linux-ext4 - Re: [PATCH v7 3/9] block: blk-crypto-fallback for Inline Encryption

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Date:   Mon, 24 Feb 2020 15:36:25 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 3/9] block: blk-crypto-fallback for Inline Encryption

On Fri, Feb 21, 2020 at 10:34:37AM -0800, Eric Biggers wrote:
> On Fri, Feb 21, 2020 at 09:35:39AM -0800, Christoph Hellwig wrote:
> > High-level question:  Does the whole keyslot manager concept even make
> > sense for the fallback?  With the work-queue we have item that exectutes
> > at a time per cpu.  So just allocatea per-cpu crypto_skcipher for
> > each encryption mode and there should never be a slot limitation.  Or
> > do I miss something?
> 
> It does make sense because if blk-crypto-fallback didn't use a keyslot manager,
> it would have to call crypto_skcipher_setkey() on the I/O path for every bio to
> ensure that the CPU's crypto_skcipher has the correct key.  That's undesirable,
> because setting a new key can be expensive with some encryption algorithms, and
> also it can require a memory allocation which can fail.  For example, with the
> Adiantum algorithm, setting a key requires encrypting ~1100 bytes of data in
> order to generate subkeys.  It's better to set a key once and use it many times.

I didn't think of such expensive operations when setting the key.
Note that you would not have to do it on every I/O, as chances are high
you'll get I/O from the same submitter and thus the same key, and we
can optimize for that case pretty easily.

But if you think the keyslot manager is better I accept that, this was
just a throught when looking over the code.