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

From: Lorenzo Stoakes
Date: Fri May 26 2023 - 05:26:13 EST

On Fri, May 26, 2023 at 10:15:26AM +0100, David Howells wrote:
> Lorenzo Stoakes <lstoakes@xxxxxxxxx> 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.

Right, I was only saying so in relation to you stating the need to audit,
for precisely this reason I wondered why you felt the need to :)

> 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.

Yes that's a very good point! Sorry, in my enthusiasm for GUP reform this
thorny aspect slipped my mind...

As Christoph said though hopefully over time we can limit the use of FOLL_GET so
this becomes easier perhaps. Larger discussion on this area in [0] :)


> David