vfs: add the RWF_HIPRI flag for preadv2/pwritev2

This adds a flag that tells the file system that this is a high priority
request for which it's worth to poll the hardware.  The flag is purely
advisory and can be ignored if not supported.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stephen Bates <stephen.bates@pmcs.com>
Tested-by: Stephen Bates <stephen.bates@pmcs.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/read_write.c

@@ -697,10 +697,12 @@ static ssize_t do_iter_readv_writev(struct file *filp, struct iov_iter *iter,
 	struct kiocb kiocb;
 	ssize_t ret;
 
-	if (flags)
+	if (flags & ~RWF_HIPRI)
 		return -EOPNOTSUPP;
 
 	init_sync_kiocb(&kiocb, filp);
+	if (flags & RWF_HIPRI)
+		kiocb.ki_flags |= IOCB_HIPRI;
 	kiocb.ki_pos = *ppos;
 
 	ret = fn(&kiocb, iter);
@@ -715,7 +717,7 @@ static ssize_t do_loop_readv_writev(struct file *filp, struct iov_iter *iter,
 {
 	ssize_t ret = 0;
 
-	if (flags)
+	if (flags & ~RWF_HIPRI)
 		return -EOPNOTSUPP;
 
 	while (iov_iter_count(iter)) {

include/linux/fs.h

@@ -320,6 +320,7 @@ struct writeback_control;
 #define IOCB_EVENTFD		(1 << 0)
 #define IOCB_APPEND		(1 << 1)
 #define IOCB_DIRECT		(1 << 2)
+#define IOCB_HIPRI		(1 << 3)
 
 struct kiocb {
 	struct file		*ki_filp;

include/uapi/linux/fs.h

@@ -305,4 +305,7 @@ struct fsxattr {
 #define SYNC_FILE_RANGE_WRITE		2
 #define SYNC_FILE_RANGE_WAIT_AFTER	4
 
+/* flags for preadv2/pwritev2: */
+#define RWF_HIPRI			0x00000001 /* high priority request, poll if possible */
+
 #endif /* _UAPI_LINUX_FS_H */