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. 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: - max_ptes_none=0: never introduce new none-pages for mTHP collapse. - 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. Co-developed-by: Dev Jain <[email protected]> Signed-off-by: Dev Jain <[email protected]> Signed-off-by: Nico Pache <[email protected]> --- mm/khugepaged.c | 124 ++++++++++++++++++++++++++++++++++-------------- 1 file changed, 88 insertions(+), 36 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index f42b55421191..283bb63854a5 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -352,51 +352,86 @@ static bool pte_none_or_zero(pte_t pte) * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse * @cc: The collapse control struct * @vma: The vma to check for userfaultfd + * @order: The folio order being collapsed to * * If we are not in khugepaged mode use HPAGE_PMD_NR to allow any - * empty page. + * empty page. For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the + * configured khugepaged_max_ptes_none value. + * + * For mTHP collapses, we currently only support khugepaged_max_pte_none values + * of 0 or (KHUGEPAGED_MAX_PTES_LIMIT). Any other value will emit a warning and + * no mTHP collapse will be attempted * * Return: Maximum number of empty 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) { if (vma && userfaultfd_armed(vma)) return 0; if (!cc->is_khugepaged) return HPAGE_PMD_NR; - return khugepaged_max_ptes_none; + if (is_pmd_order(order)) + return khugepaged_max_ptes_none; + /* Zero/non-present collapse disabled. */ + if (!khugepaged_max_ptes_none) + return 0; + if (khugepaged_max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT) + return (1 << order) - 1; + + 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 shared PTEs for collapse * @cc: The collapse control struct + * @order: The folio order being collapsed to * * If we are not in khugepaged mode use HPAGE_PMD_NR to allow any * shared page. * + * For mTHP collapses, we currently dont support collapsing memory with + * shared memory. + * * Return: Maximum number of shared PTEs allowed 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) { if (!cc->is_khugepaged) return HPAGE_PMD_NR; + if (!is_pmd_order(order)) + return 0; + return khugepaged_max_ptes_shared; } /** * collapse_max_ptes_swap - Calculate maximum allowed swap PTEs for collapse * @cc: The collapse control struct + * @order: The folio order being collapsed to * * If we are not in khugepaged mode use HPAGE_PMD_NR to allow any * swap page. * + * For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the configured + * khugepaged_max_ptes_swap value. + * + * For mTHP collapses, we currently dont support collapsing memory with + * swapped out memory. + * * Return: Maximum number of swap PTEs allowed 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) { if (!cc->is_khugepaged) return HPAGE_PMD_NR; + if (!is_pmd_order(order)) + return 0; + return khugepaged_max_ptes_swap; } @@ -590,18 +625,22 @@ 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 result; - 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)) { @@ -734,18 +773,18 @@ 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); @@ -798,13 +837,11 @@ 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; - /* * Re-establish the PMD to point to the original page table * entry. Restoring PMD needs to be done prior to releasing @@ -818,7 +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); } /* @@ -838,16 +875,16 @@ 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; @@ -866,10 +903,10 @@ 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; } @@ -1040,12 +1077,12 @@ static enum scan_result check_pmd_still_valid(struct mm_struct *mm, * 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; @@ -1077,6 +1114,19 @@ 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); @@ -1196,7 +1246,7 @@ 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; } @@ -1244,6 +1294,7 @@ 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 { @@ -1274,6 +1325,7 @@ 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)) @@ -1318,9 +1370,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, unsigned long addr; spinlock_t *ptl; int node = NUMA_NO_NODE, unmapped = 0; - unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma); - unsigned int max_ptes_shared = collapse_max_ptes_shared(cc); - unsigned int max_ptes_swap = collapse_max_ptes_swap(cc); + int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER); + unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER); + unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER); VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK); @@ -2371,8 +2423,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, int present, swap; int node = NUMA_NO_NODE; enum scan_result result = SCAN_SUCCEED; - unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL); - unsigned int max_ptes_swap = collapse_max_ptes_swap(cc); + int max_ptes_none = collapse_max_ptes_none(cc, NULL, HPAGE_PMD_ORDER); + unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER); present = 0; swap = 0; -- 2.53.0
