linux-kernel - Re: [PATCH v2] mm: re-allow pinning of zero pfns (again)

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Message-ID: <033c2e8d-8ba9-3e0b-9dee-7d0973c708fa@nvidia.com>
Date:   Sat, 27 Aug 2022 17:59:32 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        akpm@...ux-foundation.org
Cc:     alex.sierra@....com, willy@...radead.org, hch@....de,
        Felix.Kuehling@....com, apopple@...dia.com, david@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: re-allow pinning of zero pfns (again)

On 8/10/22 09:53, 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.
> 
> To avoid further subtle mistakes, split the logic into discrete tests.
> 
> Suggested-by: Matthew Wilcox <willy@...radead.org>
> Suggested-by: Felix Kuehling <felix.kuehling@....com>
> 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 |   17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
Hi Alex,

Looks good. I'm suggesting a simpler comment, below, because
even though the VFIO folks are thinking about VFIO, here we
are deep in the mm layer and there are lots of non-VFIO callers
that may pin the zero page.

> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 18e01474cf6b..835106a9718f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1544,9 +1544,20 @@ 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)));
> +	/*
> +	 * The zero page might reside in a movable zone, however it may not
> +	 * be migrated and can therefore be pinned.  The vfio subsystem pins
> +	 * user mappings including the zero page for IOMMU translation.
> +	 */

Those notes are all about (some of) the callers. But it's a simple
answer, really, so how about just this:

	/* The zero page is always allowed to be pinned. */

?

> +	if (is_zero_pfn(page_to_pfn(page)))
> +		return true;
> +
> +	/* Coherent device memory must always allow eviction. */
> +	if (is_device_coherent_page(page))
> +		return false;
> +
> +	/* Otherwise, non-movable zone pages can be pinned. */
> +	return !is_zone_movable_page(page);
>  }
>  #else
>  static inline bool is_longterm_pinnable_page(struct page *page)
> 
> 
> 

Reviewed-by: John Hubbard <jhubbard@...dia.com>


thanks,

-- 
John Hubbard
NVIDIA