[PATCH RFC 04/16] fs: Add RWF_ATOMIC and IOCB_ATOMIC flags for atomic write support

[John Garry]

From: Prasad Singamsetty <prasad.singamsetty at oracle.com>

Userspace may add flag RWF_ATOMIC to pwritev2() to indicate that the
write is to be issued atomically, according to special alignment and
length rules.

For any syscall interface utilizing struct iocb, add IOCB_ATOMIC for
iocb->ki_flags field to indicate the same.

A call to statx will give the relevant atomic write info:
- atomic_write_unit_min
- atomic_write_unit_max

Both values are a power-of-2.

Applications can avail of atomic write feature by ensuring that its data
blocks are a power-of-2 in size and also sized between
atomic_write_unit_min and atomic_write_unit_max, inclusive. Applications
must ensure that data blocks are naturally aligned also. If these rules
are followed then the kernel will guarantee to write each data block
atomically.

Not following these rules mean that there is no guarantee that data
will be written atomically.

Signed-off-by: Prasad Singamsetty <prasad.singamsetty at oracle.com>
Signed-off-by: John Garry <john.g.garry at oracle.com>
---
 include/linux/fs.h            | 1 +
 include/uapi/linux/fs.h       | 5 ++++-
 tools/include/uapi/linux/fs.h | 5 ++++-
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c85916e9f7db..5bace817c041 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -329,6 +329,7 @@ enum rw_hint {
 #define IOCB_SYNC		(__force int) RWF_SYNC
 #define IOCB_NOWAIT		(__force int) RWF_NOWAIT
 #define IOCB_APPEND		(__force int) RWF_APPEND
+#define IOCB_ATOMIC		(__force int) RWF_ATOMIC
 
 /* non-RWF related bits - start at 16 */
 #define IOCB_EVENTFD		(1 << 16)
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index b7b56871029c..e3b4f5bc6860 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -301,8 +301,11 @@ typedef int __bitwise __kernel_rwf_t;
 /* per-IO O_APPEND */
 #define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)
 
+/* Atomic Write */
+#define RWF_ATOMIC	((__force __kernel_rwf_t)0x00000020)
+
 /* mask of flags supported by the kernel */
 #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
-			 RWF_APPEND)
+			 RWF_APPEND | RWF_ATOMIC)
 
 #endif /* _UAPI_LINUX_FS_H */
diff --git a/tools/include/uapi/linux/fs.h b/tools/include/uapi/linux/fs.h
index b7b56871029c..e3b4f5bc6860 100644
--- a/tools/include/uapi/linux/fs.h
+++ b/tools/include/uapi/linux/fs.h
@@ -301,8 +301,11 @@ typedef int __bitwise __kernel_rwf_t;
 /* per-IO O_APPEND */
 #define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)
 
+/* Atomic Write */
+#define RWF_ATOMIC	((__force __kernel_rwf_t)0x00000020)
+
 /* mask of flags supported by the kernel */
 #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
-			 RWF_APPEND)
+			 RWF_APPEND | RWF_ATOMIC)
 
 #endif /* _UAPI_LINUX_FS_H */
-- 
2.31.1