Re: [PATCH v3 00/10] ensure bios aren't split in middle of crypto data unit

[Eric Biggers]

On Fri, Jun 04, 2021 at 07:58:50PM +0000, 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. So we need to support cases where the data unit size is larger than
> the logical block size.

It's worth explaining the motivation for this more clearly.  Currently 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 only multiples of the logical block
size.  So the block layer needs to start handling this case appropriately.

- Eric