Re: [PATCH v3 2/2] arm64: pageattr: Enable huge-vmalloc permission change

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

On 25/06/25 4:38 pm, Ryan Roberts wrote:
> 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@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?

Okay.