linux-kernel - Re: [PATCH] mm: re-allow pinning of zero pfns (again)

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Message-ID: <YvJTi0fAgl/T4WA4@casper.infradead.org>
Date:   Tue, 9 Aug 2022 13:31:07 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     alex.sierra@....com, akpm@...ux-foundation.org, hch@....de,
        Felix.Kuehling@....com, apopple@...dia.com, david@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: re-allow pinning of zero pfns (again)

On Mon, Aug 08, 2022 at 10:42:24PM -0600, Alex Williamson wrote:
> The below referenced commit makes the same error as 1c563432588d ("mm: fix
> is_pinnable_page against a cma page"), re-interpreting the logic to exclude
> pinning of the zero page, which breaks device assignment with vfio.

Perhaps we need to admit we're not as good at boolean logic as we think
we are.

	if (is_device_coherent_page(page))
		return false;
	if (is_zone_movable_page(page))
		return false;
	return is_zero_pfn(page_to_pfn(page));

(or whatever the right logic is ... I just woke up and I'm having
trouble parsing it).

> Link: https://lore.kernel.org/all/165490039431.944052.12458624139225785964.stgit@omen
> Fixes: f25cbb7a95a2 ("mm: add zone device coherent type memory support")
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>  include/linux/mm.h |    5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 18e01474cf6b..772279ed7010 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1544,9 +1544,8 @@ static inline bool is_longterm_pinnable_page(struct page *page)
>  	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
>  		return false;
>  #endif
> -	return !(is_device_coherent_page(page) ||
> -		 is_zone_movable_page(page) ||
> -		 is_zero_pfn(page_to_pfn(page)));
> +	return !(is_device_coherent_page(page) || is_zone_movable_page(page)) ||
> +	       is_zero_pfn(page_to_pfn(page));
>  }
>  #else
>  static inline bool is_longterm_pinnable_page(struct page *page)
> 
> 
>