[PATCH v6 4/4] 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 paths, use block_write_begin_iocb() in
blkdev_write_begin() to thread the kiocb through so that buffer_head
writeback gets dropbehind support.

CONFIG_BUFFER_HEAD=n paths are handled by the previously added iomap
BIO_COMPLETE_IN_TASK support.

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

Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
---
 block/fops.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/fops.c b/block/fops.c
index bb6642b45937..31b073181d87 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -504,7 +504,8 @@ static int blkdev_write_begin(const struct kiocb *iocb,
 			      unsigned len, struct folio **foliop,
 			      void **fsdata)
 {
-	return block_write_begin(mapping, pos, len, foliop, blkdev_get_block);
+	return block_write_begin_iocb(iocb, mapping, pos, len, foliop,
+				     blkdev_get_block);
 }
 
 static int blkdev_write_end(const struct kiocb *iocb,
@@ -966,7 +967,7 @@ 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