Subject: Re: [PATCH 1/4] mm: Add optional close() to struct vm_special_mapping

From: Michael Ellerman <[email-masked]>
Date: Mon, 12 Aug 2024 18:22:44 +1000

"Liam R. Howlett" <[email-masked]> writes:
> * Michael Ellerman <[email-masked]> [240807 08:41]:
>> Add an optional close() callback to struct vm_special_mapping. It will
>> be used, by powerpc at least, to handle unmapping of the VDSO.
>> 
>> Suggested-by: Linus Torvalds <[email-masked]>
>> Signed-off-by: Michael Ellerman <[email-masked]>
>> ---
>>  include/linux/mm_types.h | 2 ++
>>  mm/mmap.c                | 3 +++
>>  2 files changed, 5 insertions(+)
>> 
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 485424979254..ef32d87a3adc 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -1313,6 +1313,8 @@ struct vm_special_mapping {
>>  
>>  	int (*mremap)(const struct vm_special_mapping *sm,
>>  		     struct vm_area_struct *new_vma);
>
> nit: missing new line?

Ack.

>> +	void (*close)(const struct vm_special_mapping *sm,
>> +		      struct vm_area_struct *vma);
>>  };
>>  
>>  enum tlb_flush_reason {
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index d0dfc85b209b..24bd6aa9155c 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -3624,6 +3624,9 @@ static vm_fault_t special_mapping_fault(struct vm_fault *vmf);
>>   */
>
> The above comment should probably be expanded to explain what this is
> about, or removed.

I expanded it slightly, happy for others to wordsmith it further.

>>  static void special_mapping_close(struct vm_area_struct *vma)
>>  {
>> +	const struct vm_special_mapping *sm = vma->vm_private_data;
>> +	if (sm->close)
>> +		sm->close(sm, vma);
>
> Right now we have the same sort of situation for mremap calls on
> special: we have a call to the specific vma mremap() function.
> ...
> So, are we missing an opportunity to avoid every arch having the same
> implementation here (that will evolve into random bugs existing in some
> archs for years before someone realises the cloned code wasn't fixed)?
> Do we already have a fix in ppc for the size checking that doesn't exist
> in the other archs in the case of mremap?

I took this as more of a meta comment/rant :)

Yes I agree the implementation should eventually be generic, but this series
is just about moving the existing powerpc behaviour from arch_unmap()
into this hook. 

cheers