If a file system supports uncached buffered IO, it may set FOP_UNCACHED
and enable RWF_UNCACHED. If RWF_UNCACHED is attempted without the file
system supporting it, it'll get errored with -EOPNOTSUPP.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/fs.h      | 14 +++++++++++++-
 include/uapi/linux/fs.h |  6 +++++-
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3559446279c1..45510d0b8de0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -320,6 +320,7 @@ struct readahead_control;
 #define IOCB_NOWAIT		(__force int) RWF_NOWAIT
 #define IOCB_APPEND		(__force int) RWF_APPEND
 #define IOCB_ATOMIC		(__force int) RWF_ATOMIC
+#define IOCB_UNCACHED		(__force int) RWF_UNCACHED
 
 /* non-RWF related bits - start at 16 */
 #define IOCB_EVENTFD		(1 << 16)
@@ -354,7 +355,8 @@ struct readahead_control;
 	{ IOCB_SYNC,		"SYNC" }, \
 	{ IOCB_NOWAIT,		"NOWAIT" }, \
 	{ IOCB_APPEND,		"APPEND" }, \
-	{ IOCB_ATOMIC,		"ATOMIC"}, \
+	{ IOCB_ATOMIC,		"ATOMIC" }, \
+	{ IOCB_UNCACHED,	"UNCACHED" }, \
 	{ IOCB_EVENTFD,		"EVENTFD"}, \
 	{ IOCB_DIRECT,		"DIRECT" }, \
 	{ IOCB_WRITE,		"WRITE" }, \
@@ -2116,6 +2118,8 @@ struct file_operations {
 #define FOP_HUGE_PAGES		((__force fop_flags_t)(1 << 4))
 /* Treat loff_t as unsigned (e.g., /dev/mem) */
 #define FOP_UNSIGNED_OFFSET	((__force fop_flags_t)(1 << 5))
+/* File system supports uncached read/write buffered IO */
+#define FOP_UNCACHED		((__force fop_flags_t)(1 << 6))
 
 /* Wrap a directory iterator that needs exclusive inode access */
 int wrap_directory_iterator(struct file *, struct dir_context *,
@@ -3532,6 +3536,14 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags,
 		if (!(ki->ki_filp->f_mode & FMODE_CAN_ATOMIC_WRITE))
 			return -EOPNOTSUPP;
 	}
+	if (flags & RWF_UNCACHED) {
+		/* file system must support it */
+		if (!(ki->ki_filp->f_op->fop_flags & FOP_UNCACHED))
+			return -EOPNOTSUPP;
+		/* DAX mappings not supported */
+		if (IS_DAX(ki->ki_filp->f_mapping->host))
+			return -EOPNOTSUPP;
+	}
 	kiocb_flags |= (__force int) (flags & RWF_SUPPORTED);
 	if (flags & RWF_SYNC)
 		kiocb_flags |= IOCB_DSYNC;
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 753971770733..dc77cd8ae1a3 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -332,9 +332,13 @@ typedef int __bitwise __kernel_rwf_t;
 /* Atomic Write */
 #define RWF_ATOMIC	((__force __kernel_rwf_t)0x00000040)
 
+/* buffered IO that drops the cache after reading or writing data */
+#define RWF_UNCACHED	((__force __kernel_rwf_t)0x00000080)
+
 /* mask of flags supported by the kernel */
 #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
-			 RWF_APPEND | RWF_NOAPPEND | RWF_ATOMIC)
+			 RWF_APPEND | RWF_NOAPPEND | RWF_ATOMIC |\
+			 RWF_UNCACHED)
 
 #define PROCFS_IOCTL_MAGIC 'f'
 
-- 
2.45.2