Re: [PATCH v4] arm64: Enable permission change on arm64 kernel block mappings

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

On 24/07/25 1:49 pm, Catalin Marinas wrote:
> On Thu, Jul 03, 2025 at 08:44:41PM +0530, Dev Jain wrote:
>> This patch paves the path to enable huge mappings in vmalloc space and
>> linear map space by default on arm64. For this we must ensure that we can
>> handle any permission games on the kernel (init_mm) pagetable. Currently,
>> __change_memory_common() uses apply_to_page_range() which does not support
>> changing permissions for block mappings. We attempt to move away from this
>> by using the pagewalk API, similar to what riscv does right now;
> RISC-V seems to do the splitting as well and then use
> walk_page_range_novma().
>
>> however,
>> it is the responsibility of the caller to ensure that we do not pass a
>> range overlapping a partial block mapping or cont mapping; in such a case,
>> the system must be able to support range splitting.
> How does the caller know what the underlying mapping is? It can't really
> be its responsibility, so we must support splitting at least at the
> range boundaries. If you meant the caller of the internal/static
> update_range_prot(), that's an implementation detail where a code

Yes, I have added the comment in update_range_prot().

> comment would suffice. But you can't require such awareness from the
> callers of the public set_memory_*() API.

Yes, it converges to the architecture being aware that, since it supports
block mappings, a block mapping may be partially changed, so the internal
API must split at the start and end.

>
>> This patch 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
>> patch; 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)
>> split_range(end);
>> __change_memory_common(start, end);
> This makes sense if that's the end goal but it's not part of this patch.
>
>> However, this patch 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 patch provides the
>> mechanism for enabling huge mappings for various kernel mappings
>> like linear map and vmalloc.
> Does this patch actually have any user without Yang's series?
> can_set_direct_map() returns true only if the linear map uses page
> granularity, so I doubt it can even be tested on its own. I'd rather see
> this patch included with the actual user or maybe add it later as an
> optimisation to avoid splitting the full range.

Hmmm. Makes sense. Although I don't think that this patch is an optimization;
post Yang's series (without this patch), if we change permissions on a partial
linear map block mapping, set_memory_* will return -EINVAL, causing problems.
  

>
>> ---------------------
>> Implementation
>> ---------------------
>>
>> arm64 currently changes permissions on vmalloc objects locklessly, via
>> apply_to_page_range, whose limitation is to deny changing permissions for
>> block mappings. Therefore, we move away to use the generic pagewalk API,
>> thus paving the path for enabling huge mappings by default on kernel space
>> mappings, thus leading to more efficient TLB usage. However, the API
>> currently enforces the init_mm.mmap_lock to be held. To avoid the
>> unnecessary bottleneck of the mmap_lock for our usecase, this patch
>> extends this generic API to be used locklessly, so as to retain the
>> existing behaviour for changing permissions.
> Is it really a significant bottleneck if we take the lock? I suspect if
> we want to make this generic and allow splitting, we'll need a lock
> anyway (though maybe for shorter intervals if we only split the edges).

The splitting primitive may or may not require a lock, Ryan and Yang had
some discussion on the linear map block mapping thread. I am assuming
that since we didn't need a lock in the past, there is no need to take it now,
since we are only changing the pagetable walk API being used.

>