linux-kernel - Re: [PATCH v2] dm-verity: remove support for asynchronous hashes

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Message-ID: <20250710161012.GB604603@google.com>
Date: Thu, 10 Jul 2025 16:10:12 +0000
From: Eric Biggers <ebiggers@...nel.org>
To: Milan Broz <gmazyland@...il.com>
Cc: dm-devel@...ts.linux.dev, Alasdair Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...nel.org>,
	Mikulas Patocka <mpatocka@...hat.com>, linux-kernel@...r.kernel.org,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	"Jason A . Donenfeld" <Jason@...c4.com>,
	Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v2] dm-verity: remove support for asynchronous hashes

On Thu, Jul 10, 2025 at 10:28:55AM +0200, Milan Broz wrote:
> On 7/9/25 9:09 PM, Eric Biggers wrote:
> > The support for asynchronous hashes in dm-verity has outlived its
> > usefulness.  It adds significant code complexity and opportunity for
> > bugs.  I don't know of anyone using it in practice.  (The original
> > submitter of the code possibly was, but that was 8 years ago.)  Data I
> > recently collected for en/decryption shows that using off-CPU crypto
> > "accelerators" is consistently much slower than the CPU
> > (https://lore.kernel.org/r/20250704070322.20692-1-ebiggers@kernel.org/),
> > even on CPUs that lack dedicated cryptographic instructions.  Similar
> > results are likely to be seen for hashing.
> > 
> > I already removed support for asynchronous hashes from fsverity two
> > years ago, and no one ever complained.
> > 
> > Moreover, neither dm-verity, fsverity, nor fscrypt has ever actually
> > used the asynchronous crypto algorithms in a truly asynchronous manner.
> > The lack of interest in such optimizations provides further evidence
> > that it's only the CPU-based crypto that actually matters.
> > 
> > Historically, it's also been common for people to forget to enable the
> > optimized SHA-256 code, which could contribute to an off-CPU crypto
> > engine being perceived as more useful than it really is.  In 6.16 I
> > fixed that: the optimized SHA-256 code is now enabled by default.
> > 
> > Therefore, let's drop the support for asynchronous hashes in dm-verity.
> > 
> > Tested with verity-compat-test.
> 
> Hi,
> 
> I shortly tested it with veritysetup too, also on 32bit.
> And I like this patch (I wish we can remove the async thing from the dmcrypt too...)

IMO we should do it for dm-crypt too, though it's going to be a slightly
tougher sell there because it actually goes through the trouble of using
the async API "properly".

- Eric