[PATCH v7 5/5] block: enable RWF_DONTCACHE for block devices

[Tal Zussman]

Block device buffered reads and writes already pass through
filemap_read() and iomap_file_buffered_write() respectively, both of
which handle IOCB_DONTCACHE. Enable RWF_DONTCACHE for block device files
by setting FOP_DONTCACHE in def_blk_fops.

For CONFIG_BUFFER_HEAD=y, writeback goes through buffer_head's
__bh_submit() which sets BIO_COMPLETE_IN_TASK on dropbehind folios. For
CONFIG_BUFFER_HEAD=n, writeback goes through iomap which handles it via
BIO_COMPLETE_IN_TASK on the ioend bio.

This support is useful for databases that operate on raw block devices,
among other userspace applications.

Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
---
 block/fops.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/fops.c b/block/fops.c
index 15783a6180de..8b39c53b6393 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -941,7 +941,7 @@ const struct file_operations def_blk_fops = {
 	.splice_write	= iter_file_splice_write,
 	.fallocate	= blkdev_fallocate,
 	.uring_cmd	= blkdev_uring_cmd,
-	.fop_flags	= FOP_BUFFER_RASYNC,
+	.fop_flags	= FOP_BUFFER_RASYNC | FOP_DONTCACHE,
 };
 
 static __init int blkdev_init(void)

-- 
2.39.5