On Thu, May 21, 2026 at 01:11:18PM +0800, Vernon Yang wrote: >On Thu, May 21, 2026 at 02:46:54AM +0000, Wei Yang wrote: >> 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 <[email protected]> >> >> --- >> >> 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? > ^^^^ ^^^^^^^^^^^^^^^^^^^ > >NO :)
Got it. You are right. The logic here is all lower bits are not set, skip the rest. > >For more details, please refer to the following information. > >| Scenario | Old Behavior (order > 2) | New >Behavior ((BIT(order)-1) & enabled_orders) | >|-------------------------------------|--------------------------|------------------------------------------------| >| MADV_COLLAPSE | Splits 9,8,7,...,3 | No split > | >| khugepaged, only 2MB enabled | Splits 9,8,7,...,3 | No split > | >| khugepaged, only 2MB + 64KB enabled | Splits 9,8,7,...,3 | Splits >9,8,7,...,5 | >| khugepaged, only 32KB enabled | Splits 9,8,7,...,3 | Splits >9,8,7,...,4 | >| khugepaged, only 16KB enabled | Splits 9,8,7,...,3 | Splits >9,8,7,...,3 | >| khugepaged, all mTHP enabled | Splits 9,8,7,...,3 | Splits >9,8,7,...,3 | > >-- >Cheers, >Vernon -- Wei Yang Help you, Help me
