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

[Christoph Hellwig]

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.