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Date:   Fri, 7 Apr 2023 19:56:36 +0000
From:   Eric Biggers <ebiggers@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Andrey Albershteyn <aalbersh@...hat.com>, dchinner@...hat.com,
        hch@...radead.org, linux-xfs@...r.kernel.org,
        fsverity@...ts.linux.dev, rpeterso@...hat.com, agruenba@...hat.com,
        xiang@...nel.org, chao@...nel.org,
        damien.lemoal@...nsource.wdc.com, jth@...nel.org,
        linux-erofs@...ts.ozlabs.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel@...hat.com
Subject: Re: [PATCH v2 21/23] xfs: handle merkle tree block size != fs
 blocksize != PAGE_SIZE

On Wed, Apr 05, 2023 at 05:44:36PM -0700, Eric Biggers wrote:
> > Not vmalloc'ed, but vmapped. we allocate the pages individually, but
> > then call vm_map_page() to present the higher level code with a
> > single contiguous memory range if it is a multi-page buffer.
> > 
> > We do have the backing info held in the buffer, and that's what we
> > use for IO. If fsverity needs a page based scatter/gather list
> > for hardware offload, it could ask the filesystem to provide it
> > for that given buffer...
> > 
> > > BTW, converting fs/verity/ from ahash to shash is an option; I've really never
> > > been a fan of the scatterlist-based crypto APIs!  The disadvantage of doing
> > > this, though, would be that it would remove support for all the hardware crypto
> > > drivers.
> > >
> > > That *might* actually be okay, as that approach to crypto acceleration
> > > has mostly fallen out of favor, in favor of CPU-based acceleration.  But I do
> > > worry about e.g. someone coming out of the woodwork and saying they need to use
> > > fsverity on a low-powered ARM board that has a crypto accelerator like CAAM, and
> > > they MUST use their crypto accelerator to get acceptable performance.
> > 
> > True, but we are very unlikely to be using XFS on such small
> > systems and I don't think we really care about XFS performance on
> > android sized systems, either.
> > 
> 
> FYI, I've sent an RFC patch that converts fs/verity/ from ahash to shash:
> https://lore.kernel.org/r/20230406003714.94580-1-ebiggers@kernel.org
> 
> It would be great if we could do that.  But I need to get a better sense for
> whether anyone will complain...

FWIW, dm-verity went in the other direction.  It started with shash, and then in
2017 it was switched to ahash by https://git.kernel.org/linus/d1ac3ff008fb9a48
("dm verity: switch to using asynchronous hash crypto API").

I think that was part of my motivation for using ahash in fsverity from the
beginning.

Still, it does seem that ahash is more trouble than it's worth these days...

- Eric

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