[PATCH v4 09/23] mm/gup: introduce pin_user_pages*() and FOLL_PIN

[Ira Weiny]

> > +/**
> > + * pin_user_pages_fast() - pin user pages in memory without taking locks
> > + *
> > + * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
> > + * get_user_pages_fast() for documentation on the function arguments, because
> > + * the arguments here are identical.
> > + *
> > + * FOLL_PIN means that the pages must be released via put_user_page(). Please
> > + * see Documentation/vm/pin_user_pages.rst for further details.
> > + *
> > + * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
> > + * is NOT intended for Case 2 (RDMA: long-term pins).
> > + */
> > +int pin_user_pages_fast(unsigned long start, int nr_pages,
> > +			unsigned int gup_flags, struct page **pages)
> > +{
> > +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> > +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> > +		return -EINVAL;
> > +
> > +	gup_flags |= FOLL_PIN;
> > +	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
> > +}
> > +EXPORT_SYMBOL_GPL(pin_user_pages_fast);
> 
> I was somewhat wondering about the number of functions you add here. So we
> have:
> 
> pin_user_pages()
> pin_user_pages_fast()
> pin_user_pages_remote()
> 
> and then longterm variants:
> 
> pin_longterm_pages()
> pin_longterm_pages_fast()
> pin_longterm_pages_remote()
> 
> and obviously we have gup like:
> get_user_pages()
> get_user_pages_fast()
> get_user_pages_remote()
> ... and some other gup variants ...
> 
> I think we really should have pin_* vs get_* variants as they are very
> different in terms of guarantees and after conversion, any use of get_*
> variant in non-mm code should be closely scrutinized. OTOH pin_longterm_*
> don't look *that* useful to me and just using pin_* instead with
> FOLL_LONGTERM flag would look OK to me and somewhat reduce the number of
> functions which is already large enough? What do people think? I don't feel
> too strongly about this but wanted to bring this up.

I'm a bit concerned with the function explosion myself.  I think what you
suggest is a happy medium.  So I'd be ok with that.

Ira