Received-SPF: pass (google.com: domain of linux-ext4-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 7 Apr 2023 19:56:36 +0000
From:   Eric Biggers <ebiggers@kernel.org>
To:     Dave Chinner <david@fromorbit.com>
Cc:     "Darrick J. Wong" <djwong@kernel.org>,
        Andrey Albershteyn <aalbersh@redhat.com>, dchinner@redhat.com,
        hch@infradead.org, linux-xfs@vger.kernel.org,
        fsverity@lists.linux.dev, rpeterso@redhat.com, agruenba@redhat.com,
        xiang@kernel.org, chao@kernel.org,
        damien.lemoal@opensource.wdc.com, jth@kernel.org,
        linux-erofs@lists.ozlabs.org, linux-btrfs@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
        cluster-devel@redhat.com
Subject: Re: [PATCH v2 21/23] xfs: handle merkle tree block size != fs
 blocksize != PAGE_SIZE
Message-ID: <ZDB1dPVjon4Qthok@gmail.com>
References: <20230404145319.2057051-1-aalbersh@redhat.com>
 <20230404145319.2057051-22-aalbersh@redhat.com>
 <20230404163602.GC109974@frogsfrogsfrogs>
 <20230405160221.he76fb5b45dud6du@aalbersh.remote.csb>
 <20230405163847.GG303486@frogsfrogsfrogs>
 <ZC264FSkDQidOQ4N@gmail.com>
 <20230405222646.GR3223426@dread.disaster.area>
 <ZC38DkQVPZBuZCZN@gmail.com>
 <20230405233753.GU3223426@dread.disaster.area>
 <20230406004434.GA879@sol.localdomain>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230406004434.GA879@sol.localdomain>
Precedence: bulk

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