linux-kernel - Re: [PATCH 2/2] mm: pgtable: remove unnecessary split ptlock for kernel PMD page

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Message-ID: <d464ef84-fe06-4279-a7fb-5a0cadef4064@bytedance.com>
Date: Mon, 5 Feb 2024 10:14:28 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, arnd@...db.de, muchun.song@...ux.dev,
 david@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 linux-arch@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: pgtable: remove unnecessary split ptlock for
 kernel PMD page

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.