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

David Hildenbrand (Red Hat)
2025-12-18T09:41:10Z

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