[PATCH 04/21] fs: Add RWF_ATOMIC and IOCB_ATOMIC flags for atomic write support

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From: Prasad Singamsetty <prasad.singamsetty@xxxxxxxxxx>

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

Torn write prevention means that for a power or any other HW failure, all
or none of the data will be committed to storage, but never a mix of old
and new.

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 the total
length of a write is a power-of-2 in size and also sized between
atomic_write_unit_min and atomic_write_unit_max, inclusive. Applications
must ensure that the write is at a naturally-aligned offset in the file
wrt the total write length.

Signed-off-by: Prasad Singamsetty <prasad.singamsetty@xxxxxxxxxx>
Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
 include/linux/fs.h      | 1 +
 include/uapi/linux/fs.h | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index b528f063e8ff..898952dee8eb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -328,6 +328,7 @@ 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 */
 #endif /* _UAPI_LINUX_FS_H */

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