Re: [Patch v2] selftests/mm: check content to see whether mremap corrupt data

[David Hildenbrand <david@redhat.com>]

On 01.09.25 14:56, Zi Yan wrote:
> On 1 Sep 2025, at 3:22, David Hildenbrand wrote:
> 
>> On 31.08.25 04:27, Wei Yang wrote:
>>
>> Subject: "selftests/mm: verify page content after remapping PMD through PTEs"
>>
>>> After mremap(), add a check on content to see whether mremap corrupt
>>> data.
>>>
>>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>>>
>>> ---
>>> v2: add check on content instead of just test backed folio
>>
>> I'm confused, don't we have that exact check later in the function?
>>
>> Your v1 might have been better, unless I am missing something.
>>
>>> ---
>>>    tools/testing/selftests/mm/split_huge_page_test.c | 6 +++++-
>>>    1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
>>> index 10ae65ea032f..229b6dcabece 100644
>>> --- a/tools/testing/selftests/mm/split_huge_page_test.c
>>> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
>>> @@ -423,10 +423,14 @@ 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++)
>>> +	for (i = 0; i < pagesize * 4; i++) {
>>> +		if (pte_mapped[i] != (char)i)
>>> +			ksft_exit_fail_msg("%ld byte corrupted\n", i);
>>> +
>>>    		if (i % pagesize == 0 &&
>>>    		    is_backed_by_folio(&pte_mapped[i], pmd_order, pagemap_fd, kpageflags_fd))
>>>    			thp_size++;
>>> +	}
>>>     	if (thp_size != 4)
>>>    		ksft_exit_fail_msg("Some THPs are missing during mremap\n");
>>
>> I'm a bit confused with this test in general.
>>
>> We do a
>>
>> pte_mapped = mremap(one_page, pagesize, pagesize, MREMAP_MAYMOVE);
>>
>> which I read as a "NOP".
>>
>> Questions
>>
>> (a) Will this actually do anything? Also, maybe it does now, but can't the kernel just optimize that out in the future?
> 
> It remaps each subpage of 4 PMD THPs into a contiguous 2MB vaddr range and
> perform split on that range.

I'm afraid I am missing the "why".

I would have thought that a "split_pte_mapped_thp" test would want to pte-map THPs
to the see if they can be split.

Why is the mremap required? IOW, what exactly is the test trying to test that
exceeds "split_pte_mapped_thp" ?

-- 
Cheers

David / dhildenb