On Tue, Jul 06, 2021 at 10:29:34PM -0700, Satya Tangirala wrote:

When a bio has an encryption context, its size must be aligned to its
crypto data unit size. A bio must not be split in the middle of a data
unit. Currently, bios are split at logical block boundaries, but a crypto
data unit size might be larger than the logical block size - e.g. a machine
could be using fscrypt (which uses 4K crypto data units) with an eMMC block
device with inline encryption hardware that has a logical block size of 512
bytes.

Right now, the only user of blk-crypto is fscrypt (on ext4 and f2fs), which
(currently) only submits bios where the size of each segment is a multiple
of data_unit_size.  That happens to avoid most of the cases where bios
could be split in the middle of a data unit.  However, when support for
direct I/O on encrypted files is added, or when support for filesystem
metadata encryption is added, it will be possible for bios to have segment
lengths that are multiples of the logical block size, but not multiples of
the crypto data unit size.  So the block layer needs to start handling this
case appropriately.

I'm still not sold on this case yet.  sector size aligned I/O is an
optional feature, and I don't think it is worth this overhead.  And
while file systems metadata can be smaller than the file system block
size in a few cases (e.g. XFS log writes), that is usually an extra
performance optimization and can be trivially disabled in mkfs.