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

[David Hildenbrand (Red Hat)]

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