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

[David Hildenbrand]

On 26.05.23 11:15, 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, and even code like like 873aefb376bb ("vfio/type1: Unpin zero 
pages") would handle it transparently, because they also call 
unpin_user_page().

[we can remove 873aefb376bb even without this change way because it uses 
FOLL_LONGTERM that shouldn't return the shared zeropage anymore ]

--
Thanks,

David / dhildenb