On Fri, May 22, 2026 at 08:59:59AM -0600, Nico Pache wrote: > generalize the order of the __collapse_huge_page_* and collapse_max_* > functions to support future mTHP collapse. > > The current mechanism for determining collapse with the > khugepaged_max_ptes_none value is not designed with mTHP in mind. This > raises a key design issue: if we support user defined max_pte_none values > (even those scaled by order), a collapse of a lower order can introduces > an feedback loop, or "creep", when max_ptes_none is set to a value greater > than HPAGE_PMD_NR / 2. [1] > > With this configuration, a successful collapse to order N will populate > enough pages to satisfy the collapse condition on order N+1 on the next > scan. This leads to unnecessary work and memory churn. > > To fix this issue introduce a helper function that will limit mTHP > collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1. > This effectively supports two modes: [2] > > - max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE > that maps the shared zeropage. Consequently, no memory bloat. > - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest > available mTHP order. > > This removes the possibility of "creep", and a warning will be emitted if > any non-supported max_ptes_none value is configured with mTHP enabled. > Any intermediate value will default mTHP collapse to max_ptes_none=0. > > mTHP collapse will not honor the khugepaged_max_ptes_shared or > khugepaged_max_ptes_swap parameters, and will fail if it encounters a > shared or swapped entry. > > No functional changes in this patch; however it defines future behavior > for mTHP collapse. > > [1] - https://lore.kernel.org/all/[email protected] > [2] - > https://lore.kernel.org/all/[email protected] > > Reviewed-by: Lance Yang <[email protected]> > Co-developed-by: Dev Jain <[email protected]> > Signed-off-by: Dev Jain <[email protected]> > Signed-off-by: Nico Pache <[email protected]>
One small nit below, and with the fixpatch assumed applied, LGTM so: Reviewed-by: Lorenzo Stoakes <[email protected]> > --- > mm/khugepaged.c | 121 +++++++++++++++++++++++++++++++++++------------- > 1 file changed, 88 insertions(+), 33 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 116f39518948..e98ba5b15163 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -353,30 +353,52 @@ static bool pte_none_or_zero(pte_t pte) > * the shared zeropage for the given collapse operation. > * @cc: The collapse control struct > * @vma: The vma to check for userfaultfd > + * @order: The folio order being collapsed to > * > * Return: Maximum number of empty/shared zeropage PTEs for the collapse > operation > */ > static unsigned int collapse_max_ptes_none(struct collapse_control *cc, > - struct vm_area_struct *vma) > + struct vm_area_struct *vma, unsigned int order) > { > + unsigned int max_ptes_none = khugepaged_max_ptes_none; > + > if (vma && userfaultfd_armed(vma)) > return 0; > /* for MADV_COLLAPSE, allow any empty/shared zeropage PTEs */ > if (!cc->is_khugepaged) > return HPAGE_PMD_NR; > - /* For all other cases respect the user defined maximum */ > - return khugepaged_max_ptes_none; > + /* for PMD collapse, respect the user defined maximum */ > + if (is_pmd_order(order)) > + return max_ptes_none; > + /* > + * for mTHP collapse with the sysctl value set to > KHUGEPAGED_MAX_PTES_LIMIT, > + * scale the maximum number of PTEs to the order of the collapse. > + */ > + if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT) > + return (1 << order) - 1; > + if (!max_ptes_none) > + return 0; > + /* > + * For mTHP collapse of values other than 0 or > KHUGEPAGED_MAX_PTES_LIMIT, > + * emit a warning and return 0. > + */ > + pr_warn_once("mTHP collapse does not support max_ptes_none values" > + " other than 0 or %u, defaulting to 0.\n", > + KHUGEPAGED_MAX_PTES_LIMIT); > + return 0; > } > > /** > * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared > * anonymous pages for the given collapse operation. > * @cc: The collapse control struct > + * @order: The folio order being collapsed to > * > * Return: Maximum number of PTEs that map shared anonymous pages for the > * collapse operation > */ > -static unsigned int collapse_max_ptes_shared(struct collapse_control *cc) > +static unsigned int collapse_max_ptes_shared(struct collapse_control *cc, > + unsigned int order) > { > /* > * For MADV_COLLAPSE, do not restrict the number of PTEs that map shared > @@ -384,6 +406,13 @@ static unsigned int collapse_max_ptes_shared(struct > collapse_control *cc) > */ > if (!cc->is_khugepaged) > return HPAGE_PMD_NR; > + /* > + * for mTHP collapse do not allow collapsing anonymous memory pages that > + * are shared between processes. > + */ > + if (!is_pmd_order(order)) > + return 0; > + /* for PMD collapse, respect the user defined maximum */ > return khugepaged_max_ptes_shared; > } > > @@ -391,11 +420,13 @@ static unsigned int collapse_max_ptes_shared(struct > collapse_control *cc) > * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs > or the > * maximum allowed non-present pagecache entries for the given collapse > operation. > * @cc: The collapse control struct > + * @order: The folio order being collapsed to > * > * Return: Maximum number of non-present PTEs or the maximum allowed > non-present > * pagecache entries for the collapse operation. > */ > -static unsigned int collapse_max_ptes_swap(struct collapse_control *cc) > +static unsigned int collapse_max_ptes_swap(struct collapse_control *cc, > + unsigned int order) > { > /* > * For MADV_COLLAPSE, do not restrict the number PTEs entries or > @@ -403,6 +434,10 @@ static unsigned int collapse_max_ptes_swap(struct > collapse_control *cc) > */ > if (!cc->is_khugepaged) > return HPAGE_PMD_NR; > + /* for mTHP collapse do not allow any non-present PTEs or pagecache > entries */ > + if (!is_pmd_order(order)) > + return 0; > + /* for PMD collapse, respect the user defined maximum */ > return khugepaged_max_ptes_swap; > } > > @@ -596,10 +631,11 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte, > > static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct > *vma, > unsigned long start_addr, pte_t *pte, struct collapse_control > *cc, > - struct list_head *compound_pagelist) > + unsigned int order, struct list_head *compound_pagelist) > { > - const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma); > - const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc); > + const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, > order); > + const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, > order); > + const unsigned long nr_pages = 1UL << order; > struct page *page = NULL; > struct folio *folio = NULL; > unsigned long addr = start_addr; > @@ -607,7 +643,7 @@ static enum scan_result > __collapse_huge_page_isolate(struct vm_area_struct *vma, > int none_or_zero = 0, shared = 0, referenced = 0; > enum scan_result result = SCAN_FAIL; > > - for (_pte = pte; _pte < pte + HPAGE_PMD_NR; > + for (_pte = pte; _pte < pte + nr_pages; > _pte++, addr += PAGE_SIZE) { > pte_t pteval = ptep_get(_pte); > if (pte_none_or_zero(pteval)) { > @@ -740,18 +776,18 @@ static enum scan_result > __collapse_huge_page_isolate(struct vm_area_struct *vma, > } > > static void __collapse_huge_page_copy_succeeded(pte_t *pte, > - struct vm_area_struct *vma, > - unsigned long address, > - spinlock_t *ptl, > - struct list_head > *compound_pagelist) > + struct vm_area_struct *vma, unsigned long address, > + spinlock_t *ptl, unsigned int order, > + struct list_head *compound_pagelist) > { > - unsigned long end = address + HPAGE_PMD_SIZE; > + const unsigned long nr_pages = 1UL << order; > + unsigned long end = address + (PAGE_SIZE << order); Nit: could be address + PAGE_SIZE * nr_pages, be a little nicer as you already did the << order operation above. > struct folio *src, *tmp; > pte_t pteval; > pte_t *_pte; > unsigned int nr_ptes; > > - for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes, > + for (_pte = pte; _pte < pte + nr_pages; _pte += nr_ptes, > address += nr_ptes * PAGE_SIZE) { > nr_ptes = 1; > pteval = ptep_get(_pte); > @@ -804,11 +840,10 @@ static void __collapse_huge_page_copy_succeeded(pte_t > *pte, > } > > static void __collapse_huge_page_copy_failed(pte_t *pte, > - pmd_t *pmd, > - pmd_t orig_pmd, > - struct vm_area_struct *vma, > - struct list_head > *compound_pagelist) > + pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma, > + unsigned int order, struct list_head *compound_pagelist) > { > + const unsigned long nr_pages = 1UL << order; > spinlock_t *pmd_ptl; > > /* > @@ -824,7 +859,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte, > * Release both raw and compound pages isolated > * in __collapse_huge_page_isolate. > */ > - release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist); > + release_pte_pages(pte, pte + nr_pages, compound_pagelist); > } > > /* > @@ -844,16 +879,17 @@ static void __collapse_huge_page_copy_failed(pte_t *pte, > */ > static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio > *folio, > pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma, > - unsigned long address, spinlock_t *ptl, > + unsigned long address, spinlock_t *ptl, unsigned int order, > struct list_head *compound_pagelist) > { > + const unsigned long nr_pages = 1UL << order; > unsigned int i; > enum scan_result result = SCAN_SUCCEED; > > /* > * Copying pages' contents is subject to memory poison at any iteration. > */ > - for (i = 0; i < HPAGE_PMD_NR; i++) { > + for (i = 0; i < nr_pages; i++) { > pte_t pteval = ptep_get(pte + i); > struct page *page = folio_page(folio, i); > unsigned long src_addr = address + i * PAGE_SIZE; > @@ -872,10 +908,10 @@ static enum scan_result __collapse_huge_page_copy(pte_t > *pte, struct folio *foli > > if (likely(result == SCAN_SUCCEED)) > __collapse_huge_page_copy_succeeded(pte, vma, address, ptl, > - compound_pagelist); > + order, compound_pagelist); > else > __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma, > - compound_pagelist); > + order, compound_pagelist); > > return result; > } > @@ -1042,16 +1078,20 @@ static enum scan_result check_pmd_still_valid(struct > mm_struct *mm, > * Bring missing pages in from swap, to complete THP collapse. > * Only done if khugepaged_scan_pmd believes it is worthwhile. > * > + * For mTHP orders the function bails on the first swap entry, because > + * faulting pages back in during collapse could re-populate PTEs that > + * push a later scan over the threshold for a higher-order collapse. > + * > * Called and returns without pte mapped or spinlocks held. > * Returns result: if not SCAN_SUCCEED, mmap_lock has been released. > */ > static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm, > - struct vm_area_struct *vma, unsigned long start_addr, pmd_t > *pmd, > - int referenced) > + struct vm_area_struct *vma, unsigned long start_addr, > + pmd_t *pmd, int referenced, unsigned int order) > { > int swapped_in = 0; > vm_fault_t ret = 0; > - unsigned long addr, end = start_addr + (HPAGE_PMD_NR * PAGE_SIZE); > + unsigned long addr, end = start_addr + (PAGE_SIZE << order); > enum scan_result result; > pte_t *pte = NULL; > spinlock_t *ptl; > @@ -1083,6 +1123,19 @@ static enum scan_result > __collapse_huge_page_swapin(struct mm_struct *mm, > pte_present(vmf.orig_pte)) > continue; > > + /* > + * TODO: Support swapin without leading to further mTHP > + * collapses. Currently bringing in new pages via swapin may > + * cause a future higher order collapse on a rescan of the same > + * range. > + */ > + if (!is_pmd_order(order)) { > + pte_unmap(pte); > + mmap_read_unlock(mm); > + result = SCAN_EXCEED_SWAP_PTE; > + goto out; > + } > + > vmf.pte = pte; > vmf.ptl = ptl; > ret = do_swap_page(&vmf); > @@ -1203,7 +1256,7 @@ static enum scan_result collapse_huge_page(struct > mm_struct *mm, unsigned long a > * that case. Continuing to collapse causes inconsistency. > */ > result = __collapse_huge_page_swapin(mm, vma, address, pmd, > - referenced); > + referenced, > HPAGE_PMD_ORDER); > if (result != SCAN_SUCCEED) > goto out_nolock; > } > @@ -1251,6 +1304,7 @@ static enum scan_result collapse_huge_page(struct > mm_struct *mm, unsigned long a > pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); > if (pte) { > result = __collapse_huge_page_isolate(vma, address, pte, cc, > + HPAGE_PMD_ORDER, > &compound_pagelist); > spin_unlock(pte_ptl); > } else { > @@ -1281,6 +1335,7 @@ static enum scan_result collapse_huge_page(struct > mm_struct *mm, unsigned long a > > result = __collapse_huge_page_copy(pte, folio, pmd, _pmd, > vma, address, pte_ptl, > + HPAGE_PMD_ORDER, > &compound_pagelist); > pte_unmap(pte); > if (unlikely(result != SCAN_SUCCEED)) > @@ -1316,9 +1371,9 @@ 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); > - const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc); > - const unsigned int max_ptes_swap = collapse_max_ptes_swap(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); > pmd_t *pmd; > pte_t *pte, *_pte; > int none_or_zero = 0, shared = 0, referenced = 0; > @@ -2372,8 +2427,8 @@ static enum scan_result collapse_scan_file(struct > mm_struct *mm, > unsigned long addr, struct file *file, pgoff_t start, > struct collapse_control *cc) > { > - const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL); > - const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc); > + const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL, > HPAGE_PMD_ORDER); > + const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, > HPAGE_PMD_ORDER); > struct folio *folio = NULL; > struct address_space *mapping = file->f_mapping; > XA_STATE(xas, &mapping->i_pages, start); > -- > 2.54.0 >
