linux-kernel - Re: [PATCH v2 2/2] arm64: mmu: avoid allocating pages while installing ng-mapping for KPTI

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Message-ID: <aUL/zJlJFRz2/Ybt@e129823.arm.com>
Date: Wed, 17 Dec 2025 19:09:00 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: catalin.marinas@....com, will@...nel.org, akpm@...ux-foundation.org,
	david@...nel.org, kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
	dev.jain@....com, yang@...amperecomputing.com,
	chaitanyas.prakash@....com, bigeasy@...utronix.de,
	clrkwllms@...nel.org, rostedt@...dmis.org,
	lorenzo.stoakes@...cle.com, ardb@...nel.org, jackmanb@...gle.com,
	vbabka@...e.cz, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH v2 2/2] arm64: mmu: avoid allocating pages while
 installing ng-mapping for KPTI

Hi Ryanc,

[...]
> > -	if (!cpu) {
> > -		free_pages(alloc, order);
> > +	if (!cpu)
> >  		arm64_use_ng_mappings = true;
> > -	}
> >
> >  	return 0;
> >  }
> >
> >  void __init kpti_install_ng_mappings(void)
> >  {
> > +	int order = order_base_2(CONFIG_PGTABLE_LEVELS);
> > +	u64 alloc;
> > +
> >  	/* Check whether KPTI is going to be used */
> >  	if (!arm64_kernel_unmapped_at_el0())
> >  		return;
> > @@ -1436,8 +1434,14 @@ void __init kpti_install_ng_mappings(void)
> >  	if (arm64_use_ng_mappings)
> >  		return;
> >
> > +	alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
>
> I don't think this requires GFP_ATOMIC now?
>
> With that removed:
>
> Reviewed-by: Ryan Roberts <ryan.roberts@....com>

I think it would be better to use only __GFP_HIGH in here since
when kpti_install_ng_mappings() is called, "kswpd" doesn't created yet.
and to allocate page with assurance, It would be good to use
min_reserved to.

Am I missing something?

Thanks.

[...]

--
Sincerely,
Yeoreum Yun