linux-kernel - Re: [RFC PATCH 1/3] mm: Don't pin ZERO_PAGE in pin_user

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Date:   Thu, 25 May 2023 18:47:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

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@...hat.com>
> cc: Christoph Hellwig <hch@...radead.org>
> cc: David Hildenbrand <david@...hat.com>
> cc: Andrew Morton <akpm@...ux-foundation.org>
> cc: Jens Axboe <axboe@...nel.dk>
> cc: Al Viro <viro@...iv.linux.org.uk>
> cc: Matthew Wilcox <willy@...radead.org>
> cc: Jan Kara <jack@...e.cz>
> cc: Jeff Layton <jlayton@...nel.org>
> cc: Jason Gunthorpe <jgg@...dia.com>
> cc: Logan Gunthorpe <logang@...tatee.com>
> cc: Hillf Danton <hdanton@...a.com>
> cc: Christian Brauner <brauner@...nel.org>
> cc: Linus Torvalds <torvalds@...ux-foundation.org>
> cc: linux-fsdevel@...r.kernel.org
> cc: linux-block@...r.kernel.org
> cc: linux-kernel@...r.kernel.org
> cc: linux-mm@...ck.org
> ---
>   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