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

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Message-ID: <10ad7480-4e8a-43c1-9e18-db72e63dbe98@kernel.org>
Date: Thu, 18 Dec 2025 10:41:03 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Yeoreum Yun <yeoreum.yun@....com>
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

On 12/18/25 10:31, Yeoreum Yun wrote:
> 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.

The buddy should really be able to deal with that, no?

> 
>      - Similar reason kcompactd too.

Same as well.

We cannot expect alloc API users to know about these hidden details to 
work around them :)

> 
>      - 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.

Right, but do you really think we would ever trigger that path?

The default should always be GFP_KERNEL unless we have for very good 
reason special demands.

So, do you think in practice there is real value in NOT using GFP_KERNEL? :)

-- 
Cheers

David