linux-ext4 - Re: [PATCH v7 2/9] block: Inline encryption support for blk-mq

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Message-ID: <20200227182543.GC877@sol.localdomain>
Date:   Thu, 27 Feb 2020 10:25:43 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Satya Tangirala <satyat@...gle.com>, linux-block@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-ext4@...r.kernel.org,
        Barani Muthukumaran <bmuthuku@....qualcomm.com>,
        Kuohong Wang <kuohong.wang@...iatek.com>,
        Kim Boojin <boojin.kim@...sung.com>
Subject: Re: [PATCH v7 2/9] block: Inline encryption support for blk-mq

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