Re: [PATCH v7 2/9] block: Inline encryption support for blk-mq

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Feb 21, 2020 at 09:22:05AM -0800, Christoph Hellwig wrote:
> > +int blk_crypto_evict_key(struct request_queue *q,
> > +			 const struct blk_crypto_key *key)
> > +{
> > +	if (q->ksm && blk_ksm_crypto_mode_supported(q->ksm, key))
> > +		return blk_ksm_evict_key(q->ksm, key);
> > +
> > +	return 0;
> > +}
> 
> Is there any point in this wrapper that just has a single caller?
> Als why doesn't blk_ksm_evict_key have the blk_ksm_crypto_mode_supported
> sanity check itself?

Later in the series it's changed to:

int blk_crypto_evict_key(struct request_queue *q,
                         const struct blk_crypto_key *key)
{
        if (q->ksm && blk_ksm_crypto_mode_supported(q->ksm, key))
                return blk_ksm_evict_key(q->ksm, key);

        return blk_crypto_fallback_evict_key(key);
}

I.e. if the encryption mode is using hardware, then the key needs to be evicted
from q->ksm.  Otherwise the key needs to be evicted from the fallback.

Also keep in mind that our goal is to define a clean API for any user of the
block layer to use encryption, not just fs/crypto/.  That API includes:

	blk_crypto_init_key()
	blk_crypto_start_using_key()
	bio_crypt_set_ctx()
	blk_crypto_evict_key()

If anyone else decides to use inline encryption (e.g., if inline encryption
support were added to dm-crypt or another device-mapper target), they'll use
these same functions.

So IMO it's important to define a clean API that won't need to be refactored as
soon as anyone else starts using it, and not e.g. micro-optimize for code length
based on there currently being only one user.

- Eric