Received-SPF: pass (google.com: domain of linux-kernel+bounces-51985-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Message-ID: <d464ef84-fe06-4279-a7fb-5a0cadef4064@bytedance.com>
Date: Mon, 5 Feb 2024 10:14:28 +0800
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH 2/2] mm: pgtable: remove unnecessary split ptlock for
 kernel PMD page
Content-Language: en-US
To: Matthew Wilcox <willy@infradead.org>
Cc: akpm@linux-foundation.org, arnd@arndb.de, muchun.song@linux.dev,
 david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
 linux-arch@vger.kernel.org
References: <f023a6687b9f2109401e7522b727aa4708dc05f1.1706774109.git.zhengqi.arch@bytedance.com>
 <63f0b3d2f9124ae5076963fb5505bd36daba0393.1706774109.git.zhengqi.arch@bytedance.com>
 <Zb_dT43-oPsRplhi@casper.infradead.org>
From: Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <Zb_dT43-oPsRplhi@casper.infradead.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

Hi Matthew,

On 2024/2/5 02:54, Matthew Wilcox wrote:
> On Thu, Feb 01, 2024 at 04:05:41PM +0800, Qi Zheng wrote:
>> For kernel PMD entry, we use init_mm.page_table_lock to protect it, so
>> there is no need to allocate and initialize the split ptlock for kernel
>> PMD page.
> 
> I don't think this is a great idea.  Maybe there's no need to initialise
> it, but keeping things the same between kernel & user page tables is a
> usually better.  We don't normally allocate memory for the spinlock,
> it's only in debugging scenarios like LOCKDEP.  I would drop this unless
> you have a really compelling argument to make.

The reason I first noticed this is that we didn't allocate and
initialize the ptlock in __pte_alloc_one_kernel(). So in at the PTE
level, the implementation of kernel & user page tables is already
different.

Thanks.