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: Lorenzo Stoakes <[email protected]> Acked-by: David Hildenbrand (arm) <[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]> --- mm/khugepaged.c | 126 +++++++++++++++++++++++++++++++++++------------- 1 file changed, 93 insertions(+), 33 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index f56ab049a6c4..474ee97c54ba 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -353,30 +353,51 @@ 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) { + const 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; + /* + * For mTHP collapse of values other than 0 or KHUGEPAGED_MAX_PTES_LIMIT, + * emit a warning and return 0. + */ + if (max_ptes_none) + 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 +405,13 @@ 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 +419,13 @@ 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 +433,10 @@ 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; } @@ -599,10 +633,11 @@ 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; @@ -610,7 +645,7 @@ 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)) { @@ -650,6 +685,12 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma, /* See collapse_scan_pmd(). */ if (folio_maybe_mapped_shared(folio)) { + /* + * TODO: Support shared pages without leading to further + * mTHP collapses. Currently bringing in new pages via + * shared may cause a future higher order collapse on a + * rescan of the same range. + */ if (++shared > max_ptes_shared) { result = SCAN_EXCEED_SHARED_PTE; count_vm_event(THP_SCAN_EXCEED_SHARED_PTE); @@ -743,18 +784,18 @@ 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 * nr_pages); 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); @@ -807,11 +848,10 @@ 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; /* @@ -827,7 +867,7 @@ 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); } /* @@ -847,16 +887,17 @@ 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; @@ -875,10 +916,10 @@ 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; } @@ -1051,16 +1092,20 @@ 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; @@ -1092,6 +1137,19 @@ 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); @@ -1212,7 +1270,7 @@ 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; } @@ -1260,6 +1318,7 @@ 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 { @@ -1290,6 +1349,7 @@ 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)) @@ -1325,9 +1385,9 @@ 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; @@ -2381,8 +2441,8 @@ 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
