linux-kernel - Re: [PATCH] DAX: enable iostat for read/write

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Date:   Fri, 14 Oct 2016 10:35:05 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Toshi Kani <toshi.kani@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] DAX: enable iostat for read/write

On Fri, Oct 14, 2016 at 10:25 AM, Toshi Kani <toshi.kani@....com> wrote:
> DAX IO path does not support iostat, but its metadata IO path does.
> Therefore, iostat shows metadata IO statistics only, which has been
> confusing to users.
>
> Add iostat support to the DAX read/write path.
>
> Note, iostat still does not support the DAX mmap path as it allows
> user applications to access directly.
>
> Signed-off-by: Toshi Kani <toshi.kani@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> ---
>  fs/dax.c |   37 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 014defd..3aaaac2 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -144,6 +144,34 @@ static sector_t to_sector(const struct buffer_head *bh,
>         return sector;
>  }
>
> +static void dax_iostat_start(struct gendisk *disk, struct iov_iter *iter,
> +                            unsigned long *start)
> +{
> +       int rw = iov_iter_rw(iter);
> +       int sec = iov_iter_count(iter) >> 9;

Should this be a minimum of one sector since we allow unaligned
transfers through this interface?

...or is iov_iter_count() somehow guaranteed to be sector aligned here?