Re: [PATCH 2/3] block: mark bdev files as FMODE_NOWAIT if underlying device supports it

[Christoph Hellwig <hch@infradead.org>]

So it turns out this gets into the way of my planned cleanup to move
all the tatic FMODE_ flags out of this basically full field into a new
static one in file_operations.  Do you think it is ok to go back to
always claiming FMODE_NOWAIT for block devices and then just punt for
the drivers that don't support it like the patch below?

---
From 05a591ac066d9d2d57c4967bbd49c8bc63b04abf Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Tue, 20 Jun 2023 07:53:13 +0200
Subject: block: always set FMODE_NOWAIT

Block devices are the only file_operation that do not set FMODE_NOWAIT
unconditionall in ->open and thus get in the way of a planned cleanup to
move this flags into a static field in file_operations.   Switch to
always set FMODE_NOWAIT and just return -EAGAIN if it isn't actually
supported.  This just affects minor ->submit_bio based drivers as all
blk-mq drivers and the important remappers (dm, md, nvme-multipath)
support nowait I/O.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/fops.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/block/fops.c b/block/fops.c
index 555b1b9ecd2cb9..8068d0c85ae75b 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -505,7 +505,7 @@ static int blkdev_open(struct inode *inode, struct file *filp)
 	 * during an unstable branch.
 	 */
 	filp->f_flags |= O_LARGEFILE;
-	filp->f_mode |= FMODE_BUF_RASYNC;
+	filp->f_mode |= FMODE_BUF_RASYNC | FMODE_NOWAIT;
 
 	/*
 	 * Use the file private data to store the holder for exclusive openes.
@@ -519,9 +519,6 @@ static int blkdev_open(struct inode *inode, struct file *filp)
 	if (IS_ERR(bdev))
 		return PTR_ERR(bdev);
 
-	if (bdev_nowait(bdev))
-		filp->f_mode |= FMODE_NOWAIT;
-
 	filp->f_mapping = bdev->bd_inode->i_mapping;
 	filp->f_wb_err = filemap_sample_wb_err(filp->f_mapping);
 	return 0;
@@ -563,6 +560,9 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	if ((iocb->ki_flags & (IOCB_NOWAIT | IOCB_DIRECT)) == IOCB_NOWAIT)
 		return -EOPNOTSUPP;
 
+	if (!bdev_nowait(bdev))
+		return -EAGAIN;
+
 	size -= iocb->ki_pos;
 	if (iov_iter_count(from) > size) {
 		shorted = iov_iter_count(from) - size;
@@ -585,6 +585,9 @@ static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	ssize_t ret = 0;
 	size_t count;
 
+	if (!bdev_nowait(bdev))
+		return -EAGAIN;
+
 	if (unlikely(pos + iov_iter_count(to) > size)) {
 		if (pos >= size)
 			return 0;
-- 
2.39.2