Re: [PATCHv5 3/3] dm-crypt: allow unaligned bio_vecs for direct io

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jul 10, 2026 at 10:14:19PM -0600, Keith Busch wrote:
> On Fri, Jul 10, 2026 at 08:54:40PM -0400, Eric Biggers wrote:
> > 
> > So what is the minimum SG entry length alignment this can result in?
> > Once that get misaligned enough, crypto_skcipher_encrypt() and
> > crypto_skcipher_decrypt() start allocating bounce buffers internally.
> > That code has no mempool fallback and can fail with ENOMEM.
> 
> I'm not the one introducing ENOMEM consideration here.

You are.  This patch starts allowing misaligned segments, which trigger
the use of bounce buffers in the crypto code as I explained.  Currently,
dm-crypt relies on its alignment requirement to avoid those cases.

> Indeed, patch 2 in this series addresses this very thing ahead of what
> I'm asking for.

I don't think patch 2 is sufficient, as BLK_STS_RESOURCE is often not
going to get handled correctly.

> > I think you'll need to take the limitations of crypto_skcipher into
> > account.  See the comment above CRYPTO_ALG_ALLOCATES_MEMORY.
> 
> The block layer's queue limits is the single source of truth for what
> the backing device at any layer can accept. If you've some constraints
> that are not appropriately described there, then let's fix that.

I agree that dm-crypt's queue limits should accurately describe its
constraints.  This patch makes it no longer do so.

- Eric