
+ * queue a task_work based retry of the operation, attempting to copy the data
+ * again. If the latter fails because the page was NOT uptodate, then we will
+ * do a thread based blocking retry of the operation. That's the unexpected
+ * slow path.
+ */
 static int io_async_buf_func(struct wait_queue_entry *wait, unsigned mode,
 			     int sync, void *arg)
 {
@@ -3004,7 +3014,18 @@ static inline int kiocb_wait_page_queue_init(struct kiocb *kiocb,
 	return -EOPNOTSUPP;
 }
 
-
+/*
+ * This controls whether a given IO request should be armed for async page
+ * based retry. If we return false here, the request is handed to the async
+ * worker threads for retry. If we're doing buffered reads on a regular file,
+ * we prepare a private wait_page_queue entry and retry the operation. This
+ * will either succeed because the page is now uptodate and unlocked, or it
+ * will register a callback when the page is unlocked at IO completion. Through
+ * that callback, io_uring uses task_work to setup a retry of the operation.
+ * That retry will attempt the buffered read again. The retry will generally
+ * 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 kiocb *kiocb = &req->rw.kiocb;