On 06/19/2017 12:28 AM, Christoph Hellwig wrote:

On Sat, Jun 17, 2017 at 01:59:48PM -0600, Jens Axboe wrote:

quoted

We have a pwritev2(2) interface based on passing in flags. Add an
fcntl interface for querying these flags, and also for setting them
as well:

F_GET_RW_HINT		Returns the read/write hint set. Right now it
			will be one of the WRITE_LIFE_* values.

F_SET_RW_HINT		Pass in rw_hint type to set the read/write hint.
			Only WRITE_LIFE_* hints are currently supported.
			Returns 0 on succes, -1 otherwise.

Sample program testing/implementing basic setting/getting of write
hints is below.

/*
 * writehint.c: check or set a file/inode write hint
 */

static char *str[] = { "WRITE_LIFE_NONE", "WRITE_LIFE_SHORT",
			"WRITE_LIFE_MEDIUM", "WRITE_LIFE_LONG",
			"WRITE_LIFE_EXTREME" };

int main(int argc, char *argv[])
{
	int hint = -1, fd, ret;

	if (argc < 2) {
		fprintf(stderr, "%s: dev <hint>\n", argv[0]);
		return 1;
	}

	fd = open(argv[1], O_RDONLY);
	if (fd < 0) {
		perror("open");
		return 2;
	}

	if (argc > 2)
		hint = atoi(argv[2]);

	if (hint == -1) {
		ret = fcntl(fd, F_GET_RW_HINT);
		if (ret < 0) {
			perror("fcntl: F_GET_RW_HINT");
			return 3;
		}
		hint = ret;
	} else {
		ret = fcntl(fd, F_SET_RW_HINT, hint);
		if (ret < 0) {
			perror("fcntl: F_SET_RW_HINT");
			return 4;
		}
	}

	printf("%s: %shint %s\n", argv[1], hint != -1 ? "set " : "", str[hint]);
	close(fd);
	return 0;
}

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/fcntl.c                 | 38 ++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/fcntl.h |  6 ++++++
 2 files changed, 44 insertions(+)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index f4e7267d117f..417ce336c875 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c

@@ -243,6 +243,40 @@ static int f_getowner_uids(struct file *filp, unsigned long arg)
 }
 #endif
 
+long fcntl_rw_hint(struct file *file, unsigned int cmd, unsigned long arg)

The unsigned long arg is a little annoying because it will de different
size for 32-bit vs 64-vit architectures.

How about just doing a get_user and use a fixed-size 64-bit value?

It's just passing it in from fcnt(), my intent was for it to be a u32 on the
uapi side. But we can make it a 64-bit type, since we're going away from
the enum and making it a proper type.

-- 
Jens Axboe