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

[David Hildenbrand <david@redhat.com>]

On 01.09.25 14:54, David Hildenbrand wrote:
> 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.

Zi Yan mentions that there is a reason we are using mremap in the other 
thread. So likely best to just keep it as is unless we can understand 
that it definitely can done simpler.

-- 
Cheers

David / dhildenb