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

[Jeremy Bongio <jbongio@google.com>]

What is the advantage of using write flags instead of using an atomic
open flag (O_ATOMIC)? With an open flag, write, writev, pwritev would
all be supported for atomic writes. And this would potentially require
less application changes to take advantage of atomic writes.

On Fri, Sep 29, 2023 at 3:28 AM John Garry <john.g.garry@oracle.com> wrote:
>
> From: Prasad Singamsetty <prasad.singamsetty@oracle.com>
>
> 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@oracle.com>
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  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 */
>  #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
>