[RFC PATCH v4 09/12] fs: Introduce RWF_NOSERIAL flag to indicate parallel reads/writes

[Ojaswin Mujoo]

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) <ritesh.list@xxxxxxxxx>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
---
 include/linux/fs.h            | 10 +++++++++-
 include/uapi/linux/fs.h       |  6 +++++-
 tools/include/uapi/linux/fs.h |  5 ++++-
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index cbaa65e05d9e..6f84bb41f84a 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)
@@ -375,7 +376,8 @@ struct readahead_control;
 	{ IOCB_NOIO,		"NOIO" }, \
 	{ IOCB_ALLOC_CACHE,	"ALLOC_CACHE" }, \
 	{ IOCB_AIO_RW,		"AIO_RW" }, \
-	{ IOCB_HAS_METADATA,	"AIO_HAS_METADATA" }
+	{ IOCB_HAS_METADATA,	"AIO_HAS_METADATA" }, \
+	{ IOCB_NOSERIAL,	"IOCB_NOSERIAL" }
 
 struct kiocb {
 	struct file		*ki_filp;
@@ -3499,6 +3501,12 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags,
 		ki->ki_flags &= ~IOCB_APPEND;
 	}
 
+	/*
+	 * Currently, only writethrough supports noserial IO.
+	 */
+	if ((flags & RWF_NOSERIAL) && !(flags & RWF_WRITETHROUGH))
+		return -EOPNOTSUPP;
+
 	ki->ki_flags |= kiocb_flags;
 	return 0;
 }
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 67d8987b343d..b2ad2d8548ac 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'
 
diff --git a/tools/include/uapi/linux/fs.h b/tools/include/uapi/linux/fs.h
index 63ef6b126b8b..c7a1a1d238f0 100644
--- a/tools/include/uapi/linux/fs.h
+++ b/tools/include/uapi/linux/fs.h
@@ -347,10 +347,13 @@ 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_WRITETHROUGH)
+			 RWF_DONTCACHE | RWF_WRITETHROUGH | RWF_NOSERIAL)
 
 #define PROCFS_IOCTL_MAGIC 'f'
 
-- 
2.55.0