On Fri, Jun 5, 2026 at 5:07 AM Nico Pache <[email protected]> wrote: > > On Thu, Jun 4, 2026 at 8:45 AM Lorenzo Stoakes <[email protected]> wrote: > > > > On Fri, May 22, 2026 at 09:00:06AM -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 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 | 181 +++++++++++++++++++++++++++++++++++++++++++++--- > > > 1 file changed, 172 insertions(+), 9 deletions(-) > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > > index 64ceebc9d8a7..d3d7db8be26c 100644 > > > --- a/mm/khugepaged.c > > > +++ b/mm/khugepaged.c > > > @@ -99,6 +99,30 @@ 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; > > > > > > @@ -110,6 +134,12 @@ 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]; > > > }; > > > > > > /** > > > @@ -1411,20 +1441,137 @@ 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, struct vm_area_struct > > > *vma, > > > + 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; > > > + int 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, vma, order); > > > + > > > + 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 ((BIT(order) - 1) & enabled_orders) { > > > + const u8 next_order = order - 1; > > > + const u16 mid_offset = offset + (nr_ptes / 2); > > > + > > > + collapse_mthp_stack_push(cc, &stack_size, > > > mid_offset, > > > + next_order); > > > + collapse_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 *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; > > > - int none_or_zero = 0, shared = 0, referenced = 0; > > > + pte_t *pte, *_pte, pteval; > > > + int i; > > > + int none_or_zero = 0, shared = 0, nr_collapsed = 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; > > > > > > @@ -1436,8 +1583,19 @@ static enum scan_result collapse_scan_pmd(struct > > > mm_struct *mm, > > > goto out; > > > } > > > > > > + bitmap_zero(cc->mthp_bitmap, MAX_PTRS_PER_PTE); > > > memset(cc->node_load, 0, sizeof(cc->node_load)); > > > nodes_clear(cc->alloc_nmask); > > > + > > > + enabled_orders = collapse_allowable_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; > > > > Hmm, this is a bit odd, what if the user set max_ptes_none = 0? > > We'd still want to scan the full PMD to populate the bitmap. That way > we can find the smaller orders that contain 0 none/zero PTEs. > > > > > I assume we handle the 0/511 thing elsewhere? > > Yes in the bitmap weight check and in collapse_huge_page_isolate() > > > > > > + > > > pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl); > > > if (!pte) { > > > cc->progress++; > > > @@ -1445,11 +1603,13 @@ 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; > > > @@ -1529,6 +1689,8 @@ static enum scan_result collapse_scan_pmd(struct > > > mm_struct *mm, > > > } > > > } > > > > > > + /* Set bit for occupied pages */ > > > + __set_bit(i, cc->mthp_bitmap); > > > /* > > > * Record which node the original page is from and save this > > > * information to cc->node_load[]. > > > @@ -1587,10 +1749,11 @@ 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 */ > > > + nr_collapsed = mthp_collapse(mm, vma, start_addr, > > > referenced, > > > + unmapped, cc, enabled_orders); > > > > I guess mthp_collapse() also does PMD collapse if only PMD is enabled? > > > > It feels like this name is a bit confusing then :) > > > > But I guess we can do a follow up to think of a better name possibly.
Yeah, ideally we can clean that up later! Thank you so much for reviewing and verifying the new algorithm. The diff i sent was just a draft-- I have already added comments and cleaned up the code more. Cheers :) -- Nico > > > > > + /* mmap_lock was released above, set lock_dropped */ > > > *lock_dropped = true; > > > + result = nr_collapsed ? SCAN_SUCCEED : SCAN_FAIL; > > > } > > > out: > > > trace_mm_khugepaged_scan_pmd(mm, folio, referenced, > > > -- > > > 2.54.0 > > > > > > > Thanks, Lorenzo > >
