[PATCH v12 11/22] mm/gup: introduce pin_user_pages*() and FOLL_PIN

[Christoph Hellwig]

On Tue, Jan 07, 2020 at 02:45:47PM -0800, John Hubbard wrote:
> Introduce pin_user_pages*() variations of get_user_pages*() calls,
> and also pin_longterm_pages*() variations.
> 
> For now, these are placeholder calls, until the various call sites
> are converted to use the correct get_user_pages*() or
> pin_user_pages*() API.

What do the pure placeholders buy us?  The API itself looks ok,
but until it actually is properly implemented it doesn't help at
all, and we've had all kinds of bad experiences with these sorts
of stub APIs.