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/20260605161422.213817-5-npache@redhat.com
Link: https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com
Link: https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com
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: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
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: "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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 69d34305b21734847880cf12ba42fe2da1a28bf6..1075dadb9de3ca75ac5e42fbc05e22b5f699cc8f 100644 (file)
--- 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 @@ 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 * 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);
@@ -1217,7 +1275,7 @@ 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;
        }
@@ -1265,6 +1323,7 @@ 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 {
@@ -1295,6 +1354,7 @@ 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))
@@ -1330,9 +1390,9 @@ 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;
@@ -2386,8 +2446,8 @@ 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);