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

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

On 30/05/25 4:07 pm, Ryan Roberts wrote:
> 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.

Sorry, so consider 8M + 4K.

>> 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.

I think we are both saying the same thing; yes we will split only the start and the end,
so if the address 2Mb + 4Kb is mapped as a PMD-hugepage, we need to split this PMD into
a PTE table, which will map 2Mb till 4Mb as base pages now.

>
>> __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(-)
>>>>