On Sun, 19 Apr 2026 12:57:41 -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. > > 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(-) > Small nits. Most might not need change. > 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; > + There are 2 reads of khugepaged_max_ptes_none here. A concurrent sysctl write between reads can yield "0 then non-zero" or "LIMIT then mid-value". Would be good to just snapshot once at the start of the function and use that value? > + pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or > %u\n", > + KHUGEPAGED_MAX_PTES_LIMIT); IMO, warn_once can get lost quickly in dmesg. Maybe pr_warn_ratelimited? Not sure what others opinions are.. > + 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; Would a dedicated SCAN_INVALID_PTES_NONE make more sense here instead of 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)) { > @@ -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; > - Shouldn't remove the newline above? > /* > * 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; > - Same here? > /* > * 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) Will probably find out in later reviews, but there is tracepoint in __collapse_huge_page_swapin. Would be good to add order in that tracepoint if you are adding order here? > { > 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)) { Would it be good to introduce this in the patch that activates it? No strong preference btw. Just that its dead code in this patch itself. > + 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 > >
