Re: [PATCH] dax: Fix missed PMD wakeups

[Dan Williams]

On Thu, Jul 4, 2019 at 12:14 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jul 04, 2019 at 06:54:50PM +0200, Jan Kara wrote:
> > On Wed 03-07-19 20:27:28, Matthew Wilcox wrote:
> > > So I think we're good for all current users.
> >
> > Agreed but it is an ugly trap. As I already said, I'd rather pay the
> > unnecessary cost of waiting for pte entry and have an easy to understand
> > interface. If we ever have a real world use case that would care for this
> > optimization, we will need to refactor functions to make this possible and
> > still keep the interfaces sane. For example get_unlocked_entry() could
> > return special "error code" indicating that there's no entry with matching
> > order in xarray but there's a conflict with it. That would be much less
> > error-prone interface.
>
> This is an internal interface.  I think it's already a pretty gnarly
> interface to use by definition -- it's going to sleep and might return
> almost anything.  There's not much scope for returning an error indicator
> either; value entries occupy half of the range (all odd numbers between 1
> and ULONG_MAX inclusive), plus NULL.  We could use an internal entry, but
> I don't think that makes the interface any easier to use than returning
> a locked entry.
>
> I think this iteration of the patch makes it a little clearer.  What do you
> think?
>

Not much clearer to me. get_unlocked_entry() is now misnamed and this
arrangement allows for mismatches of @order argument vs @xas
configuration. Can you describe, or even better demonstrate with
numbers, why it's better to carry this complication than just
converging the waitqueues between the types?