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

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Message-ID: <0d08b4bf-35c5-4c63-964b-ef886b8262d9@arm.com>
Date: Thu, 18 Dec 2025 12:02:14 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Yeoreum Yun <yeoreum.yun@....com>, Michal Hocko <mhocko@...e.com>
Cc: 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 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.

> 
> --
> Sincerely,
> Yeoreum Yun