Re: [PATCH 0/3] Enable huge-vmalloc permission change

[Dev Jain <dev.jain@arm.com>]

On 30/05/25 3:33 pm, Ryan Roberts wrote:
> On 30/05/2025 10:04, Dev Jain wrote:
>> This series paves the path to enable huge mappings in vmalloc space by
>> default on arm64. > For this we must ensure that we can handle any permission
>> games on vmalloc space.
> And the linear map :)
>
>> Currently, __change_memory_common() uses
>> apply_to_page_range() which does not support changing permissions for
>> leaf mappings.
> nit: A "leaf mapping" is the lowest level entry in the page tables for a given
> address - i.e. it maps an address to some actual memory rather than to another
> pgtable. It includes what the Arm ARM calls "page mappings" (PTE level) and
> "block mappings" (PMD/PUD/.. level). apply_to_page_range() does support page
> mappings, so saying it doesn't support leaf mappings is incorrect. It doesn't
> support block mappings.

Sorry, again got confused by nomenclature : )

>
>> We attempt to move away from this by using walk_page_range_novma(),
>> similar to what riscv does right now; however, it is the responsibility
>> of the caller to ensure that we do not pass a range, or split the range
>> covering a partial leaf mapping.
>>
>> This series is tied with Yang Shi's attempt [1] at using huge mappings
>> in the linear mapping in case the system supports BBML2, in which case
>> we will be able to split the linear mapping if needed without break-before-make.
>> Thus, Yang's series, IIUC, will be one such user of my series; suppose we
>> are changing permissions on a range of the linear map backed by PMD-hugepages,
>> then the sequence of operations should look like the following:
>>
>> split_range(start, (start + HPAGE_PMD_SIZE) & ~HPAGE_PMD_MASK);
>> split_range(end & ~HPAGE_PMD_MASK, end);
> I don't understand what the HPAGE_PMD_MASK twiddling is doing? That's not right.
> It's going to give you the offset within the 2M region. You just want:
>
> split_range(start)
> split_range(end)
>
> right?

Suppose start = 2M + 4K, end = 8M + 5K. Then my sequence will compute to
split_range(2M + 4K, 3M)
split_range(8M, 8M + 5K)
__change_memory_common(2M + 4K, 8M + 5K)

So now __change_memory_common() wouldn't have to deal with splitting the
starts and ends. Please correct me if I am wrong.

>
>> __change_memory_common(start, end);
>>
>> However, this series can be used independently of Yang's; since currently
>> permission games are being played only on pte mappings (due to apply_to_page_range
>> not supporting otherwise), this series provides the mechanism for enabling
>> huge mappings for various kernel mappings like linear map and vmalloc.
> In other words, you are saying that this series is a prerequisite for Yang's
> series (and both are prerequisites for huge vmalloc by default). Your series
> adds a new capability that Yang's series will rely on (the ability to change
> permissions on block mappings).

That's right.

>
> Thanks,
> Ryan
>
>> [1] https://lore.kernel.org/all/20250304222018.615808-1-yang@os.amperecomputing.com/
>>
>> Dev Jain (3):
>>    mm: Allow pagewalk without locks
>>    arm64: pageattr: Use walk_page_range_novma() to change memory
>>      permissions
>>    mm/pagewalk: Add pre/post_pte_table callback for lazy MMU on arm64
>>
>>   arch/arm64/mm/pageattr.c | 81 +++++++++++++++++++++++++++++++++++++---
>>   include/linux/pagewalk.h |  4 ++
>>   mm/pagewalk.c            | 18 +++++++--
>>   3 files changed, 94 insertions(+), 9 deletions(-)
>>