On Mon, May 18, 2026 at 6:50 AM Wei Yang <[email protected]> wrote:
>
> On Mon, May 11, 2026 at 12:58:00PM -0600, Nico Pache wrote:
> >The following series provides khugepaged with the capability to collapse
> >anonymous memory regions to mTHPs.
> >
> >To achieve this we generalize the khugepaged functions to no longer depend
> >on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
> >pages that are occupied (!none/zero). After the PMD scan is done, we use
> >the bitmap to find the optimal mTHP sizes for the PMD range. The
> >restriction on max_ptes_none is removed during the scan, to make sure we
> >account for the whole PMD range in the bitmap. When no mTHP size is
> >enabled, the legacy behavior of khugepaged is maintained.
> >
> >We currently only support max_ptes_none values of 0 or HPAGE_PMD_NR - 1
> >(ie 511). If any other value is specified, the kernel will emit a warning
> >and no mTHP collapse will be attempted. If a mTHP collapse is attempted,
> >but contains swapped out, or shared pages, we don't perform the collapse.
> >It is now also possible to collapse to mTHPs without requiring the PMD THP
> >size to be enabled. These limitations are to prevent collapse "creep"
> >behavior. This prevents constantly promoting mTHPs to the next available
> >size, which would occur because a collapse introduces more non-zero pages
> >that would satisfy the promotion condition on subsequent scans.
> >
> >Patch 1-2: Generalize hugepage_vma_revalidate and alloc_charge_folio
> > for arbitrary orders.
> >Patch 3: Rework max_ptes_* handling into helper functions
> >Patch 4: Generalize __collapse_huge_page_* for mTHP support
> >Patch 5: Require collapse_huge_page to enter/exit with the lock dropped
> >Patch 6: Generalize collapse_huge_page for mTHP collapse
> >Patch 7: Skip collapsing mTHP to smaller orders
> >Patch 8-9: Add per-order mTHP statistics and tracepoints
> >Patch 10: Introduce collapse_allowable_orders helper function
> >Patch 11-13: Introduce bitmap and mTHP collapse support, fully enabled
> >Patch 14: Documentation
> >
> >Testing:
> >- Built for x86_64, aarch64, ppc64le, and s390x
> >- ran all arches on test suites provided by the kernel-tests project
> >- internal testing suites: functional testing and performance testing
> >- selftests mm
> >- I created a test script that I used to push khugepaged to its limits
> > while monitoring a number of stats and tracepoints. The code is
> > available here[1] (Run in legacy mode for these changes and set mthp
> > sizes to inherit)
> > The summary from my testings was that there was no significant
> > regression noticed through this test. In some cases my changes had
> > better collapse latencies, and was able to scan more pages in the same
> > amount of time/work, but for the most part the results were consistent.
> >- redis testing. I did some testing with these changes along with my defer
> > changes (see followup [2] post for more details). We've decided to get
> > the mTHP changes merged first before attempting the defer series.
> >- some basic testing on 64k page size.
> >- lots of general use.
> >
>
> Two links are missing. I got them from previous version.
>
> [1] - https://gitlab.com/npache/khugepaged_mthp_test
> [2] - https://lore.kernel.org/lkml/[email protected]/
Oh whoops, ill make sure they are there in the followup
>
> And the test in [1] is a performance test. I am thinking whether we want a
> functional test in selftests.
It also works as a functional test in some regards. The reason i never
pursued self-tests is that I naively thought this was getting merged
6(?) months ago and at the time the selftests infrastructure didn't
support it well. Baolin included patches to clean that up in his shmem
mTHP support patches and added tests for both features. Let's repost
and re-merge this first; then, I will follow up in one or two weeks
regarding self-tests. I'm currently on PTO and only have time to
complete, test, and return the v18 changes to Andrew before they
create a huge merge headache and we miss yet another window.
>
> I did a quick try with following change and some hack.
Thanks Ill use that as a base!
>
> @@ -744,6 +765,51 @@ static void collapse_max_ptes_none(struct
> collapse_context *c, struct mem_ops *o
> ksft_test_result_report(exit_status, "%s\n", __func__);
> }
>
> +static void collapse_mth_ptes(struct collapse_context *c, struct mem_ops
> *ops)
> +{
> + struct thp_settings settings = *thp_current_settings();
> + void *p;
> + int i;
> +
> + /* Disable mthp on fault */
> + for (i = 0; i < NR_ORDERS; i++) {
> + settings.hugepages[i].enabled = THP_NEVER;
> + }
> + thp_push_settings(&settings);
> +
> + p = ops->setup_area(1);
> +
> + ops->fault(p, 0, hpage_pmd_size);
> +
> + /* Expect all order-0 folio after fault */
> + memset(expected_orders, 0, sizeof(int) * (pmd_order + 1));
> + expected_orders[0] = hpage_pmd_nr;
> + if (check_folio_orders(p, hpage_pmd_size, pagemap_fd,
> + kpageflags_fd, expected_orders,
> + (pmd_order + 1)))
> + ksft_exit_fail_msg("Unexpected huge page at fault\n");
> +
> + /* Enable mthp before collapse */
> + thp_pop_settings();
> + settings.hugepages[2].enabled = THP_ALWAYS;
> + thp_push_settings(&settings);
> +
> + c->collapse("Collapse fully populated PTE table with order 2", p, 1,
> + ops, true);
> +
> + /* Expect all order-2 folio after collapse */
> + memset(expected_orders, 0, sizeof(int) * (pmd_order + 1));
> + expected_orders[2] = 1 << (pmd_order - 2);
> + if (check_folio_orders(p, hpage_pmd_size, pagemap_fd,
> + kpageflags_fd, expected_orders,
> + (pmd_order + 1)))
> + ksft_exit_fail_msg("Unexpected page order\n");
> +
> + ops->cleanup_area(p, hpage_pmd_size);
> + thp_pop_settings();
> + ksft_test_result_report(exit_status, "%s\n", __func__);
> +}
> +
> static void collapse_swapin_single_pte(struct collapse_context *c, struct
> mem_ops *ops)
> {
> void *p;
>
> This leverage check_after_split_folio_orders() in split_huge_page_test.c to
> check folio order in PMD range.
>
> --
> Wei Yang
> Help you, Help me
>
