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

On Tue, Sep 02, 2025 at 09:49:48AM +0200, David Hildenbrand wrote:

...

...

• /*

• * 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.
  

• */
  

This confuse me a little. How about move "and trigger the split from that smaller area" to the end? So that I know mremap convert to pte-mapping and leave thp unsplit. Then we do the split.

...

...

• 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.

Thanks for clarifying. One nit to the comment above.