Re: [man-pages RFC PATCH v2] statx.2, open.2: document STATX_DIOALIGN

Darrick J. Wong
2022-07-22T16:31:58Z

On Fri, Jul 22, 2022 at 12:42:28AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> Document the proposed STATX_DIOALIGN support for statx()
> (https://lore.kernel.org/linux-fsdevel/20220722071228.146690-1-ebiggers@kernel.org/T/#u).
>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>
> v2: rebased onto man-pages master branch, mentioned xfs, and updated
> link to patchset
>
> man2/open.2 | 43 ++++++++++++++++++++++++++++++++-----------
> man2/statx.2 | 29 +++++++++++++++++++++++++++++
> 2 files changed, 61 insertions(+), 11 deletions(-)
>
> diff --git a/man2/open.2 b/man2/open.2
> index d1485999f..ef29847c3 100644
> --- a/man2/open.2
> +++ b/man2/open.2
> @@ -1732,21 +1732,42 @@ of user-space buffers and the file offset of I/Os.
> In Linux alignment
> restrictions vary by filesystem and kernel version and might be
> absent entirely.
> -However there is currently no filesystem\-independent
> -interface for an application to discover these restrictions for a given
> -file or filesystem.
> -Some filesystems provide their own interfaces
> -for doing so, for example the
> +The handling of misaligned
> +.B O_DIRECT
> +I/Os also varies; they can either fail with
> +.B EINVAL
> +or fall back to buffered I/O.
> +.PP
> +Since Linux 5.20,
> +.B O_DIRECT
> +support and alignment restrictions for a file can be queried using
> +.BR statx (2),
> +using the
> +.B STATX_DIOALIGN
> +flag.
> +Support for
> +.B STATX_DIOALIGN
> +varies by filesystem; see
> +.BR statx (2).
> +.PP
> +Some filesystems provide their own interfaces for querying
> +.B O_DIRECT
> +alignment restrictions, for example the
> .B XFS_IOC_DIOINFO
> operation in
> .BR xfsctl (3).
> +.B STATX_DIOALIGN
> +should be used instead when it is available.
> .PP
> -Under Linux 2.4, transfer sizes, the alignment of the user buffer,
> -and the file offset must all be multiples of the logical block size
> -of the filesystem.
> -Since Linux 2.6.0, alignment to the logical block size of the
> -underlying storage (typically 512 bytes) suffices.
> -The logical block size can be determined using the
> +If none of the above is available, then direct I/O support and alignment
> +restrictions can only be assumed from known characteristics of the filesystem,
> +the individual file, the underlying storage device(s), and the kernel version.
> +In Linux 2.4, most block device based filesystems require that the file offset
> +and the length and memory address of all I/O segments be multiples of the
> +filesystem block size (typically 4096 bytes).
> +In Linux 2.6.0, this was relaxed to the logical block size of the block device
> +(typically 512 bytes).
> +A block device's logical block size can be determined using the
> .BR ioctl (2)
> .B BLKSSZGET
> operation or from the shell using the command:
> diff --git a/man2/statx.2 b/man2/statx.2
> index 0326e9af0..ea38ec829 100644
> --- a/man2/statx.2
> +++ b/man2/statx.2
> @@ -61,7 +61,12 @@ struct statx {
> containing the filesystem where the file resides */
> __u32 stx_dev_major; /* Major ID */
> __u32 stx_dev_minor; /* Minor ID */
> +
> __u64 stx_mnt_id; /* Mount ID */
> +
> + /* Direct I/O alignment restrictions */
> + __u32 stx_dio_mem_align;
> + __u32 stx_dio_offset_align;
> };
> .EE
> .in
> @@ -247,6 +252,8 @@ STATX_BTIME Want stx_btime
> STATX_ALL The same as STATX_BASIC_STATS | STATX_BTIME.
> It is deprecated and should not be used.
> STATX_MNT_ID Want stx_mnt_id (since Linux 5.8)
> +STATX_DIOALIGN Want stx_dio_mem_align and stx_dio_offset_align
> + (since Linux 5.20; support varies by filesystem)
> .TE
> .in
> .PP
> @@ -407,6 +414,28 @@ This is the same number reported by
> .BR name_to_handle_at (2)
> and corresponds to the number in the first field in one of the records in
> .IR /proc/self/mountinfo .
> +.TP
> +.I stx_dio_mem_align
> +The alignment (in bytes) required for user memory buffers for direct I/O
> +.BR "" ( O_DIRECT )
> +on this file. or 0 if direct I/O is not supported on this file.

Nit: "..on this file, or 0 if direct..."

> +.IP
> +.B STATX_DIOALIGN
> +.IR "" ( stx_dio_mem_align
> +and
> +.IR stx_dio_offset_align )
> +is supported on block devices since Linux 5.20.
> +The support on regular files varies by filesystem; it is supported by ext4,
> +f2fs, and xfs since Linux 5.20.
> +.TP
> +.I stx_dio_offset_align
> +The alignment (in bytes) required for file offsets and I/O segment lengths for
> +direct I/O
> +.BR "" ( O_DIRECT )
> +on this file, or 0 if direct I/O is not supported on this file.

On this last part -- userspace can only conclude that directio is not
supported on the file if ((STATX_DIOALIGN & stx_mask) &&
stx_dio_offset_align == 0), right?

IOWs, if (STATX_DIOALIGN & stx_mask)==0 then userspace can't draw any
conclusions from stx_dio_offset_align, correct?

If the answers are yes and yes, then I think I've understood all this
and can say
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> +This will only be nonzero if
> +.I stx_dio_mem_align
> +is nonzero, and vice versa.
> .PP
> For further information on the above fields, see
> .BR inode (7).
>
> base-commit: f9f25914e4ed393ac284ab921876e8a78722c504
> --
> 2.37.0
>