Re: [PATCH v1 2/5] s390/uv: convert gmap_make_secure() to work on folios

[David Hildenbrand]

[...]

> > +	if (rc == -E2BIG) {
> > +		/*
> > +		 * Splitting might fail with -EBUSY due to unexpected folio
> > +		 * references, just like make_folio_secure(). So handle it
> > +		 * ahead of time without the PTL being held.
> > +		 */
> > +		folio_lock(folio);
> > +		rc = split_folio(folio);
>
> if split_folio returns -EAGAIN...
>
> > +		folio_unlock(folio);
> > +		folio_put(folio);
> > +	}
> > +
> >   	if (rc == -EAGAIN) {
>
> ... we will not skip this ...
>
> >   		/*
> >   		 * If we are here because the UVC returned busy or partial
> >   		 * completion, this is just a useless check, but it is safe.
> >   		 */
> > -		wait_on_page_writeback(page);
> > -		put_page(page);
> > +		folio_wait_writeback(folio);
> > +		folio_put(folio);
>
> ... and we will do one folio_put() too many
>
> >   	} else if (rc == -EBUSY) {
> >   		/*
> >   		 * If we have tried a local drain and the page refcount
>
> are we sure that split_folio() can never return -EAGAIN now and in the
> future too?

Yes, and in contrast to documentation, that can actually happen! The 
documentation is even wrong: we return -EAGAIN if there are unexpected 
folio references (e.g., pinned), thanks for raising that.

> maybe just change it to  } else if (...   ?


I think I'll make it all clearer by handling split_folio() return values 
separately.

--
Cheers,

David / dhildenb