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

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Message-ID: <Zb_dT43-oPsRplhi@casper.infradead.org>
Date: Sun, 4 Feb 2024 18:54:07 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Qi Zheng <zhengqi.arch@...edance.com>
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

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.