Re: [PATCH v4 2/3] readv.2: Document RWF_ATOMIC flag

Darrick J. Wong
2024-07-17T21:44:24Z

On Wed, Jul 17, 2024 at 09:36:18AM +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>
> ---
> man/man2/readv.2 | 76 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 76 insertions(+)
>
> diff --git a/man/man2/readv.2 b/man/man2/readv.2
> index eecde06dc..9c8a11324 100644
> --- a/man/man2/readv.2
> +++ b/man/man2/readv.2
> @@ -193,6 +193,66 @@ which provides lower latency, but may use additional resources.
> .B O_DIRECT
> flag.)
> .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,

Nit: these could be two sentences

"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 32KB at a file offset of 32KB is permitted,
> +however a write of length 32KB at a file offset of 48KB is not permitted.

Pickier nit: KiB, not KB.

> +The upper limit of
> +.I iovcnt
> +for
> +.BR pwritev2 ()
> +is in

"is given by" ?

> +.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,
> +.BR fdatasync (2),
> +or
> +.BR fsync (2),
> +or
> +.BR open (2)
> +and either
> +.B O_SYNC
> +or
> +.B O_DSYNC,
> +or
> +.B pwritev2 ()
> +and either
> +.B RWF_SYNC
> +or
> +.B RWF_DSYNC
> +is required. Flags

This sentence ^^ should start on a new line.

> +.B O_SYNC
> +or
> +.B RWF_SYNC
> +provide the strongest guarantees for
> +.BR RWF_ATOMIC,
> +in that all data and also file metadata updates will be persisted for a
> +successfully completed write.
> +Just using either flags
> +.B O_DSYNC
> +or
> +.B RWF_DSYNC
> +means that all data and any file updates will be persisted for a successfully
> +completed write.

"any file updates" ? I /think/ the difference between O_SYNC and
O_DSYNC is that O_DSYNC persists all data and file metadata updates for
the file range that was written, whereas O_SYNC persists all data and
file metadata updates for the entire file.

Perhaps everything between "Flags O_SYNC or RWF_SYNC..." and "...for a
successfully completed write." should instead refer readers to the notes
about synchronized I/O flags in the openat manpage?

> +Not using any sync flags means that there is no guarantee that data or
> +filesystem updates are persisted.
> +.TP
> .BR RWF_SYNC " (since Linux 4.7)"
> .\" commit e864f39569f4092c2b2bc72c773b6e486c7e3bd9
> Provide a per-write equivalent of the
> @@ -279,10 +339,26 @@ values overflows an
> .I ssize_t
> value.
> .TP
> +.B EINVAL
> + For
> +.BR RWF_ATOMIC
> +set,

"If RWF_ATOMIC is specified..." ?

(to be a bit more consistent with the language around the AT_* flags in
openat)

> +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.
> +For
> +.BR RWF_ATOMIC
> +set, this maximum is in

(same)

--D

> +.I stx_atomic_write_segments_max
> +from
> +.I statx.
> .TP
> .B EOPNOTSUPP
> An unknown flag is specified in \fIflags\fP.
> --
> 2.31.1
>