Re: [RFC PATCH v2 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

David Howells
2023-05-26T09:16:24Z

Lorenzo Stoakes <lstoakes@gmail.com> wrote:

> > iov_iter_extract_pages(), on the other hand, is only used in two places
> > with these patches and the pins are always released with
> > unpin_user_page*() so it's a lot easier to audit.
>
> Thanks for the clarification. I guess these are the cases where you're
> likely to see zero page usage, but since this is changing all PUP*() callers
> don't you need to audit all of those too?

I don't think it should be necessary. This only affects pages obtained from
gup with FOLL_PIN - and, so far as I know, those always have to be released
with unpin_user_page*() which is part of the gup API and thus it should be
transparent to the users.

Pages obtained FOLL_GET, on the other hand, aren't freed through the gup API -
and there are a bunch of ways of releasing them - and getting additional refs
too.

David