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

[David Hildenbrand]

On 25.05.23 17:51, David Howells wrote:
> Make pin_user_pages*() leave the ZERO_PAGE unpinned if it extracts a
> pointer to it from the page tables and make unpin_user_page*()
> correspondingly ignore the ZERO_PAGE when unpinning.  We don't want to risk
> overrunning the zero page's refcount as we're only allowed ~2 million pins
> on it - something that userspace can conceivably trigger.

As Linus raised, the ZERO_PAGE(0) checks should probably be 
is_zero_pfn(page_to_pfn(page)).

> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> cc: David Hildenbrand <david@xxxxxxxxxx>
> cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> cc: Jens Axboe <axboe@xxxxxxxxx>
> cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> cc: Jan Kara <jack@xxxxxxx>
> cc: Jeff Layton <jlayton@xxxxxxxxxx>
> cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
> cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> cc: Hillf Danton <hdanton@xxxxxxxx>
> cc: Christian Brauner <brauner@xxxxxxxxxx>
> cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> cc: linux-fsdevel@xxxxxxxxxxxxxxx
> cc: linux-block@xxxxxxxxxxxxxxx
> cc: linux-kernel@xxxxxxxxxxxxxxx
> cc: linux-mm@xxxxxxxxx
> ---
>   mm/gup.c | 25 ++++++++++++++++++++++++-
>   1 file changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index bbe416236593..d2662aa8cf01 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -51,7 +51,8 @@ static inline void sanity_check_pinned_pages(struct page **pages,
>   		struct page *page = *pages;
>   		struct folio *folio = page_folio(page);
>   
> -		if (!folio_test_anon(folio))
> +		if (page == ZERO_PAGE(0) ||
> +		    !folio_test_anon(folio))
>   			continue;
>   		if (!folio_test_large(folio) || folio_test_hugetlb(folio))
>   			VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page);
> @@ -131,6 +132,13 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>   	else if (flags & FOLL_PIN) {
>   		struct folio *folio;
>   
> +		/*
> +		 * Don't take a pin on the zero page - it's not going anywhere
> +		 * and it is used in a *lot* of places.
> +		 */
> +		if (page == ZERO_PAGE(0))
> +			return page_folio(ZERO_PAGE(0));

With the fixed check, this should be
	return page_folio(page);

I guess.

--
Thanks,

David / dhildenb