Re: [RFC v6 04/10] iomap: Add iomap_get_folio helper

[Dave Chinner <david@fromorbit.com>]

On Wed, Jan 11, 2023 at 07:36:26PM +0000, Matthew Wilcox wrote:
> On Tue, Jan 10, 2023 at 07:24:27AM -0800, Christoph Hellwig wrote:
> > On Tue, Jan 10, 2023 at 01:34:16PM +0000, Matthew Wilcox wrote:
> > > > Exactly.  And as I already pointed out in reply to Dave's original
> > > > patch what we really should be doing is returning an ERR_PTR from
> > > > __filemap_get_folio instead of reverse-engineering the expected
> > > > error code.
> > > 
> > > Ouch, we have a nasty problem.
> > > 
> > > If somebody passes FGP_ENTRY, we can return a shadow entry.  And the
> > > encodings for shadow entries overlap with the encodings for ERR_PTR,
> > > meaning that some shadow entries will look like errors.  The way I
> > > solved this in the XArray code is by shifting the error values by
> > > two bits and encoding errors as XA_ERROR(-ENOMEM) (for example).
> > > 
> > > I don't _object_ to introducing XA_ERROR() / xa_err() into the VFS,
> > > but so far we haven't, and I'd like to make that decision intentionally.
> > 
> > So what would be an alternative way to tell the callers why no folio
> > was found instead of trying to reverse engineer that?  Return an errno
> > and the folio by reference?  The would work, but the calling conventions
> > would be awful.
> 
> Agreed.  How about an xa_filemap_get_folio()?
> 
> (there are a number of things to fix here; haven't decided if XA_ERROR
> should return void *, or whether i should use a separate 'entry' and
> 'folio' until I know the entry is actually a folio ...)

That's awful. Exposing internal implementation details in the API
that is supposed to abstract away the internal implementation
details from users doesn't seem like a great idea to me.

Exactly what are we trying to fix here?  Do we really need to punch
a hole through the abstraction layers like this just to remove half
a dozen lines of -slow path- context specific error handling from a
single caller?

If there's half a dozen cases that need this sort of handling, then
maybe it's the right thing to do. But for a single calling context
that only needs to add a null return check in one specific case?
There's absolutely no need to make generic infrastructure violate
layering abstractions to handle that...

-Dave.

-- 
Dave Chinner
david@fromorbit.com