linux-ext4 - Re: [PATCH v2 09/23] iomap: allow filesystem to implement read path verification

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Date:   Wed, 5 Apr 2023 08:48:17 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Andrey Albershteyn <aalbersh@...hat.com>,
        Christoph Hellwig <hch@...radead.org>, dchinner@...hat.com,
        ebiggers@...nel.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 09/23] iomap: allow filesystem to implement read path
 verification

On Wed, Apr 05, 2023 at 08:06:27AM -0700, Darrick J. Wong wrote:
> > > I wonder if that also makes sense and keep all the deferral in the
> > > file system.  We'll need that for the btrfs iomap conversion anyway,
> > > and it seems more flexible.  The ioend processing would then move into
> > > XFS.
> > > 
> > 
> > Not sure what you mean here.
> 
> I /think/ Christoph is talking about allowing callers of iomap pagecache
> operations to supply a custom submit_bio function and a bio_set so that
> filesystems can add in their own post-IO processing and appropriately
> sized (read: minimum you can get away with) bios.  I imagine btrfs has
> quite a lot of (read) ioend processing they need to do, as will xfs now
> that you're adding fsverity.

Exactly.

> I think the point is that this is a general "check what we just read"
> hook, so it could be in readpage_ops since we're never going to need to
> re-validate verity contents, right?  Hence it could be in readpage_ops
> instead of the general iomap_folio_ops.
> 
> <shrug> Is there a use case for ->verify_folio that isn't a read post-
> processing step?

Yes.  In fact I wonder if the verification might actually be done
in the per-bio end_io handler in the file system.  But I'll need
to find some more time to read through the XFS parts of series to
come up with a more intelligent suggestion on that.