[PATCH v3 2/2] arm64: pageattr: Enable huge-vmalloc permission change
Ryan Roberts
ryan.roberts at arm.com
Wed Jun 25 04:08:59 PDT 2025
On 13/06/2025 14:43, Dev Jain wrote:
> Commit fcf8dda8cc48 ("arm64: pageattr: Explicitly bail out when changing
> permissions for vmalloc_huge mappings") disallowed changing permissions
> for vmalloc-huge mappings. The motivation of this was to enforce an API
> requirement and explicitly tell the caller that it is unsafe to change
> permissions for block mappings since splitting may be required, which
> cannot be handled safely on an arm64 system in absence of BBML2.
>
> This patch effectively partially reverts this commit, since patch 1
> will now enable permission changes on kernel block mappings, thus,
> through change_memory_common(), enable permission changes for vmalloc-huge
> mappings. Any caller "misusing" the API, in the sense, calling it for
> a partial block mapping, will receive an error code of -EINVAL via
> the pagewalk callbacks, thus reverting to the behaviour of the API
> itself returning -EINVAL, through apply_to_page_range returning -EINVAL
> in case of block mappings, the difference now being, the -EINVAL is
> restricted to playing permission games on partial block mappings
> courtesy of patch 1.
>
> Signed-off-by: Dev Jain <dev.jain at arm.com>
> ---
> arch/arm64/mm/pageattr.c | 4 +---
> 1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index cfc5279f27a2..66676f7f432a 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -195,8 +195,6 @@ static int change_memory_common(unsigned long addr, int numpages,
> * we are operating on does not result in such splitting.
> *
> * Let's restrict ourselves to mappings created by vmalloc (or vmap).
> - * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
> - * mappings are updated and splitting is never needed.
> *
> * So check whether the [addr, addr + size) interval is entirely
> * covered by precisely one VM area that has the VM_ALLOC flag set.
> @@ -204,7 +202,7 @@ static int change_memory_common(unsigned long addr, int numpages,
> area = find_vm_area((void *)addr);
> if (!area ||
> end > (unsigned long)kasan_reset_tag(area->addr) + area->size ||
> - ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
> + !(area->flags & VM_ALLOC))
> return -EINVAL;
>
> if (!numpages)
I'd be inclined to leave this restriction in place for now. It is not useful
until we have context of the full vmalloc-huge-by-default series, I don't think?
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