linux-kernel - [PATCH] block: enable RWF_DONTCACHE for block device buffered I/O

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Message-ID: <20251219071707.3295393-1-den@valinux.co.jp>
Date: Fri, 19 Dec 2025 16:17:07 +0900
From: Koichiro Den <den@...inux.co.jp>
To: axboe@...nel.dk
Cc: linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] block: enable RWF_DONTCACHE for block device buffered I/O

Set FOP_DONTCACHE for def_blk_fops so preadv2()/pwritev2() can accept
RWF_DONTCACHE on block device files. The generic page cache and iomap
infrastructure already handles RWF_DONTCACHE semantics for the buffered
I/O, so simply enabling the flag suffices.

This lets userspace request drop-behind behaviour and is particularly
useful for avoiding double caching in virtualization stacks that do not
use O_DIRECT (e.g. virtio-disk backend for xen).

Signed-off-by: Koichiro Den <den@...inux.co.jp>
---
 block/fops.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/fops.c b/block/fops.c
index 4d32785b31d9..9db41dada818 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -967,7 +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)
-- 
2.51.0