[PATCH 04/11] mm: support async buffered reads in generic_file_buffered_read()

[Jens Axboe]

Use the async page locking infrastructure, if IOCB_WAITQ is set in the
passed in iocb. The caller must expect an -EIOCBQUEUED return value,
which means that IO is started but not done yet. This is similar to how
O_DIRECT signals the same operation. Once the callback is received by
the caller for IO completion, the caller must retry the operation.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
 mm/filemap.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 0bc77f431bea..d9d65c74b2f0 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2074,17 +2074,25 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 					index, last_index - index);
 		}
 		if (!PageUptodate(page)) {
-			if (iocb->ki_flags & IOCB_NOWAIT) {
-				put_page(page);
-				goto would_block;
-			}
-
 			/*
 			 * See comment in do_read_cache_page on why
 			 * wait_on_page_locked is used to avoid unnecessarily
 			 * serialisations and why it's safe.
 			 */
-			error = wait_on_page_locked_killable(page);
+			if (iocb->ki_flags & IOCB_WAITQ) {
+				if (written) {
+					put_page(page);
+					goto out;
+				}
+				error = wait_on_page_locked_async(page,
+								iocb->private);
+			} else {
+				if (iocb->ki_flags & IOCB_NOWAIT) {
+					put_page(page);
+					goto would_block;
+				}
+				error = wait_on_page_locked_killable(page);
+			}
 			if (unlikely(error))
 				goto readpage_error;
 			if (PageUptodate(page))
@@ -2172,7 +2180,10 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 page_not_up_to_date:
 		/* Get exclusive access to the page ... */
-		error = lock_page_killable(page);
+		if (iocb->ki_flags & IOCB_WAITQ)
+			error = lock_page_async(page, iocb->private);
+		else
+			error = lock_page_killable(page);
 		if (unlikely(error))
 			goto readpage_error;
 
-- 
2.26.2