On Fri, Jun 05, 2026 at 10:14:18AM -0600, Nico Pache wrote:
>Enable khugepaged to collapse to mTHP orders. This patch implements the
>main scanning logic using a bitmap to track occupied pages and the
>algorithm to find optimal collapse sizes.
>
>Previous to this patch, PMD collapse had 3 main phases, a light weight
>scanning phase (mmap_read_lock) that determines a potential PMD
>collapse, an alloc phase (mmap unlocked), then finally heavier collapse
>phase (mmap_write_lock).
>
>To enabled mTHP collapse we make the following changes:
>
>During PMD scan phase, track occupied pages in a bitmap. When mTHP
>orders are enabled, we remove the restriction of max_ptes_none during the
>scan phase to avoid missing potential mTHP collapse candidates. Once we
>have scanned the full PMD range and updated the bitmap to track occupied
>pages, we use the bitmap to find the optimal mTHP size.
>
>Implement mthp_collapse() to walk forward through the bitmap and
>determine the best eligible order for each naturally-aligned region. The
>algorithm starts at the beginning of the PMD range and, for each offset,
>tries the highest order that fits the alignment. If the number of
>occupied PTEs in that region satisfies the max_ptes_none threshold for
>that order, a collapse is attempted. On failure, the order is
>decremented and the same offset is retried at the next smaller size. Once
>the smallest enabled order is exhausted (or a collapse succeeds), the
>offset advances past the region just processed, and the next attempt
>starts at the highest order permitted by the new offset's natural
>alignment.
>
>The algorithm works as follows:
> 1) set offset=0 and order=HPAGE_PMD_ORDER
> 2) if the order is not enabled, go to step (5)
> 3) count occupied PTEs in the (offset, order) range using
> bitmap_weight_from()
> 4) if the count satisfies the max_ptes_none threshold, attempt
> collapse; on success, advance to step (6)
> 5) if a smaller enabled order exists, decrement order and retry
> from step (2) at the same offset
> 6) advance offset past the current region and compute the next
> order from the new offset's natural alignment via __ffs(offset),
> capped at HPAGE_PMD_ORDER
> 7) repeat from step (2) until the full PMD range is covered
>
>mTHP collapses reject regions containing swapped out or shared pages.
>This is because adding new entries can lead to new none pages, and these
>may lead to constant promotion into a higher order mTHP. A similar
>issue can occur with "max_ptes_none > HPAGE_PMD_NR/2" due to a collapse
>introducing at least 2x the number of pages, and on a future scan will
>satisfy the promotion condition once again. This issue is prevented via
>the collapse_max_ptes_none() function which imposes the max_ptes_none
>restrictions above.
>
>We currently only support mTHP collapse for max_ptes_none values of 0
>and HPAGE_PMD_NR - 1. resulting in the following behavior:
>
> - max_ptes_none=0: Never introduce new empty pages during collapse
> - max_ptes_none=HPAGE_PMD_NR-1: Always try collapse to the highest
> available mTHP order
>
>Any other max_ptes_none value will emit a warning and default mTHP
>collapse to max_ptes_none=0. There should be no behavior change for PMD
>collapse.
>
>Once we determine what mTHP sizes fits best in that PMD range a collapse
>is attempted. A minimum collapse order of 2 is used as this is the lowest
>order supported by anon memory as defined by THP_ORDERS_ALL_ANON.
>
>Currently madv_collapse is not supported and will only attempt PMD
>collapse.
>
>We can also remove the check for is_khugepaged inside the PMD scan as
>the collapse_max_ptes_none() function handles this logic now.
>
>Signed-off-by: Nico Pache <[email protected]>
>---
> mm/khugepaged.c | 146 +++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 138 insertions(+), 8 deletions(-)
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index ec886a031952..430047316f43 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -99,6 +99,8 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash,
>MM_SLOTS_HASH_BITS);
>
> static struct kmem_cache *mm_slot_cache __ro_after_init;
>
>+#define KHUGEPAGED_MIN_MTHP_ORDER 2
>+
> struct collapse_control {
> bool is_khugepaged;
>
>@@ -110,6 +112,9 @@ struct collapse_control {
>
> /* nodemask for allocation fallback */
> nodemask_t alloc_nmask;
>+
>+ /* Each bit represents a single occupied (!none/zero) page. */
>+ DECLARE_BITMAP(mthp_present_ptes, MAX_PTRS_PER_PTE);
> };
>
> /**
>@@ -1440,20 +1445,130 @@ static enum scan_result collapse_huge_page(struct
>mm_struct *mm, unsigned long s
> return result;
> }
>
>+/* Return the highest naturally aligned order that fits at @offset within a
>PMD. */
>+static unsigned int max_order_from_offset(unsigned int offset)
>+{
>+ if (offset == 0)
>+ return HPAGE_PMD_ORDER;
>+
>+ return min_t(unsigned int, __ffs(offset), HPAGE_PMD_ORDER);
>+}
>+
>+/*
>+ * mthp_collapse() consumes the bitmap that is generated during
>+ * collapse_scan_pmd() to determine what regions and mTHP orders fit best.
>+ *
>+ * Each bit in cc->mthp_present_ptes represents a single occupied (!none/zero)
>+ * page. We start at the PMD order and check if it is eligible for collapse;
>+ * if not, we check the left and right halves of the PTE page table we are
>+ * examining at a lower order.
>+ *
>+ * For each of these, we determine how many PTE entries are occupied in the
>+ * range of PTE entries we propose to collapse, then we compare this to a
>+ * threshold number of PTE entries which would need to be occupied for a
>+ * collapse to be permitted at that order (accounting for max_ptes_none).
>+ *
>+ * If a collapse is permitted, we attempt to collapse the PTE range into a
>+ * mTHP.
>+ */
>+static enum scan_result mthp_collapse(struct mm_struct *mm,
>+ unsigned long address, int referenced, int unmapped,
>+ struct collapse_control *cc, unsigned long enabled_orders)
>+{
>+ unsigned int nr_occupied_ptes, nr_ptes, max_ptes_none;
>+ enum scan_result last_result = SCAN_FAIL;
>+ int collapsed = 0;
>+ bool alloc_failed = false;
>+ unsigned long collapse_address;
>+ unsigned int offset = 0;
>+ unsigned int order = HPAGE_PMD_ORDER;
>+
>+ while (offset < HPAGE_PMD_NR) {
>+ nr_ptes = 1UL << order;
>+
>+ if (!test_bit(order, &enabled_orders))
>+ goto next_order;
>+
>+ max_ptes_none = collapse_max_ptes_none(cc, NULL, order);
>+ nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes,
>offset,
>+ offset + nr_ptes);
>+
>+ if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
Looks broken for swap PTEs in PMD collapse ...
collapse_scan_pmd() allows them up to max_ptes_swap and record them in
unmapped, but they don't get a bit in mthp_present_ptes. And then
mthp_collapse() does the check above:
nr_occupied_ptes >= nr_ptes - max_ptes_none
So max_ptes_none=0 + 511 present PTEs + one allowed swap PTE won't even
call collapse_huge_page() for PMD order.
Shouldn't we account for them in the PMD-order check? Something like:
if (is_pmd_order(order))
nr_occupied_ptes += unmapped;
Cheers, Lance
>+ enum scan_result ret;
>+
>+ collapse_address = address + offset * PAGE_SIZE;
>+ ret = collapse_huge_page(mm, collapse_address,
>referenced,
>+ unmapped, cc, order);
>+ switch (ret) {
>+ /* Cases where we continue to next collapse candidate */
>+ case SCAN_SUCCEED:
>+ collapsed += nr_ptes;
>+ fallthrough;
>+ case SCAN_PTE_MAPPED_HUGEPAGE:
>+ goto next_offset;
>+ /* Cases where lower orders might still succeed */
>+ case SCAN_ALLOC_HUGE_PAGE_FAIL:
>+ alloc_failed = true;
>+ last_result = ret;
>+ goto next_order;
>+ /* Cases where no further collapse is possible */
>+ case SCAN_PMD_MAPPED:
>+ fallthrough;
>+ default:
>+ last_result = ret;
>+ goto done;
>+ }
>+ }
>+
>+next_order:
>+ /*
>+ * Continue with the next smaller order if there is still
>+ * any smaller order enabled. When at the smallest order
>+ * we must always move to the next offset.
>+ */
>+ if (order > KHUGEPAGED_MIN_MTHP_ORDER &&
>+ (enabled_orders & GENMASK(order - 1, 0))) {
>+ order--;
>+ continue;
>+ }
>+next_offset:
>+ /*
>+ * Advance past the region we just processed and determine the
>+ * highest order we can attempt next. Since huge pages must be
>+ * naturally aligned, the max order we can attempt next is
>+ * limited by the alignment of the new offset.
>+ * E.g. if we collapsed a order-2 mTHP at offset 0, offset
>+ * becomes 4 and __ffs(4) == 2, so the next attempt starts at
>+ * order 2.
>+ */
>+ offset += nr_ptes;
>+ order = max_order_from_offset(offset);
>+ }
>+done:
>+ if (collapsed)
>+ return SCAN_SUCCEED;
>+ if (alloc_failed)
>+ return SCAN_ALLOC_HUGE_PAGE_FAIL;
>+ return last_result;
>+}
>+
> static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> struct vm_area_struct *vma, unsigned long start_addr,
> bool *lock_dropped, struct collapse_control *cc)
> {
>- const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma,
>HPAGE_PMD_ORDER);
> const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc,
> HPAGE_PMD_ORDER);
> const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc,
> HPAGE_PMD_ORDER);
>+ unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma,
>HPAGE_PMD_ORDER);
>+ enum tva_type tva_flags = cc->is_khugepaged ? TVA_KHUGEPAGED :
>TVA_FORCED_COLLAPSE;
> pmd_t *pmd;
>- pte_t *pte, *_pte;
>+ pte_t *pte, *_pte, pteval;
>+ int i;
> int none_or_zero = 0, shared = 0, referenced = 0;
> enum scan_result result = SCAN_FAIL;
> struct page *page = NULL;
> struct folio *folio = NULL;
> unsigned long addr;
>+ unsigned long enabled_orders;
> spinlock_t *ptl;
> int node = NUMA_NO_NODE, unmapped = 0;
>
>@@ -1465,8 +1580,19 @@ static enum scan_result collapse_scan_pmd(struct
>mm_struct *mm,
> goto out;
> }
>
>+ bitmap_zero(cc->mthp_present_ptes, MAX_PTRS_PER_PTE);
> memset(cc->node_load, 0, sizeof(cc->node_load));
> nodes_clear(cc->alloc_nmask);
>+
>+ enabled_orders = collapse_possible_orders(vma, vma->vm_flags,
>tva_flags);
>+
>+ /*
>+ * If PMD is the only enabled order, enforce max_ptes_none, otherwise
>+ * scan all pages to populate the bitmap for mTHP collapse.
>+ */
>+ if (enabled_orders != BIT(HPAGE_PMD_ORDER))
>+ max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
>+
> pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
> if (!pte) {
> cc->progress++;
>@@ -1474,11 +1600,13 @@ static enum scan_result collapse_scan_pmd(struct
>mm_struct *mm,
> goto out;
> }
>
>- for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
>- _pte++, addr += PAGE_SIZE) {
>+ for (i = 0; i < HPAGE_PMD_NR; i++) {
>+ _pte = pte + i;
>+ addr = start_addr + i * PAGE_SIZE;
>+ pteval = ptep_get(_pte);
>+
> cc->progress++;
>
>- pte_t pteval = ptep_get(_pte);
> if (pte_none_or_zero(pteval)) {
> if (++none_or_zero > max_ptes_none) {
> result = SCAN_EXCEED_NONE_PTE;
>@@ -1558,6 +1686,8 @@ static enum scan_result collapse_scan_pmd(struct
>mm_struct *mm,
> }
> }
>
>+ /* Set bit for occupied pages */
>+ __set_bit(i, cc->mthp_present_ptes);
> /*
> * Record which node the original page is from and save this
> * information to cc->node_load[].
>@@ -1616,9 +1746,9 @@ static enum scan_result collapse_scan_pmd(struct
>mm_struct *mm,
> if (result == SCAN_SUCCEED) {
> /* collapse_huge_page expects the lock to be dropped before
> calling */
> mmap_read_unlock(mm);
>- result = collapse_huge_page(mm, start_addr, referenced,
>- unmapped, cc, HPAGE_PMD_ORDER);
>- /* collapse_huge_page will return with the mmap_lock released */
>+ result = mthp_collapse(mm, start_addr, referenced,
>+ unmapped, cc, enabled_orders);
>+ /* mmap_lock was released above, set lock_dropped */
> *lock_dropped = true;
> }
> out:
>--
>2.54.0
>
>
