linux-kernel - Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking

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Message-ID: <c99e60d8-57c3-4607-a0ea-b12ed6765b73@arm.com>
Date: Wed, 11 Jun 2025 09:15:28 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, catalin.marinas@....com, will@...nel.org,
 Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com,
 mhocko@...e.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 suzuki.poulose@....com, steven.price@....com, gshan@...hat.com,
 linux-arm-kernel@...ts.infradead.org, yang@...amperecomputing.com,
 ryan.roberts@....com, anshuman.khandual@....com
Subject: Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking


On 10/06/25 6:57 pm, Lorenzo Stoakes wrote:
> On Tue, Jun 10, 2025 at 03:24:16PM +0200, David Hildenbrand wrote:
>> On 10.06.25 14:07, Lorenzo Stoakes wrote:
>>> OK so I think the best solution here is to just update check_ops_valid(), which
>>> was kind of sucky anyway (we check everywhere but walk_page_range_mm() to
>>> enforce the install pte thing).
>>>
>>> Let's do something like:
>>>
>>> #define OPS_MAY_INSTALL_PTE	(1<<0)
>>> #define OPS_MAY_AVOID_LOCK	(1<<1)
>>>
>>> and update check_ops_valid() to take a flags or maybe 'capabilities' field.
>>>
>>> Then check based on this e.g.:
>>>
>>> if (ops->install_pte && !(capabilities & OPS_MAY_INSTALL_PTE))
>>> 	return false;
>>>
>>> if (ops->walk_lock == PGWALK_NOLOCK && !(capabilities & OPS_MAY_AVOID_LOCK))
>>> 	return false;
>>>
>> Hm. I mean, we really only want to allow this lockless check for
>> walk_kernel_page_table_range(), right?
>>
>> Having a walk_kernel_page_table_range_lockeless() might (or might not) be
>> better, to really only special-case this specific path.
> Agree completely, Dev - let's definitely do this.

Makes sense.

>
>> So, I am wondering if we should further start splitting the
>> kernel-page-table walker up from the mm walker, at least on the "entry"
>> function for now.
> How do you mean?
>
>> --
>> Cheers,
>>
>> David / dhildenb
>>