Re: [PATCH v1 01/10] mm: Expose clear_huge_page() unconditionally

Ryan Roberts
2023-06-27T07:21:22Z

On 27/06/2023 02:55, Yu Zhao wrote:
> On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> In preparation for extending vma_alloc_zeroed_movable_folio() to
>> allocate a arbitrary order folio, expose clear_huge_page()
>> unconditionally, so that it can be used to zero the allocated folio in
>> the generic implementation of vma_alloc_zeroed_movable_folio().
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>> include/linux/mm.h | 3 ++-
>> mm/memory.c | 2 +-
>> 2 files changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 7f1741bd870a..7e3bf45e6491 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
>> */
>> extern const struct attribute_group memory_failure_attr_group;
>>
>> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>> extern void clear_huge_page(struct page *page,
>> unsigned long addr_hint,
>> unsigned int pages_per_huge_page);
>> +
>> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>
> We might not want to depend on THP eventually. Right now, we still
> have to, unless splitting is optional, which seems to contradict
> 06/10. (deferred_split_folio() is a nop without THP.)

Yes, I agree - for large anon folios to work, we depend on THP. But I don't
think that helps us here.

In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
parameter. So the generic/default version of the function now needs a way to
clear a compound page.

I guess I could do something like:

static inline
struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
unsigned long vaddr, gfp_t gfp, int order)
{
struct folio *folio;

folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
order, vma, vaddr, false);
if (folio) {
#ifdef CONFIG_LARGE_FOLIO
clear_huge_page(&folio->page, vaddr, 1U << order);
#else
BUG_ON(order != 0);
clear_user_highpage(&folio->page, vaddr);
#endif
}

return folio;
}

But that's pretty messy and there's no reason why other users might come along
that pass order != 0 and will be surprised by the BUG_ON.