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SubjectRe: [PATCH v11 4/7] dm: enable synchronous dax
On Mon, Jun 10 2019 at  5:07am -0400,
Pankaj Gupta <pagupta@redhat.com> 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 <pagupta@redhat.com>
> ---
> 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

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