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

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Message-ID: <970e1b84-7f47-4a51-ad99-4df9026b8f7d@redhat.com>
Date: Tue, 10 Jun 2025 15:24:16 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Dev Jain <dev.jain@....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 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.

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.

-- 
Cheers,

David / dhildenb