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Date:   Sat, 21 May 2022 05:02:43 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Chih-En Lin <shiyn.lin@gmail.com>
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Subject: Re: [RFC PATCH 3/6] mm, pgtable: Add ownership for the PTE table
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 <20220519183127.3909598-4-shiyn.lin@gmail.com>
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On Fri, May 20, 2022 at 02:31:24AM +0800, Chih-En Lin wrote:
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 8834e38c06a4..5dcbd7f6c361 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -221,6 +221,7 @@ struct page {
>  #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
>  	int _last_cpupid;
>  #endif
> +	pmd_t *cow_pte_owner; /* cow pte: pmd */

This is definitely the wrong place.  I think it could replace _pt_pad_1,
since it's a pointer to a PMD and so the bottom bit will definitely
be clear.