On Mon, 11 May 2026 12:58:04 -0600 Nico Pache <[email protected]> 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 possiblilty of "creep", while not modifying any uAPI > expectations. A warning will be emitted if any non-supported > max_ptes_none value is configured with mTHP enabled. > > 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] > > Co-developed-by: Dev Jain <[email protected]> > Signed-off-by: Dev Jain <[email protected]> > Signed-off-by: Nico Pache <[email protected]> > --- > include/trace/events/huge_memory.h | 3 +- > mm/khugepaged.c | 117 ++++++++++++++++++++--------- > 2 files changed, 85 insertions(+), 35 deletions(-) > > diff --git a/include/trace/events/huge_memory.h > b/include/trace/events/huge_memory.h > index bcdc57eea270..443e0bd13fdb 100644 > --- a/include/trace/events/huge_memory.h > +++ b/include/trace/events/huge_memory.h > @@ -39,7 +39,8 @@ > EM( SCAN_STORE_FAILED, "store_failed") \ > EM( SCAN_COPY_MC, "copy_poisoned_page") \ > EM( SCAN_PAGE_FILLED, "page_filled") \ > - EMe(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback") > + EM(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback") \ > + EMe(SCAN_INVALID_PTES_NONE, "invalid_ptes_none") > > #undef EM > #undef EMe > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index f68853b3caa7..27465161fa6d 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -61,6 +61,7 @@ enum scan_result { > SCAN_COPY_MC, > SCAN_PAGE_FILLED, > SCAN_PAGE_DIRTY_OR_WRITEBACK, > + SCAN_INVALID_PTES_NONE, > }; > > #define CREATE_TRACE_POINTS > @@ -353,37 +354,60 @@ static bool pte_none_or_zero(pte_t pte) > * PTEs 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 none-page or zero-page PTEs allowed for the > * collapse operation. > */ > -static unsigned int collapse_max_ptes_none(struct collapse_control *cc, > - struct vm_area_struct *vma) > +static int collapse_max_ptes_none(struct collapse_control *cc, > + struct vm_area_struct *vma, unsigned int order) > { > + unsigned int max_ptes_none = khugepaged_max_ptes_none; > // If the vma is userfaultfd-armed, allow no none-page or zero-page > PTEs. > if (vma && userfaultfd_armed(vma)) > return 0; > // for MADV_COLLAPSE, allow any none-page or zero-page PTEs. > if (!cc->is_khugepaged) > return HPAGE_PMD_NR; > - // For all other cases repect 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; > + /* Zero/non-present collapse disabled. */ > + if (!max_ptes_none) > + return 0; > + // 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; > + > + // We currently only support max_ptes_none values of 0 or > KHUGEPAGED_MAX_PTES_LIMIT. > + // Emit a warning and return -EINVAL. > + pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or > %u\n", > + KHUGEPAGED_MAX_PTES_LIMIT); > + return -EINVAL; > } > > /** > * 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 > // anonymous pages. > 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; > } > > @@ -391,16 +415,22 @@ 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 > // pagecache entries that are non-present. > 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; > } > > @@ -594,18 +624,22 @@ 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 long nr_pages = 1UL << order; > struct page *page = NULL; > struct folio *folio = NULL; > unsigned long addr = start_addr; > pte_t *_pte; > int none_or_zero = 0, shared = 0, referenced = 0; > enum scan_result result = SCAN_FAIL; > - unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma); > - unsigned int max_ptes_shared = collapse_max_ptes_shared(cc); > + int max_ptes_none = collapse_max_ptes_none(cc, vma, order); > + unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, order); > + > + if (max_ptes_none < 0) > + return SCAN_INVALID_PTES_NONE; > > - 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)) { > @@ -738,18 +772,18 @@ 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); > 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); > @@ -802,11 +836,10 @@ 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; > > /* > @@ -822,7 +855,7 @@ 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); > } > > /* > @@ -842,16 +875,17 @@ 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; > @@ -870,10 +904,10 @@ 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; > } > @@ -1044,12 +1078,12 @@ static enum scan_result check_pmd_still_valid(struct > mm_struct *mm, > * Returns result: if not SCAN_SUCCEED, mmap_lock has been released. > */ Can you add a comment above __collapse_huge_page_swapin function that says its only done for PMD size only? Something like: For PMD-order collapse this faults in any swap entries it finds. For mTHP orders the function bails on the first swap entry with SCAN_EXCEED_SWAP_PTE, because faulting pages back in during a lower-order collapse could re-populate PTEs that push a later scan over the threshold for a higher-order collapse. > 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; > @@ -1081,6 +1115,19 @@ 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); > @@ -1200,7 +1247,7 @@ 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; > } > @@ -1248,6 +1295,7 @@ 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 { > @@ -1278,6 +1326,7 @@ 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)) > @@ -1313,9 +1362,9 @@ 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 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; > @@ -2369,8 +2418,8 @@ 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 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 > >
