linux-kernel - Re: [PATCH v3 1/2] arm64: pageattr: Use pagewalk API to change memory permissions

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Message-ID: <956f6ebe-606f-4575-a0a5-7841c95b5371@arm.com>
Date: Wed, 25 Jun 2025 12:04:09 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Mike Rapoport <rppt@...nel.org>, Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, catalin.marinas@....com,
 will@...nel.org, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 vbabka@...e.cz, 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, anshuman.khandual@....com
Subject: Re: [PATCH v3 1/2] arm64: pageattr: Use pagewalk API to change memory
 permissions

On 15/06/2025 08:32, Mike Rapoport wrote:
> On Fri, Jun 13, 2025 at 07:13:51PM +0530, Dev Jain wrote:
>> -/*
>> - * This function assumes that the range is mapped with PAGE_SIZE pages.
>> - */
>> -static int __change_memory_common(unsigned long start, unsigned long size,
>> +static int ___change_memory_common(unsigned long start, unsigned long size,
>>  				pgprot_t set_mask, pgprot_t clear_mask)
>>  {
>>  	struct page_change_data data;
>> @@ -61,9 +140,28 @@ static int __change_memory_common(unsigned long start, unsigned long size,
>>  	data.set_mask = set_mask;
>>  	data.clear_mask = clear_mask;
>>  
>> -	ret = apply_to_page_range(&init_mm, start, size, change_page_range,
>> -					&data);
>> +	arch_enter_lazy_mmu_mode();
>> +
>> +	/*
>> +	 * The caller must ensure that the range we are operating on does not
>> +	 * partially overlap a block mapping. Any such case should either not
>> +	 * exist, or must be eliminated by splitting the mapping - which for
>> +	 * kernel mappings can be done only on BBML2 systems.
>> +	 *
>> +	 */
>> +	ret = walk_kernel_page_table_range_lockless(start, start + size,
>> +						    &pageattr_ops, NULL, &data);
> 
> x86 has a cpa_lock for set_memory/set_direct_map to ensure that there's on
> concurrency in kernel page table updates. I think arm64 has to have such
> lock as well.

We don't have a lock today, using apply_to_page_range(); we are expecting that
the caller has exclusive ownership of the portion of virtual memory - i.e. the
vmalloc region or linear map. So I don't think this patch changes that requirement?

Where it does get a bit more hairy is when we introduce the support for
splitting. In that case, 2 non-overlapping areas of virtual memory may share a
large leaf mapping that needs to be split. But I've been discussing that with
Yang Shi at [1] and I think we can handle that locklessly too.

Perhaps I'm misunderstanding something?

[1] https://lore.kernel.org/all/f036acea-1bd1-48a7-8600-75ddd504b8db@arm.com/

Thanks,
Ryan

> 
>> +	arch_leave_lazy_mmu_mode();
>> +
>> +	return ret;
>> +}
>