linux-kernel - Re: [PATCH 2/2] arm64: mmu: use pagetable_alloc_nolock() while stop

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Message-ID: <aUPw1YFNLf7ONqe9@tiehlicka>
Date: Thu, 18 Dec 2025 13:17:25 +0100
From: Michal Hocko <mhocko@...e.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Yeoreum Yun <yeoreum.yun@....com>, akpm@...ux-foundation.org,
	david@...nel.org, lorenzo.stoakes@...cle.com,
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Subject: Re: [PATCH 2/2] arm64: mmu: use pagetable_alloc_nolock() while
 stop_machine()

On Thu 18-12-25 12:02:14, Ryan Roberts wrote:
> On 18/12/2025 09:36, Yeoreum Yun wrote:
> > Hi,
> >> On Fri 12-12-25 16:18:32, Yeoreum Yun wrote:
> >>> linear_map_split_to_ptes() and __kpti_install_ng_mappings()
> >>> are called as callback of stop_machine().
> >>> That means these functions context are preemption disabled.
> >>>
> >>> Unfortunately, under PREEMPT_RT, the pagetable_alloc() or
> >>> __get_free_pages() couldn't be called in this context
> >>> since spin lock that becomes sleepable on RT,
> >>> potentially causing a sleep during page allocation.
> >>>
> >>> To address this, pagetable_alloc_nolock().
> >>
> >> As you cannot tolerate allocation failure and this is pretty much
> >> permanent allocation (AFAIU) why don't you use a static allocation?
> > 
> > Because of when bbl2_noabort is supported, that pages doesn't need to.
> > If static alloc, that would be a waste in the system where bbl2_noabort
> > is supported.
> > 
> > When I tested, these extra pages are more than 40 in my FVP.
> > So, it would be better dynamic allocation and I think since it's quite a
> > early time, it's probably not failed that's why former code runs as it
> > is.
> 
> The required allocation size is also a function of the size of the installed RAM
> so a static worst case allocation would consume all the RAM on small systems.

Understood. But is it possible to pre-allocate early on so that the
allocation itself doesn't have to happen from a constrained context.
-- 
Michal Hocko
SUSE Labs