linux-kernel - Re: [PATCH] mm: Re-allow pinning of zero pfns

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Message-ID: <YqPgIJ2+OaltxRmV@google.com>
Date:   Fri, 10 Jun 2022 17:21:52 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, paulmck@...nel.org,
        jhubbard@...dia.com, joaodias@...gle.com, jgg@...pe.ca,
        david@...hat.com
Subject: Re: [PATCH] mm: Re-allow pinning of zero pfns

On Fri, Jun 10, 2022 at 04:35:13PM -0600, Alex Williamson wrote:
> The commit referenced below subtly and inadvertently changed the logic
> to disallow pinning of zero pfns.  This breaks device assignment with
> vfio and potentially various other users of gup.  Exclude the zero page
> test from the negation.
> 
> Fixes: 1c563432588d ("mm: fix is_pinnable_page against a cma page")
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
> 
> At least I assume this was inadvertent...  If there's a better fix,
> please run with it as I'm out of the office the 1st half of next
> week and would like to see this fixed ASAP.  Thanks!
> 
>  include/linux/mm.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index bc8f326be0ce..781fae17177d 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1600,7 +1600,7 @@ static inline bool is_pinnable_page(struct page *page)
>  	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
>  		return false;
>  #endif
> -	return !(is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page)));
> +	return !is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page));

Thanks for catching!

I don't think zero pfn could stay in the movable zone or CMA area.

Acked-by: Minchan Kim <minchan@...nel.org>