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

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Message-ID: <fb5086b6-8041-402a-b1c7-7197ca188147@arm.com>
Date: Thu, 19 Jun 2025 09:40:27 +0530
From: Dev Jain <dev.jain@....com>
To: Mike Rapoport <rppt@...nel.org>
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, ryan.roberts@....com, anshuman.khandual@....com
Subject: Re: [PATCH v3 1/2] arm64: pageattr: Use pagewalk API to change memory
 permissions


On 15/06/25 1:02 pm, 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.

My understanding is that it is guaranteed that the set_memory_* caller has
exclusive access to the range it is changing permissions for.
The x86 comment is

Serialize cpa() (for !DEBUG_PAGEALLOC which uses large identity mappings) using cpa_lock.
So that we don't allow any other cpu, with stale large tlb entries change the page attribute in
parallel to some other cpu splitting a large page entry along with changing the attribute.

On arm64 we are doing flush_tlb_kernel_range in __change_memory_common; and also, the caller
of __change_memory_common is required to first split the start and end before changing permissions,
so the splitting and permission change won't happen in parallel as described by the comment.

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