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

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Message-ID: <bb120b4a-e6f6-de81-35f0-9803acf9b0be@redhat.com>
Date:   Sat, 11 Jun 2022 20:29:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        akpm@...ux-foundation.org, minchan@...nel.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        paulmck@...nel.org, jhubbard@...dia.com, joaodias@...gle.com,
        jgg@...pe.ca
Subject: Re: [PATCH] mm: Re-allow pinning of zero pfns

On 11.06.22 00:35, 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.

I wonder which setups can reliably work with a long-term pin on a shared
zeropage. In a MAP_PRIVATE mapping, any write access via the page tables
will end up replacing the shared zeropage with an anonymous page.
Something similar should apply in MAP_SHARED mappings, when lazily
allocating disk blocks.

In the future, we might trigger unsharing when taking a R/O pin for the
shared zeropage, just like we do as of now upstream for shared anonymous
pages (!PageAnonExclusive). And something similar could then be done
when finding a !anon page in a MAP_SHARED mapping.

> 
> Fixes: 1c563432588d ("mm: fix is_pinnable_page against a cma page")

Having that said, it indeed looks like that was an unintended change.

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

> 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));
>  }
>  #else
>  static inline bool is_pinnable_page(struct page *page)
> 
> 


-- 
Thanks,

David / dhildenb