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

[Ryan Roberts <ryan.roberts@arm.com>]

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.

> 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?

> __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).

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(-)
>