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

[David Hildenbrand <david@redhat.com>]

>> +	/*
>> +	 * To challenge spitting code, we will mremap page[x] of the
>> +	 * thp[x] into a smaller area, and trigger the split from that
>> +	 * smaller area. This will end up replacing the PMD mappings in
>> +	 * the thp_area by PTE mappings first, leaving the THPs unsplit.
>> +	 */
>> +	page_area = mmap(NULL, page_area_size, PROT_READ | PROT_WRITE,
>> +			MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>> +	if (page_area == MAP_FAILED) {
>> +		ksft_test_result_fail("Fail to allocate memory: %s\n", strerror(errno));
>> +		goto out;
>> +	}
>> -		if (i % pagesize == 0 &&
>> -		    !is_backed_by_folio(&pte_mapped[i], 0, pagemap_fd, kpageflags_fd))
>> -			thp_size++;
>> +	for (i = 0; i < nr_thps; i++) {
>> +		tmp = mremap(thp_area + pmd_pagesize * i + pagesize * i,
>> +			     pagesize, pagesize, MREMAP_MAYMOVE|MREMAP_FIXED,
>> +			     page_area + pagesize * i);
> 
> Would this create one hole at the beginning of each 2M range and cause
> splitting underlining THP?

Yes, it will create a hole (this also happens in the old code).

As the comment above it now states: "leaving the THPs unsplit"

There is a check verifying that after this mremap code.

-- 
Cheers

David / dhildenb