Re: [PATCH v3 1/8] statx: add direct I/O alignment information

Darrick J. Wong
2022-06-23T15:58:38Z

On Thu, Jun 16, 2022 at 01:14:59PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> Traditionally, the conditions for when DIO (direct I/O) is supported
> were fairly simple. For both block devices and regular files, DIO had
> to be aligned to the logical block size of the block device.
>
> However, due to filesystem features that have been added over time (e.g.
> multi-device support, data journalling, inline data, encryption, verity,
> compression, checkpoint disabling, log-structured mode), the conditions
> for when DIO is allowed on a regular file have gotten increasingly
> complex. Whether a particular regular file supports DIO, and with what
> alignment, can depend on various file attributes and filesystem mount
> options, as well as which block device(s) the file's data is located on.
>
> Moreover, the general rule of DIO needing to be aligned to the block
> device's logical block size is being relaxed to allow user buffers (but
> not file offsets) aligned to the DMA alignment instead
> (https://lore.kernel.org/linux-block/20220610195830.3574005-1-kbusch@fb.com/T/#u).
>
> XFS has an ioctl XFS_IOC_DIOINFO that exposes DIO alignment information.
> Uplifting this to the VFS is one possibility. However, as discussed
> (https://lore.kernel.org/linux-fsdevel/20220120071215.123274-1-ebiggers@kernel.org/T/#u),
> this ioctl is rarely used and not known to be used outside of
> XFS-specific code. It was also never intended to indicate when a file
> doesn't support DIO at all, nor was it intended for block devices.
>
> Therefore, let's expose this information via statx(). Add the
> STATX_DIOALIGN flag and two new statx fields associated with it:
>
> * stx_dio_mem_align: the alignment (in bytes) required for user memory
> buffers for DIO, or 0 if DIO is not supported on the file.
>
> * stx_dio_offset_align: the alignment (in bytes) required for file
> offsets and I/O segment lengths for DIO, or 0 if DIO is not supported
> on the file. This will only be nonzero if stx_dio_mem_align is
> nonzero, and vice versa.
>
> Note that as with other statx() extensions, if STATX_DIOALIGN isn't set
> in the returned statx struct, then these new fields won't be filled in.
> This will happen if the file is neither a regular file nor a block
> device, or if the file is a regular file and the filesystem doesn't
> support STATX_DIOALIGN. It might also happen if the caller didn't
> include STATX_DIOALIGN in the request mask, since statx() isn't required
> to return unrequested information.
>
> This commit only adds the VFS-level plumbing for STATX_DIOALIGN. For
> regular files, individual filesystems will still need to add code to
> support it. For block devices, a separate commit will wire it up too.
>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> fs/stat.c | 2 ++
> include/linux/stat.h | 2 ++
> include/uapi/linux/stat.h | 4 +++-
> 3 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/stat.c b/fs/stat.c
> index 9ced8860e0f35..a7930d7444830 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -611,6 +611,8 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
> tmp.stx_dev_major = MAJOR(stat->dev);
> tmp.stx_dev_minor = MINOR(stat->dev);
> tmp.stx_mnt_id = stat->mnt_id;
> + tmp.stx_dio_mem_align = stat->dio_mem_align;
> + tmp.stx_dio_offset_align = stat->dio_offset_align;
>
> return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
> }
> diff --git a/include/linux/stat.h b/include/linux/stat.h
> index 7df06931f25d8..ff277ced50e9f 100644
> --- a/include/linux/stat.h
> +++ b/include/linux/stat.h
> @@ -50,6 +50,8 @@ struct kstat {
> struct timespec64 btime; /* File creation time */
> u64 blocks;
> u64 mnt_id;
> + u32 dio_mem_align;
> + u32 dio_offset_align;

Hmm. Does the XFS port of XFS_IOC_DIOINFO to STATX_DIOALIGN look like
this?

struct xfs_buftarg *target = xfs_inode_buftarg(ip);

kstat.dio_mem_align = target->bt_logical_sectorsize;
kstat.dio_offset_align = target->bt_logical_sectorsize;
kstat.result_mask |= STATX_DIOALIGN;

And I guess you're tabling the "optimal" IO discussions for now, because
there are too many variants of what that means?

--D

> };
>
> #endif
> diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
> index 1500a0f58041a..7cab2c65d3d7f 100644
> --- a/include/uapi/linux/stat.h
> +++ b/include/uapi/linux/stat.h
> @@ -124,7 +124,8 @@ struct statx {
> __u32 stx_dev_minor;
> /* 0x90 */
> __u64 stx_mnt_id;
> - __u64 __spare2;
> + __u32 stx_dio_mem_align; /* Memory buffer alignment for direct I/O */
> + __u32 stx_dio_offset_align; /* File offset alignment for direct I/O */
> /* 0xa0 */
> __u64 __spare3[12]; /* Spare space for future expansion */
> /* 0x100 */
> @@ -152,6 +153,7 @@ struct statx {
> #define STATX_BASIC_STATS 0x000007ffU /* The stuff in the normal stat struct */
> #define STATX_BTIME 0x00000800U /* Want/got stx_btime */
> #define STATX_MNT_ID 0x00001000U /* Got stx_mnt_id */
> +#define STATX_DIOALIGN 0x00002000U /* Want/got direct I/O alignment info */
>
> #define STATX__RESERVED 0x80000000U /* Reserved for future struct statx expansion */
>
> --
> 2.36.1
>