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.

Making blk-crypto-fallback use the keyslot manager also allows the keyslot
manager to be tested by routine filesystem regression testing, e.g.
'gce-xfstests -c ext4/encrypt -g auto -m inlinecrypt'.

- Eric