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

David Hildenbrand
2023-05-26T09:25:27Z

On 26.05.23 11:15, David Howells wrote:
> 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.

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