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Date:   Sat, 21 May 2022 05:08:09 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Chih-En Lin <shiyn.lin@gmail.com>
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Subject: Re: [RFC PATCH 5/6] mm, pgtable: Add the reference counter for COW
 PTE
Message-ID: <YohlqalOBlJHAT1u@casper.infradead.org>
References: <20220519183127.3909598-1-shiyn.lin@gmail.com>
 <20220519183127.3909598-6-shiyn.lin@gmail.com>
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On Fri, May 20, 2022 at 02:31:26AM +0800, Chih-En Lin wrote:
> +++ b/include/linux/mm_types.h
> @@ -221,6 +221,7 @@ struct page {
>  #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
>  	int _last_cpupid;
>  #endif
> +	atomic_t cow_pgtable_refcount; /* COW page table */
>  	pmd_t *cow_pte_owner; /* cow pte: pmd */
>  } _struct_page_alignment;

Oh.  You need another 4 bytes.  Hmm.

Can you share _refcount?

Using _pt_pad_2 should be possible, but some care will be needed to make
sure it's (a) in a union with an unsigned long to keep the alignment
as expected, and (b) is definitely zero before the page is freed (or
the page allocator will squawk at you).