diff options
| author | Nico Pache <npache@redhat.com> | 2026-05-22 08:59:59 -0600 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2026-05-28 21:31:33 -0700 |
| commit | 5f2f3b76b10f8d62bd1932b6f4755d3417519ffd (patch) | |
| tree | ebf962f14d4138e3af2acaa2def4a3ed5172c98b /mm | |
| parent | 102ae3e16b6b3c5f770a110d0d61292a71db4abd (diff) | |
| download | linux-next-history-5f2f3b76b10f8d62bd1932b6f4755d3417519ffd.tar.gz | |
mm/khugepaged: generalize __collapse_huge_page_* for mTHP support
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.
Link: https://lore.kernel.org/20260522150009.121603-5-npache@redhat.com
Link: https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com [1]
Link: https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com [2]
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rafael Aquini <raquini@redhat.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shivank Garg <shivankg@amd.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Takashi Iwai (SUSE) <tiwai@suse.de>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/khugepaged.c | 121 |
1 files changed, 88 insertions, 33 deletions
diff --git a/mm/khugepaged.c b/mm/khugepaged.c index f7ebb4fe744be..702e1d47da859 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; } @@ -599,10 +634,11 @@ 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 +646,7 @@ 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)) { @@ -743,18 +779,18 @@ out: } 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); @@ -807,11 +843,10 @@ 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 +862,7 @@ 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 +882,17 @@ 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 +911,10 @@ 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; } @@ -1045,16 +1081,20 @@ 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; @@ -1086,6 +1126,19 @@ 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); @@ -1206,7 +1259,7 @@ 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; } @@ -1254,6 +1307,7 @@ 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 { @@ -1284,6 +1338,7 @@ 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)) @@ -1319,9 +1374,9 @@ 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; @@ -2375,8 +2430,8 @@ 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); |