Re: [PATCH] selftests/mm: directly add pagesize instead of increase until page size

[David Hildenbrand <david@redhat.com>]

On 01.09.25 14:51, Wei Yang wrote:
> On Mon, Sep 01, 2025 at 11:32:11AM +0200, David Hildenbrand wrote:
>> On 31.08.25 03:32, Zi Yan wrote:
>>> On 29 Aug 2025, at 22:31, Wei Yang wrote:
>>>
>>>> The check of is_backed_by_folio() is done on each page.
>>>>
>>>> Directly move pointer to next page instead of increase one and check if
>>>> it is page size aligned.
>>>>
>>>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>>>> ---
>>>>    tools/testing/selftests/mm/split_huge_page_test.c | 5 ++---
>>>>    1 file changed, 2 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
>>>> index 10ae65ea032f..7f7016ba4054 100644
>>>> --- a/tools/testing/selftests/mm/split_huge_page_test.c
>>>> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
>>>> @@ -423,9 +423,8 @@ static void split_pte_mapped_thp(void)
>>>>
>>>>    	/* smap does not show THPs after mremap, use kpageflags instead */
>>>>    	thp_size = 0;
>>>> -	for (i = 0; i < pagesize * 4; i++)
>>>> -		if (i % pagesize == 0 &&
>>>> -		    is_backed_by_folio(&pte_mapped[i], pmd_order, pagemap_fd, kpageflags_fd))
>>>> +	for (i = 0; i < pagesize * 4; i += pagesize)
>>>> +		if (is_backed_by_folio(&pte_mapped[i], pmd_order, pagemap_fd, kpageflags_fd))
>>>>    			thp_size++;
>>>>
>>>>    	if (thp_size != 4)
>>>
>>> It might be better to add
>>>
>>> if (pte_mapped[i] != (char)i)
>>> 	ksft_exit_fail_msg("%ld byte corrupted\n", i);
>>>
>>> instead to make sure mremap() does not change pte_mapped[] values.
>>
>> We do have a corruption check later in that function, so I think we can just
>> keep it simple here.
>>
>> So this as is LGTM
>>
>> Acked-by: David Hildenbrand <david@redhat.com>
>>
>> As noted, we should just move from mremap() to mprotect() or sth like that
>> which has clearer semantics.
>>
> 
> If my understanding is correct, we should
> 
>     mmap 4 PMD_SIZE region with RW
>     madvise and fault in to allocate pmd-mapped thp
>     mprotect first page of each PMD to read-only to split to pte-mapped thp
>     check whether the page is backed by pmd-order folio
> 
> Is this the correct way?

Yeah, I would just mprotect(PROT_READ) the first page of each PMD. That 
will trigger a PTE-mapping of the THP reliably.

-- 
Cheers

David / dhildenb