Re: [PATCH v7 04/19] iov_iter: Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable

[Al Viro]

On Fri, Aug 27, 2021 at 11:57:19AM -0700, Linus Torvalds wrote:
> On Fri, Aug 27, 2021 at 11:53 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > I really disagree with these calling conventions.  "Number not faulted in"
> > is bloody useless
> 
> It's what we already have for copy_to/from_user(), so it's actually
> consistent with that.

After copy_to/copy_from you've got the data copied and it's not going
anywhere.  After fault-in you still have to copy, and it still can give
you less data than fault-in had succeeded for.  So you must handle short
copies separately, no matter how much you've got from fault-in.

> And it avoids changing all the existing tests where people really
> cared only about the "everything ok" case.

The thing is, the checks tend to be wrong.  We can't rely upon the full
fault-in to expect the full copy-in/copy-out, so the checks downstream
are impossible to avoid anyway.  And fault-in failure is always a slow
path, so we are not saving time here.

And for the memory poisoining we end up aborting a copy potentially
a lot earlier than we should.

> Andreas' first patch did that changed version, and was ugly as hell.
> 
> But if you have a version that avoids the ugliness...

I'll need to dig my notes out...