Re: [PATCH v5 2/3] readv.2: Document RWF_ATOMIC flag

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jul 22, 2024 at 09:57:22AM +0000, John Garry wrote:
> From: Himanshu Madhani <himanshu.madhani@oracle.com>
> 
> Add RWF_ATOMIC flag description for pwritev2().
> 
> Signed-off-by: Himanshu Madhani <himanshu.madhani@oracle.com>
> [jpg: complete rewrite]
> Signed-off-by: John Garry <john.g.garry@oracle.com>

Sounds good to me now!  Thanks for taking care of the documentation!
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  man/man2/readv.2 | 61 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 61 insertions(+)
> 
> diff --git a/man/man2/readv.2 b/man/man2/readv.2
> index eecde06dc..7737eb65c 100644
> --- a/man/man2/readv.2
> +++ b/man/man2/readv.2
> @@ -237,6 +237,50 @@ the data is always appended to the end of the file.
>  However, if the
>  .I offset
>  argument is \-1, the current file offset is updated.
> +.TP
> +.BR RWF_ATOMIC " (since Linux 6.11)"
> +Requires that writes to regular files in block-based filesystems be issued with
> +torn-write protection.
> +Torn-write protection means that for a power or any other hardware failure,
> +all or none of the data from the write will be stored,
> +but never a mix of old and new data.
> +This flag is meaningful only for
> +.BR pwritev2 (),
> +and its effect applies only to the data range written by the system call.
> +The total write length must be power-of-2 and must be sized in the range
> +.RI [ stx_atomic_write_unit_min ,
> +.IR stx_atomic_write_unit_max ].
> +The write must be at a naturally-aligned offset within the file with respect to
> +the total write length.
> +For example,
> +a write of length 32KiB at a file offset of 32KiB is permitted,
> +however a write of length 32KiB at a file offset of 48KiB is not permitted.
> +The upper limit of
> +.I iovcnt
> +for
> +.BR pwritev2 ()
> +is given by the value in
> +.I stx_atomic_write_segments_max.
> +Torn-write protection only works with
> +.B O_DIRECT
> +flag,
> +i.e. buffered writes are not supported.
> +To guarantee consistency from the write between a file's in-core state with the
> +storage device,
> +.B O_SYNC
> +or
> +.B O_DSYNC
> +must be specified for
> +.BR open (2).
> +The same synchronized I/O guarantees as described in
> +.BR open (2)
> +are provided when these flags or their equivalent flags and system calls are
> +used (e.g.
> +if
> +.BR RWF_SYNC
> +is specified for
> +.BR pwritev2 ()
> +).
>  .SH RETURN VALUE
>  On success,
>  .BR readv (),
> @@ -280,9 +324,26 @@ values overflows an
>  value.
>  .TP
>  .B EINVAL
> +If
> +.BR RWF_ATOMIC
> +is specified,
> +the combination of the sum of the
> +.I iov_len
> +values and the
> +.I offset
> +value does not comply with the length and offset torn-write protection rules.
> +.TP
> +.B EINVAL
>  The vector count,
>  .IR iovcnt ,
>  is less than zero or greater than the permitted maximum.
> +If
> +.BR RWF_ATOMIC
> +is specified,
> +this maximum is given by the
> +.I stx_atomic_write_segments_max
> +value from
> +.I statx.
>  .TP
>  .B EOPNOTSUPP
>  An unknown flag is specified in \fIflags\fP.
> -- 
> 2.31.1
> 
>