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

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Message-ID: <aUPJ1N99LEVZ77co@e129823.arm.com>
Date: Thu, 18 Dec 2025 09:31:00 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Ryan Roberts <ryan.roberts@....com>, catalin.marinas@....com,
	will@...nel.org, akpm@...ux-foundation.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 David,

> On 12/18/25 09:37, Yeoreum Yun wrote:
> > [...]
> > > > 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?
> > >
> > > Personally I think we should just use "GFP_KERNEL | __GFP_ZERO". Anything else
> > > would make this allocation look special, which it is not. If we fail to allocate
> > > at this point in boot, we have bigger problems.
> >
> > But I'm not sure *HOW effective* to use GFP_KERNEL in here.
> > Since it's before the any filesystem inited.
> > IOW, in this context, almost there would be no *page cache*
> > and I think it seems meaningless to use "GFP_KERNEL" and "direct
> > reclaim"
> >
> > So to get success for allocation, __GFP_HIGH | _GFP_ZERO seems much
> > better.
>
> Unless there is a real reason to confuse readers why this is very special,
> just go with "GFP_KERNEL | __GFP_ZERO", really.
>
> In particular if it doesn't matter in practice? Or does it and we are not
> getting your point?

My worries was
    - kpti_install_ng_mappings() is called while in "smp_init()" which is
      before creating the kswapd thread via module_init().
      Just wondered whether it allows to call wakeup_kswapd() before
      kswapd is created.

    - Similar reason kcompactd too.

    - Just wonder how much direct reclaim is effecitve since
      when kpti_install_ng_mappings() called before each
      filesystem initialised where not much of page cache in usage.

TBH (1) and (2) seems fine since each wakeup function checks
the waitqueue. but because of (3),
I think not GFP_KERNEL but __GFP_HIGH | __GFP_DIRECT_RECLAIM | __GFP_ZERO (?)

Am I missing?

--
Sincerely,
Yeoreum Yun