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

[Ryan Roberts]

On 30/05/2025 11:10, Dev Jain wrote:
> 
> 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

8M + 5K is not a valid split point. It has to be at least page aligned.

> split_range(2M + 4K, 3M)
> split_range(8M, 8M + 5K)

We just want to split at start and end. What are the 3M and 8M params supposed
to be? Anyway, this is off-topic for this series.

> __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 at 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(-)
>>>