Re: [PATCH mm-unstable v17 11/14] mm/khugepaged: Introduce mTHP collapse support

[Wei Yang <richard.weiyang@gmail.com>]

On Thu, May 21, 2026 at 10:36:15AM +0800, Vernon Yang wrote:
>On Mon, May 11, 2026 at 12:58:11PM -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 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, 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 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. This also
>> prevents a traditional recursive approach when the kernel stack struct is
>> limited. 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
>>
>> 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 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.
>>
>> 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 <npache@redhat.com>
>> ---
>>  mm/khugepaged.c | 182 +++++++++++++++++++++++++++++++++++++++++++++---
>>  1 file changed, 174 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 3492b135d667..39bf7ea8a6e8 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -100,6 +100,30 @@ 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
>> +/*
>> + * mthp_collapse() does an iterative DFS over a binary tree, from
>> + * HPAGE_PMD_ORDER down to KHUGEPAGED_MIN_MTHP_ORDER. The max stack
>> + * size needed for a DFS on a binary tree is height + 1, where
>> + * height = HPAGE_PMD_ORDER - KHUGEPAGED_MIN_MTHP_ORDER.
>> + *
>> + * ilog2 is used in place of HPAGE_PMD_ORDER because some architectures
>> + * (e.g. ppc64le) do not define HPAGE_PMD_ORDER until after build time.
>> + */
>> +#define MTHP_STACK_SIZE	(ilog2(MAX_PTRS_PER_PTE) - KHUGEPAGED_MIN_MTHP_ORDER + 1)
>> +
>> +/*
>> + * Defines a range of PTE entries in a PTE page table which are being
>> + * considered for mTHP collapse.
>> + *
>> + * @offset: the offset of the first PTE entry in a PMD range.
>> + * @order: the order of the PTE entries being considered for collapse.
>> + */
>> +struct mthp_range {
>> +	u16 offset;
>> +	u8 order;
>> +};
>> +
>>  struct collapse_control {
>>  	bool is_khugepaged;
>>
>> @@ -111,6 +135,12 @@ struct collapse_control {
>>
>>  	/* nodemask for allocation fallback */
>>  	nodemask_t alloc_nmask;
>> +
>> +	/* Each bit represents a single occupied (!none/zero) page. */
>> +	DECLARE_BITMAP(mthp_bitmap, MAX_PTRS_PER_PTE);
>> +	/* A mask of the current range being considered for mTHP collapse. */
>> +	DECLARE_BITMAP(mthp_bitmap_mask, MAX_PTRS_PER_PTE);
>> +	struct mthp_range mthp_bitmap_stack[MTHP_STACK_SIZE];
>>  };
>>
>>  /**
>> @@ -1404,20 +1434,140 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
>>  	return result;
>>  }
>>
>> +static void collapse_mthp_stack_push(struct collapse_control *cc, int *stack_size,
>> +				     u16 offset, u8 order)
>> +{
>> +	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 collapse_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 collapse_mthp_count_present(struct collapse_control *cc,
>> +						u16 offset, unsigned int nr_ptes)
>> +{
>> +	bitmap_zero(cc->mthp_bitmap_mask, MAX_PTRS_PER_PTE);
>> +	bitmap_set(cc->mthp_bitmap_mask, offset, nr_ptes);
>> +	return bitmap_weight_and(cc->mthp_bitmap, cc->mthp_bitmap_mask, MAX_PTRS_PER_PTE);
>> +}
>> +
>> +/*
>> + * 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,
>> +		unsigned long enabled_orders)
>> +{
>> +	unsigned int nr_occupied_ptes, nr_ptes;
>> +	int max_ptes_none, collapsed = 0, stack_size = 0;
>> +	unsigned long collapse_address;
>> +	struct mthp_range range;
>> +	u16 offset;
>> +	u8 order;
>> +
>> +	collapse_mthp_stack_push(cc, &stack_size, 0, HPAGE_PMD_ORDER);
>> +
>> +	while (stack_size) {
>> +		range = collapse_mthp_stack_pop(cc, &stack_size);
>> +		order = range.order;
>> +		offset = range.offset;
>> +		nr_ptes = 1UL << order;
>> +
>> +		if (!test_bit(order, &enabled_orders))
>> +			goto next_order;
>> +
>> +		max_ptes_none = collapse_max_ptes_none(cc, NULL, order);
>> +
>> +		if (max_ptes_none < 0)
>> +			return collapsed;
>> +
>> +		nr_occupied_ptes = collapse_mthp_count_present(cc, offset,
>> +							       nr_ptes);
>> +
>> +		if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
>> +			int ret;
>> +
>> +			collapse_address = address + offset * PAGE_SIZE;
>> +			ret = collapse_huge_page(mm, collapse_address, referenced,
>> +						 unmapped, cc, order);
>> +			if (ret == SCAN_SUCCEED) {
>> +				collapsed += nr_ptes;
>> +				continue;
>> +			}
>> +		}
>> +
>> +next_order:
>> +		if (order > KHUGEPAGED_MIN_MTHP_ORDER) {
>
>Hi Nico, thank you very much for your contributions to this series.
>
>I found a minor issue, for MADV_COLLAPSE, if collapse_huge_page() fails
>for some reason (e.g. allocate folio), it goes to next_order and
>continues splitting to the next small order. However, enabled_orders
>only supports HPAGE_PMD_ORDER, so it keeps runing the split operations
>without any effective work until KHUGEPAGED_MIN_MTHP_ORDER is reached
>before exiting. For khugepaged, e.g. setting only 2MB to always, also
>same phenomenon.

Yes, but it does no actual work since it is checked after pop up.

>
>This does not affect the overall functionality of mthp collapse, just
>redundant.
>
>The redundant operations can be easily skipped with the following
>modification. If I miss some thing, please let me know. Thanks!
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index 1a25af3d6d0f..fa407cce525c 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -1574,7 +1574,7 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address,
> 		}
>
> next_order:
>-		if (order > KHUGEPAGED_MIN_MTHP_ORDER) {
>+		if ((BIT(order) - 1) & enabled_orders) {
> 			const u8 next_order = order - 1;
> 			const u16 mid_offset = offset + (nr_ptes / 2);
>

This would stop the iteration if there are other lower enabled order, right?

>Cheers,
>Vernon

-- 
Wei Yang
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