linux-kernel - Re: [PATCH v4 7/9] dm: handle error from blk_ksm

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Date:   Fri, 23 Jul 2021 10:26:48 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Satya Tangirala <satyaprateek2357@...il.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        Satya Tangirala <satyat@...gle.com>
Subject: Re: [PATCH v4 7/9] dm: handle error from blk_ksm_register()

On Tue, Jul 06, 2021 at 10:29:41PM -0700, Satya Tangirala wrote:
> From: Satya Tangirala <satyat@...gle.com>
> 
> Handle any error from blk_ksm_register() in the callers. Previously,
> the callers ignored the return value because blk_ksm_register() wouldn't
> fail as long as the request_queue didn't have integrity support too, but
> as this is no longer the case, it's safer for the callers to just handle
> the return value appropriately.
> 
> Signed-off-by: Satya Tangirala <satyat@...gle.com>
> ---
>  drivers/md/dm-table.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 29cbfc3e3c4b..c79c0fbe80dd 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -1343,6 +1343,20 @@ static int dm_table_construct_keyslot_manager(struct dm_table *t)
>  	 */
>  	t->ksm = ksm;
>  
> +	/*
> +	 * At this point, t->ksm is either NULL or *not* empty (i.e. will support
> +	 * at least 1 crypto capability), the request queue doesn't support
> +	 * integrity, and it also satisfies all the block layer constraints
> +	 * "sufficiently" (as in the constraints of the DM device's request queue
> +	 * won't preclude any of the intersection of the supported capabilities
> +	 * of the underlying devices, since if some capability were precluded by
> +	 * the DM device's request queue's constraints, that capability would
> +	 * also have been precluded by one of the child device's request queues).
> +	 *
> +	 * Hence any future attempt to call blk_ksm_register() on t->ksm (if it's
> +	 * not NULL) will succeed.
> +	 */
> +
>  	return 0;

I don't think this properly answers the question I had on the previous version
of this patch, which was not just how we know that blk_ksm_register() will
succeed later, but also how we know that the blk_ksm_is_superset() check done
above is valid when some of the crypto capabilities may be cleared by
blk_ksm_register() later.  Is the assumption actually that in the device-mapper
case, blk_ksm_register() will never clear any crypto capabilities at all?
If so, can that be explained properly?

- Eric