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

Yeoreum Yun
2025-12-18T09:32:13Z

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