Re: [Qemu-devel] [PATCH v11 4/7] dm: enable synchronous dax

[Mike Snitzer]

On Mon, Jun 10 2019 at  5:07am -0400,
Pankaj Gupta <address@hidden> wrote:

>  This patch sets dax device 'DAXDEV_SYNC' flag if all the target
>  devices of device mapper support synchrononous DAX. If device
>  mapper consists of both synchronous and asynchronous dax devices,
>  we don't set 'DAXDEV_SYNC' flag.
> 
> Signed-off-by: Pankaj Gupta <address@hidden>
> ---
>  drivers/md/dm-table.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 350cf0451456..c5160d846fe6 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -890,10 +890,17 @@ static int device_supports_dax(struct dm_target *ti, 
> struct dm_dev *dev,
>                       start, len);
>  }
>  
> +static int device_synchronous(struct dm_target *ti, struct dm_dev *dev,
> +                                    sector_t start, sector_t len, void *data)
> +{
> +     return dax_synchronous(dev->dax_dev);
> +}
> +
>  bool dm_table_supports_dax(struct dm_table *t, int blocksize)
>  {
>       struct dm_target *ti;
>       unsigned i;
> +     bool dax_sync = true;
>  
>       /* Ensure that all targets support DAX. */
>       for (i = 0; i < dm_table_get_num_targets(t); i++) {
> @@ -906,7 +913,14 @@ bool dm_table_supports_dax(struct dm_table *t, int 
> blocksize)
>                   !ti->type->iterate_devices(ti, device_supports_dax,
>                           &blocksize))
>                       return false;
> +
> +             /* Check devices support synchronous DAX */
> +             if (dax_sync &&
> +                 !ti->type->iterate_devices(ti, device_synchronous, NULL))
> +                     dax_sync = false;
>       }
> +     if (dax_sync)
> +             set_dax_synchronous(t->md->dax_dev);
>  
>       return true;
>  }
> -- 
> 2.20.1
> 

dm_table_supports_dax() is called multiple times (from
dm_table_set_restrictions and dm_table_determine_type).  It is strange
to have a getter have a side-effect of being a setter too.  Overloading
like this could get you in trouble in the future.

Are you certain this is what you want?

Or would it be better to refactor dm_table_supports_dax() to take an
iterate_devices_fn arg and have callers pass the appropriate function?
Then have dm_table_set_restrictions() caller do:

     if (dm_table_supports_dax(t, device_synchronous, NULL))
          set_dax_synchronous(t->md->dax_dev);

(NULL arg implies dm_table_supports_dax() refactoring would take a int
*data pointer rather than int type).

Mike