[PATCH V2] block: ignore RWF_HIPRI hint for sync dio

[Ming Lei <ming.lei@redhat.com>]

So far bio is marked as REQ_POLLED if RWF_HIPRI/IOCB_HIPRI is passed
from userspace sync io interface, then block layer tries to poll until
the bio is completed. But the current implementation calls
blk_io_schedule() if bio_poll() returns 0, and this way causes io hang or
timeout easily.

But looks no one reports this kind of issue, which should have been
triggered in normal io poll sanity test or blktests block/007 as
observed by Changhui, that means it is very likely that no one uses it
or no one cares it.

Also after io_uring is invented, io poll for sync dio becomes legacy
interface.

So ignore RWF_HIPRI hint for sync dio.

CC: linux-mm@kvack.org
Cc: linux-xfs@vger.kernel.org
Reported-by: Changhui Zhong <czhong@redhat.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V2:
	- avoid to break io_uring async polling as pointed by Chritoph

 block/fops.c         | 22 +---------------------
 fs/iomap/direct-io.c |  7 +++----
 mm/page_io.c         |  4 +---
 3 files changed, 5 insertions(+), 28 deletions(-)

diff --git a/block/fops.c b/block/fops.c
index e3643362c244..b9b83030e0df 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -44,14 +44,6 @@ static unsigned int dio_bio_write_op(struct kiocb *iocb)
 
 #define DIO_INLINE_BIO_VECS 4
 
-static void blkdev_bio_end_io_simple(struct bio *bio)
-{
-	struct task_struct *waiter = bio->bi_private;
-
-	WRITE_ONCE(bio->bi_private, NULL);
-	blk_wake_io_task(waiter);
-}
-
 static ssize_t __blkdev_direct_IO_simple(struct kiocb *iocb,
 		struct iov_iter *iter, unsigned int nr_pages)
 {
@@ -83,8 +75,6 @@ static ssize_t __blkdev_direct_IO_simple(struct kiocb *iocb,
 		bio_init(&bio, bdev, vecs, nr_pages, dio_bio_write_op(iocb));
 	}
 	bio.bi_iter.bi_sector = pos >> SECTOR_SHIFT;
-	bio.bi_private = current;
-	bio.bi_end_io = blkdev_bio_end_io_simple;
 	bio.bi_ioprio = iocb->ki_ioprio;
 
 	ret = bio_iov_iter_get_pages(&bio, iter);
@@ -97,18 +87,8 @@ static ssize_t __blkdev_direct_IO_simple(struct kiocb *iocb,
 
 	if (iocb->ki_flags & IOCB_NOWAIT)
 		bio.bi_opf |= REQ_NOWAIT;
-	if (iocb->ki_flags & IOCB_HIPRI)
-		bio_set_polled(&bio, iocb);
 
-	submit_bio(&bio);
-	for (;;) {
-		set_current_state(TASK_UNINTERRUPTIBLE);
-		if (!READ_ONCE(bio.bi_private))
-			break;
-		if (!(iocb->ki_flags & IOCB_HIPRI) || !bio_poll(&bio, NULL, 0))
-			blk_io_schedule();
-	}
-	__set_current_state(TASK_RUNNING);
+	submit_bio_wait(&bio);
 
 	bio_release_pages(&bio, should_dirty);
 	if (unlikely(bio.bi_status))
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 62da020d02a1..80f9b047aa1b 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -56,7 +56,8 @@ static void iomap_dio_submit_bio(const struct iomap_iter *iter,
 {
 	atomic_inc(&dio->ref);
 
-	if (dio->iocb->ki_flags & IOCB_HIPRI) {
+	/* Sync dio can't be polled reliably */
+	if ((dio->iocb->ki_flags & IOCB_HIPRI) && !is_sync_kiocb(dio->iocb)) {
 		bio_set_polled(bio, dio->iocb);
 		dio->submit.poll_bio = bio;
 	}
@@ -653,9 +654,7 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 			if (!READ_ONCE(dio->submit.waiter))
 				break;
 
-			if (!dio->submit.poll_bio ||
-			    !bio_poll(dio->submit.poll_bio, NULL, 0))
-				blk_io_schedule();
+			blk_io_schedule();
 		}
 		__set_current_state(TASK_RUNNING);
 	}
diff --git a/mm/page_io.c b/mm/page_io.c
index 89fbf3cae30f..3fbdab6a940e 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -360,7 +360,6 @@ int swap_readpage(struct page *page, bool synchronous)
 	 * attempt to access it in the page fault retry time check.
 	 */
 	if (synchronous) {
-		bio->bi_opf |= REQ_POLLED;
 		get_task_struct(current);
 		bio->bi_private = current;
 	}
@@ -372,8 +371,7 @@ int swap_readpage(struct page *page, bool synchronous)
 		if (!READ_ONCE(bio->bi_private))
 			break;
 
-		if (!bio_poll(bio, NULL, 0))
-			blk_io_schedule();
+		blk_io_schedule();
 	}
 	__set_current_state(TASK_RUNNING);
 	bio_put(bio);
-- 
2.31.1