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

From: Christoph Hellwig
Date: Fri May 26 2023 - 04:18:19 EST

On Fri, May 26, 2023 at 09:10:33AM +0100, Lorenzo Stoakes wrote:
> On Thu, May 25, 2023 at 11:39:51PM +0100, David Howells wrote:
> > Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a pointer
> > to it from the page tables and make unpin_user_page*() correspondingly
> > ignore a ZERO_PAGE when unpinning. We don't want to risk overrunning a
> > zero page's refcount as we're only allowed ~2 million pins on it -
> > something that userspace can conceivably trigger.
> I guess we're not quite as concerned about FOLL_GET because FOLL_GET should
> be ephemeral and FOLL_PIN (horrifically) adds GUP_PIN_COUNTING_BIAS each
> time?

I think FOLL_GET would be just as useful. But given that we have
a few places that release pins while gets just do a put_page it would
be a lot more effort to audit all of them. Maybe it's better do only
do this once we've converted all the places that should do pin and
have very few FOLL_GET users left.