[RFC PATCH v3 08/11] fs: Introduce RWF_NOSERIAL flag to indicate parallel reads/writes

Introduce RWF_NOSERIAL flag to indicate that the application is
okay with its reads and writes going in parallel to other read/writes.
This flag will allow writes and read to go in parallel (for eg, under a
shared lock) increasing performance at the cost of losing the (loosely
implemented) POSIX guarantees wrt to R/W serialization that various
filesystems provide.

In this patch we just introduce the flag and in upcoming patches we will
use it to implement parallel writes to increase performance of
RWF_WRITETHROUGH

Co-developed-by: Ritesh Harjani (IBM) <[email protected]>
Signed-off-by: Ritesh Harjani (IBM) <[email protected]>
Signed-off-by: Ojaswin Mujoo <[email protected]>
---
 include/linux/fs.h      | 10 ++++++++++
 include/uapi/linux/fs.h |  6 +++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index bff09a5f90e8..685ffe8da6ea 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -345,6 +345,7 @@ struct readahead_control;
 #define IOCB_DONTCACHE		(__force int) RWF_DONTCACHE
 #define IOCB_NOSIGNAL		(__force int) RWF_NOSIGNAL
 #define IOCB_WRITETHROUGH	(__force int) RWF_WRITETHROUGH
+#define IOCB_NOSERIAL		(__force int) RWF_NOSERIAL
 
 /* non-RWF related bits - start at 16 */
 #define IOCB_EVENTFD		(1 << 16)
@@ -3475,6 +3476,15 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags,
 		if (IS_DAX(ki->ki_filp->f_mapping->host))
 			return -EOPNOTSUPP;
 	}
+
+	/*
+	 * For now we don't allow users to pass the NOSERIAL flag
+	 * TODO: Once the semantics of this flag are finalized we can lift this
+	 * restriction.
+	 */
+	if (flags & IOCB_NOSERIAL)
+		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 9c8d91b926a7..cde8c9492da6 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -454,10 +454,14 @@ typedef int __bitwise __kernel_rwf_t;
 /* buffered IO that is asynchronously written through to disk after write */
 #define RWF_WRITETHROUGH	((__force __kernel_rwf_t)0x00000200)
 
+/* buffered IO writes that are non sequential because they use a shared lock */
+#define RWF_NOSERIAL	((__force __kernel_rwf_t)0x00000400)
+
 /* 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_DONTCACHE | RWF_NOSIGNAL | RWF_WRITETHROUGH)
+			 RWF_DONTCACHE | RWF_NOSIGNAL | RWF_WRITETHROUGH |\
+			 RWF_NOSERIAL)
 
 #define PROCFS_IOCTL_MAGIC 'f'
 
-- 
2.55.0