Re: [PATCH v7 12/16] arm: mm: define clear_user_highpages()

[Ankur Arora <ankur.a.arora@oracle.com>]

David Hildenbrand <david@redhat.com> writes:

> On 24.09.25 00:25, Ankur Arora wrote:
>> David Hildenbrand <david@redhat.com> writes:
>>
>>> On 17.09.25 17:24, Ankur Arora wrote:
>>>> For configurations with CONFIG_MMU we do not define clear_user_page().
>>>> This runs into issues for configurations with !CONFIG_HIGHMEM, because
>>>> clear_user_highpages() expects to clear_user_page() (via a default
>>>> version of clear_user_pages()).
>>>
>>> I'm confused. Can you elaborate once more why we cannot take care of that in
>>> common code?
>> So my definition of clear_user_highpages,
>>      +#ifndef clear_user_highpages
>>      +static inline void clear_user_highpages(struct page *page, unsigned long vaddr,
>>      +					unsigned int npages)
>>      +{
>>      +	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
>>      +		void *base = page_address(page);
>>      +		clear_user_pages(base, vaddr, page, npages);
>>      +		return;
>>      +	}
>>      +
>>      +	do {
>>      +		clear_user_highpage(page, vaddr);
>>      +		vaddr += PAGE_SIZE;
>>      +		page++;
>>      +	} while (--npages);
>>      +}
>>      +#endif
>> assumes one of the following:
>>    1. clear_user_highpages is defined by the architecture or,
>>    2. HIGHMEM => arch defines clear_user_highpage or clear_user_page
>>    3. !HIGHMEM => arch defines clear_user_pages or clear_user_page
>> Case 2 is fine, since ARM has clear_user_highpage().
>> Case 3 runs into a problem since ARM doesn't have clear_user_pages()
>> or clear_user_page() (it does have the second, but only with !CONFIG_MMU).
>
> I think we should look into having a generic fallback version in common code
> instead for that case, and not require the arch to implement such a loop around
> clear_user_highpage().

So, as you suggested, I moved clear_user_pages() to mm/utils.c and
conditioned it on clear_user_page() also existing.

  #if defined(clear_user_page) && !defined(clear_user_pages)
  void clear_user_pages(void *addr, unsigned long vaddr, struct page *page,
                        unsigned int npages) {
                      ...
  }
  #endif

That fixed this issue as well since there's no more bogus reference to
clear_user_page().

Are there cases in which (TRANSPARENT_HUGEPAGE || HUGETLB) might be enabled
on ARM?

--
ankur