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

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Message-ID: <201d0fa3-2ebe-4df2-926c-9e11607a6f07@kernel.org>
Date: Thu, 18 Dec 2025 09:46:10 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Yeoreum Yun <yeoreum.yun@....com>, Ryan Roberts <ryan.roberts@....com>
Cc: 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

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?

-- 
Cheers

David