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

[Qi Zheng]

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.