fs: add RWF_APPEND

This is the per-I/O equivalent of O_APPEND to support atomic append
operations on any open file.

If a file is opened with O_APPEND, pwrite() ignores the offset and
always appends data to the end of the file. RWF_APPEND enables atomic
append and pwrite() with offset on a single file descriptor.

Signed-off-by: Jürg Billeter <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

include/linux/fs.h

···
 		ki->ki_flags |= IOCB_DSYNC;
 	if (flags & RWF_SYNC)
 		ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
+	if (flags & RWF_APPEND)
+		ki->ki_flags |= IOCB_APPEND;
 	return 0;
 }
 

include/uapi/linux/fs.h

···
 /* per-IO, return -EAGAIN if operation would block */
 #define RWF_NOWAIT	((__force __kernel_rwf_t)0x00000008)
 
+/* per-IO O_APPEND */
+#define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)
+
 /* mask of flags supported by the kernel */
-#define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT)
+#define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
+			 RWF_APPEND)
 
 #endif /* _UAPI_LINUX_FS_H */