Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

[David Hildenbrand <david@redhat.com>]

On 26.05.23 23:41, 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.

2 millions pins (FOLL_PIN, which increments the refcount by 1024) or 2 
million references ?

> 
> Add a pair of functions to test whether a page or a folio is a ZERO_PAGE.
> 

[...]

> diff --git a/mm/gup.c b/mm/gup.c
> index bbe416236593..ad28261dcafd 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 (is_zero_page(page) ||
> +		    !folio_test_anon(folio))

Nit: Fits into a single line (without harming readability IMHO).

>   			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 (is_zero_page(page))
> +			return page_folio(page);
> +
>   		/*
>   		 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
>   		 * right zone, so fail and let the caller fall back to the slow
> @@ -180,6 +188,8 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>   static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
>   {
>   	if (flags & FOLL_PIN) {
> +		if (is_zero_folio(folio))
> +			return;
>   		node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
>   		if (folio_test_large(folio))
>   			atomic_sub(refs, &folio->_pincount);
> @@ -224,6 +234,13 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
>   	if (flags & FOLL_GET)
>   		folio_ref_inc(folio);
>   	else if (flags & FOLL_PIN) {
> +		/*
> +		 * Don't take a pin on the zero page - it's not going anywhere
> +		 * and it is used in a *lot* of places.
> +		 */
> +		if (is_zero_page(page))
> +			return 0;
> +
>   		/*
>   		 * Similar to try_grab_folio(): be sure to *also*
>   		 * increment the normal page refcount field at least once,
> @@ -3079,6 +3096,9 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
>    *
>    * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
>    * see Documentation/core-api/pin_user_pages.rst for further details.
> + *
> + * Note that if a zero_page is amongst the returned pages, it will not have
> + * pins in it and unpin_user_page() will not remove pins from it.
>    */

"it will not have pins in it" sounds fairly weird to a non-native speaker.

"Note that the refcount of any zero_pages returned among the pinned 
pages will not be incremented, and unpin_user_page() will similarly not 
decrement it."


Acked-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb