Re: [RFC PATCH 5/6] mm, pgtable: Add the reference counter for COW PTE

From: Matthew Wilcox <willy@infradead.org>
To: Chih-En Lin <shiyn.lin@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Christian Brauner <brauner@kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	William Kucharski <william.kucharski@oracle.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Yunsheng Lin <linyunsheng@huawei.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Suren Baghdasaryan <surenb@google.com>,
	Colin Cross <ccross@google.com>, Feng Tang <feng.tang@intel.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Mike Rapoport <rppt@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Daniel Axtens <dja@axtens.net>,
	Jonathan Marek <jonathan@marek.ca>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Pasha Tatashin <pasha.tatashin@soleen.com>,
	Peter Xu <peterx@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Fenghua Yu <fenghua.yu@intel.com>,
	David Hildenbrand <david@redhat.com>,
	linux-kernel@vger.kernel.org, Kaiyang Zhao <zhao776@purdue.edu>,
	Huichun Feng <foxhoundsk.tw@gmail.com>,
	Jim Huang <jserv.tw@gmail.com>
Subject: Re: [RFC PATCH 5/6] mm, pgtable: Add the reference counter for COW PTE
Date: Sat, 21 May 2022 05:08:09 +0100	[thread overview]
Message-ID: <YohlqalOBlJHAT1u@casper.infradead.org> (raw)
In-Reply-To: <20220519183127.3909598-6-shiyn.lin@gmail.com>

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).