Re: [PATCH mm-unstable v15 10/13] mm/khugepaged: Introduce mTHP collapse support

David Hildenbrand (Arm)
2026-03-12T21:17:12Z

On 2/26/26 04:26, 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 a stack
> structure that allows us 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, a 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 collapse_scan_bitmap() to perform binary recursion on the bitmap
> and determine the best eligible order for the collapse. A stack structure
> is used instead of traditional recursion to manage the search. The
> algorithm recursively splits the bitmap into smaller chunks to find the
> highest order mTHPs that satisfy the collapse criteria. We start by
> attempting the PMD order, then moved on the consecutively lower orders
> (mTHP collapse). The stack maintains a pair of variables (offset, order),
> indicating the number of PTEs from the start of the PMD, and the order of
> the potential collapse candidate.
>
> The algorithm for consuming the bitmap works as such:
> 1) push (0, HPAGE_PMD_ORDER) onto the stack
> 2) pop the stack
> 3) check if the number of set bits in that (offset,order) pair
> statisfy the max_ptes_none threshold for that order
> 4) if yes, attempt collapse
> 5) if no (or collapse fails), push two new stack items representing
> the left and right halves of the current bitmap range, at the
> next lower order
> 6) repeat at step (2) until stack is empty.
>
> Below is a diagram representing the algorithm and stack items:
>
> offset mid_offset
> | |
> | |
> v v
> ____________________________________
> | PTE Page Table |
> --------------------------------------
> <-------><------->
> order-1 order-1
>
> 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 skip mTHP collapse
> attempts. 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.
>
> 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 (m)THP. 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.
>
> 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.
>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---

[...]

> /**
> @@ -1361,17 +1392,138 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
> return result;
> }
>
> +static void mthp_stack_push(struct collapse_control *cc, int *stack_size,
> + u16 offset, u8 order)

Nit: indentation. Same for other functions.

Wondering if you'd want to call these functions

collapse_mthp_*

> +{
> + const int size = *stack_size;
> + struct mthp_range *stack = &cc->mthp_bitmap_stack[size];
> +
> + VM_WARN_ON_ONCE(size >= MTHP_STACK_SIZE);
> + stack->order = order;
> + stack->offset = offset;
> + (*stack_size)++;
> +}
> +
> +static struct mthp_range mthp_stack_pop(struct collapse_control *cc, int *stack_size)
> +{
> + const int size = *stack_size;
> +
> + VM_WARN_ON_ONCE(size <= 0);
> + (*stack_size)--;
> + return cc->mthp_bitmap_stack[size - 1];
> +}
> +
> +static unsigned int mthp_nr_occupied_pte_entries(struct collapse_control *cc,
> + u16 offset, unsigned long nr_pte_entries)

s/pte_entries/ptes/ ?

> +{
> + bitmap_zero(cc->mthp_bitmap_mask, HPAGE_PMD_NR);
> + bitmap_set(cc->mthp_bitmap_mask, offset, nr_pte_entries);
> + return bitmap_weight_and(cc->mthp_bitmap, cc->mthp_bitmap_mask, HPAGE_PMD_NR);
> +}
> +
> +/*
> + * 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_bitmap represents a single occupied (!none/zero) page.
> + * A stack structure cc->mthp_bitmap_stack is used to check different regions
> + * of the bitmap for collapse eligibility. The stack maintains a pair of
> + * variables (offset, order), indicating the number of PTEs from the start of
> + * the PMD, and the order of the potential collapse candidate respectively. We
> + * start at the PMD order and check if it is eligible for collapse; if not, we
> + * add two entries to the stack at a lower order to represent the left and right
> + * halves of the PTE page table we are examining.
> + *
> + * offset mid_offset
> + * | |
> + * | |
> + * v v
> + * --------------------------------------
> + * | cc->mthp_bitmap |
> + * --------------------------------------
> + * <-------><------->
> + * order-1 order-1
> + *
> + * 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 int mthp_collapse(struct mm_struct *mm, unsigned long address,
> + int referenced, int unmapped, struct collapse_control *cc,
> + bool *mmap_locked, unsigned long enabled_orders)
> +{
> + unsigned int max_ptes_none, nr_occupied_ptes;
> + struct mthp_range range;
> + unsigned long collapse_address;
> + int collapsed = 0, stack_size = 0;
> + unsigned long nr_pte_entries;

"nr_ptes" ? Any reason for that to be an unsigned long?

> + u16 offset;
> + u8 order;
> +
> + mthp_stack_push(cc, &stack_size, 0, HPAGE_PMD_ORDER);
> +
> + while (stack_size > 0) {
> + range = mthp_stack_pop(cc, &stack_size);
> + order = range.order;
> + offset = range.offset;
> + nr_pte_entries = 1UL << order;
> +
> + if (!test_bit(order, &enabled_orders))
> + goto next_order;
> +
> + if (cc->is_khugepaged)
> + max_ptes_none = collapse_max_ptes_none(order);
> + else
> + max_ptes_none = COLLAPSE_MAX_PTES_LIMIT;
> +
> + if (max_ptes_none == -EINVAL)
> + return collapsed;

With the previous suggested rework, you could likely make this

max_ptes_none = collapse_max_ptes_none(cc, NULL, order);
if (max_ptes_none < 0)
return collapsed;

> +
> + nr_occupied_ptes = mthp_nr_occupied_pte_entries(cc, offset, nr_pte_entries);
> +
> + if (nr_occupied_ptes >= nr_pte_entries - max_ptes_none) {
> + int ret;
> +
> + collapse_address = address + offset * PAGE_SIZE;
> + ret = collapse_huge_page(mm, collapse_address, referenced,
> + unmapped, cc, mmap_locked,
> + order);
> + if (ret == SCAN_SUCCEED) {
> + collapsed += nr_pte_entries;
> + continue;
> + }
> + }
> +
> +next_order:
> + if (order > KHUGEPAGED_MIN_MTHP_ORDER) {
> + const u8 next_order = order - 1;
> + const u16 mid_offset = offset + (nr_pte_entries / 2);
> +
> + mthp_stack_push(cc, &stack_size, mid_offset, next_order);
> + mthp_stack_push(cc, &stack_size, offset, next_order);
> + }
> + }
> + return collapsed;
> +}
> +
> static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> struct vm_area_struct *vma, unsigned long start_addr, bool *mmap_locked,
> unsigned int *cur_progress, struct collapse_control *cc)
> {
> pmd_t *pmd;
> pte_t *pte, *_pte;
> - int none_or_zero = 0, shared = 0, referenced = 0;
> + int i;
> + int none_or_zero = 0, shared = 0, nr_collapsed = 0, referenced = 0;
> enum scan_result result = SCAN_FAIL;
> struct page *page = NULL;
> + unsigned int max_ptes_none;
> struct folio *folio = NULL;
> unsigned long addr;
> + unsigned long enabled_orders;
> spinlock_t *ptl;
> int node = NUMA_NO_NODE, unmapped = 0;
>
> @@ -1384,8 +1536,21 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> goto out;
> }
>
> + bitmap_zero(cc->mthp_bitmap, HPAGE_PMD_NR);
> memset(cc->node_load, 0, sizeof(cc->node_load));
> nodes_clear(cc->alloc_nmask);
> +
> + enabled_orders = collapse_allowable_orders(vma, vma->vm_flags, cc->is_khugepaged);
> +
> + /*
> + * If PMD is the only enabled order, enforce max_ptes_none, otherwise
> + * scan all pages to populate the bitmap for mTHP collapse.
> + */
> + if (cc->is_khugepaged && enabled_orders == BIT(HPAGE_PMD_ORDER))
> + max_ptes_none = collapse_max_ptes_none(HPAGE_PMD_ORDER);
> + else
> + max_ptes_none = COLLAPSE_MAX_PTES_LIMIT;
> +

I assume that code to change as well. If you need help figuring out how
to make it work, please shout.

[...]

--
Cheers,

David