Re: [PATCH v4 0/9] ensure bios aren't split in middle of crypto data unit

[Christoph Hellwig]

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.