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Date:   Thu, 2 Nov 2023 12:43:31 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     agk@redhat.com, snitzer@kernel.org, dm-devel@lists.linux.dev,
        linux-crypto@vger.kernel.org, gilad@benyossef.com,
        samitolvanen@google.com
Subject: Re: [PATCH] dm-verity: hash blocks with shash import+finup when
 possible
Message-ID: <ZUMo87EMeYxiCZLX@gondor.apana.org.au>
References: <20231030023351.6041-1-ebiggers@kernel.org>
 <ZUHZQ3tJH0WSV9dX@gondor.apana.org.au>
 <20231101054856.GA140941@sol.localdomain>
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Precedence: bulk

On Tue, Oct 31, 2023 at 10:48:56PM -0700, Eric Biggers wrote:
>
> Note that dm-verity also has to use an ugly and error-prone workaround to use
> ahash at all, since its hash blocks can be cached in vmalloc memory; see
> verity_hash_update().  shash handles vmalloc memory much more naturally, since
> no translation from vmalloc address to page to linear address is needed.

So why not drop ahash and always use shash? Does anybody care about
offload for dm-verity?

Alternatively, we could incorporate this into ahash itself.  Then
you could have an optimised code path that does not do SGs if the
underlying algorithm is shash.

I really do not wish to see this ahash/shash paradigm proliferate.

> > On a side note, if we're going to use shash for bulk data then we
> > should reintroduce the can/cannot sleep flag.
> 
> This patch limits the use of shash to blocks of at most PAGE_SIZE (*), which is
> the same as what ahash does internally.
> 
> (*) Except when the data block size is <= PAGE_SIZE but the hash block size is
>     > PAGE_SIZE.  I think that's an uncommon configuration that's not worth
>     worrying too much about, but it could be excluded from the shash-based code.

OK.  But we do still have the module signature verification code
path and I think that one still needs the can-sleep flag.

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt