Re: [RFC PATCH] blk-integrity: support arbitrary buffer alignment

[Keith Busch <kbusch@kernel.org>]

On Wed, Nov 05, 2025 at 03:15:04PM +0100, Christoph Hellwig wrote:
> I have a hard time understanding how this actually works, mostly due to
> "union pi_tuple".  What does that union of two pointers buy us over just
> passing a void pointer and deriving the type we need deeper down?  Also
> given that taking the address and the comparing it (at thbeginning of
> blk_tuple_remap_end) how is this actually going to work?

It's a good question.

This generally does pass 'void *' to contiguous protection data. If the
actual protection payload is spread across multiple segments, though, we
don't have contiguous data, so we have to bounce it through something.
Declaring the union on the stack provides this memory for that unusual
case. If the bip segment is big enough, we point the 'void *' directly
at it; if not, we point it to the temporary onstack allocation. Using
a union ensures that it's definitely big enough for any checksum type.

Again, it's a weird corner case. The kernel auto-pi wouldn't allocate
that way unless you have very weird metadata format, like with an odd
number of bytes preceding or following the DIF (never encountered such a
thing, but the specs allow it). User space can send weird alignemnts
with io_uring now, too.