On Mon, Sep 01, 2025 at 09:10:57PM +0200, David Hildenbrand wrote:
>> > > > (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" ?
>>
>> IMHO, it is an interesting test case for splitting a THP when only a subpage
>> is mapped into a vaddr range and in a contiguous vaddr each page comes from
>> different THPs.
>
>Right. Slightly similar to just MAV_DONTNEED'ing the other PTEs and trying to
>split the bigger range.
>
>Of course, if you involve more mremap, the RMAP logic of installing migration
>ptes will get stressed more.
>
>So yes, there are various ways on how to stress the RMAP walk when splitting.
>
>> The mprotect test case you are mentioning would still have all
>> subpages mapped by contiguous vaddrs.
>
>Right, it would not stress RMAP as much.
>
>>
>> But if you think both are just testing PTE-mapped THPs, feel free to replace
>> the
>> existing one with the mprotect test case. In addition, is_backed_by_folio()
>> can be reverted back to its prior version, since it no longer needs to handle
>> the case where subpages from different THPs can be mapped into a vaddr range.
>
>Oh, the is_backed_by_folio() change is actually really valuable.
>
>
>I think I was confused by the implementation that works on a single virtual
>address
>range with multiple different variables, questioning why we mremap at all.
>
>I tried cleaning up that test myself and ended up with the following (it
>escalated a bit). If that looks cleaner to you as well, I can submit that as a
>patch.
>
>diff --git a/tools/testing/selftests/mm/split_huge_page_test.c
>b/tools/testing/selftests/mm/split_huge_page_test.c
>index 10ae65ea032f6..aa0f0502efa06 100644
>--- a/tools/testing/selftests/mm/split_huge_page_test.c
>+++ b/tools/testing/selftests/mm/split_huge_page_test.c
>@@ -390,67 +390,88 @@ static void split_pmd_thp_to_order(int order)
> static void split_pte_mapped_thp(void)
> {
>- char *one_page, *pte_mapped, *pte_mapped2;
>- size_t len = 4 * pmd_pagesize;
>- uint64_t thp_size;
>+ const size_t nr_thps = 4;
>+ const size_t thp_area_size = nr_thps * pmd_pagesize;
>+ const size_t page_area_size = nr_thps * pagesize;
>+ char *thp_area, *page_area = NULL, *tmp;
> size_t i;
>- one_page = mmap((void *)(1UL << 30), len, PROT_READ | PROT_WRITE,
>+ thp_area = mmap((void *)(1UL << 30), thp_area_size, PROT_READ |
>PROT_WRITE,
> MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>- if (one_page == MAP_FAILED)
>- ksft_exit_fail_msg("Fail to allocate memory: %s\n",
>strerror(errno));
>+ if (thp_area == MAP_FAILED) {
>+ ksft_test_result_fail("Fail to allocate memory: %s\n",
>strerror(errno));
>+ goto out;
>+ }
>- madvise(one_page, len, MADV_HUGEPAGE);
>+ madvise(thp_area, thp_area_size, MADV_HUGEPAGE);
>- for (i = 0; i < len; i++)
>- one_page[i] = (char)i;
>+ for (i = 0; i < thp_area_size; i++)
>+ thp_area[i] = (char)i;
>- if (!check_huge_anon(one_page, 4, pmd_pagesize))
>- ksft_exit_fail_msg("No THP is allocated\n");
>+ if (!check_huge_anon(thp_area, nr_thps, pmd_pagesize)) {
>+ ksft_test_result_skip("Not all THPs allocated\n");
>+ goto out;
>+ }
>- /* remap the first pagesize of first THP */
>- pte_mapped = mremap(one_page, pagesize, pagesize, MREMAP_MAYMOVE);
>-
>- /* remap the Nth pagesize of Nth THP */
>- for (i = 1; i < 4; i++) {
>- pte_mapped2 = mremap(one_page + pmd_pagesize * i + pagesize * i,
>- pagesize, pagesize,
>- MREMAP_MAYMOVE|MREMAP_FIXED,
>- pte_mapped + pagesize * i);
>- if (pte_mapped2 == MAP_FAILED)
>- ksft_exit_fail_msg("mremap failed: %s\n",
>strerror(errno));
>- }
>-
>- /* 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))
>- thp_size++;
>-
>- if (thp_size != 4)
>- ksft_exit_fail_msg("Some THPs are missing during mremap\n");
>-
>- /* split all remapped THPs */
>- write_debugfs(PID_FMT, getpid(), (uint64_t)pte_mapped,
>- (uint64_t)pte_mapped + pagesize * 4, 0);
>-
>- /* smap does not show THPs after mremap, use kpageflags instead */
>- thp_size = 0;
>- for (i = 0; i < pagesize * 4; i++) {
>- if (pte_mapped[i] != (char)i)
>- ksft_exit_fail_msg("%ld byte corrupted\n", i);
>+ /*
>+ * 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?
>+ if (tmp != MAP_FAILED)
>+ continue;
>+ ksft_test_result_fail("mremap failed: %s\n", strerror(errno));
>+ goto out;
>+ }
>+
>+ /*
>+ * Verify that our THPs were not split yet. Note that
>+ * check_huge_anon() cannot be used as it checks for PMD mappings.
>+ */
>+ for (i = 0; i < nr_thps; i++) {
>+ if (is_backed_by_folio(page_area + i * pagesize, pmd_order,
>+ pagemap_fd, kpageflags_fd))
>+ continue;
>+ ksft_test_result_fail("THP %zu missing after mremap\n", i);
>+ goto out;
> }
>- if (thp_size)
>- ksft_exit_fail_msg("Still %ld THPs not split\n", thp_size);
>+ /* Split all THPs through the remapped pages. */
>+ write_debugfs(PID_FMT, getpid(), (uint64_t)page_area,
>+ (uint64_t)page_area + page_area_size, 0);
>+
>+ /* Corruption during mremap or split? */
>+ for (i = 0; i < page_area_size; i++) {
>+ if (page_area[i] == (char)i)
>+ continue;
>+ ksft_test_result_fail("%zu byte corrupted\n", i);
>+ goto out;
>+ }
>+
>+ /* Split failed? */
>+ for (i = 0; i < nr_thps; i++) {
>+ if (is_backed_by_folio(&page_area[i], 0, pagemap_fd,
>kpageflags_fd))
>+ continue;
>+ ksft_test_result_fail("THP %zu not split\n", i);
>+ }
> ksft_test_result_pass("Split PTE-mapped huge pages successful\n");
>- munmap(one_page, len);
>+out:
>+ munmap(thp_area, thp_area_size);
>+ if (page_area)
>+ munmap(page_area, page_area_size);
> }
> static void split_file_backed_thp(int order)
>--
>2.50.1
>
>
>--
>Cheers
>
>David / dhildenb
--
Wei Yang
Help you, Help me
