[PATCH v2 07/12] mm/khugepaged: add flag to ignore khugepaged_max_ptes_*

["Zach O'Keefe" <zokeefe@xxxxxxxxxx>]

Add enforce_pte_scan_limits flag to struct collapse_control that allows
context to ignore sysfs-controlled knobs:
khugepaged_max_ptes_[none|swap|shared].  Set this flag in khugepaged
collapse context to preserve existing khugepaged behavior and unset the
flag in madvise collapse context since the user presumably has reason to
believe the collapse will be beneficial.

Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>
---
 mm/khugepaged.c | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 716ba465b356..2f95f60431aa 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -87,6 +87,9 @@ static struct kmem_cache *mm_slot_cache __read_mostly;
 #define MAX_PTE_MAPPED_THP 8
 
 struct collapse_control {
+	/* Respect khugepaged_max_ptes_[none|swap|shared] */
+	bool enforce_pte_scan_limits;
+
 	/* Num pages scanned per node */
 	int node_load[MAX_NUMNODES];
 
@@ -631,6 +634,7 @@ static bool is_refcount_suitable(struct page *page)
 static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 					unsigned long address,
 					pte_t *pte,
+					struct collapse_control *cc,
 					struct list_head *compound_pagelist)
 {
 	struct page *page = NULL;
@@ -644,7 +648,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		if (pte_none(pteval) || (pte_present(pteval) &&
 				is_zero_pfn(pte_pfn(pteval)))) {
 			if (!userfaultfd_armed(vma) &&
-			    ++none_or_zero <= khugepaged_max_ptes_none) {
+			    (++none_or_zero <= khugepaged_max_ptes_none ||
+			     !cc->enforce_pte_scan_limits)) {
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -664,8 +669,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 		VM_BUG_ON_PAGE(!PageAnon(page), page);
 
-		if (page_mapcount(page) > 1 &&
-				++shared > khugepaged_max_ptes_shared) {
+		if (cc->enforce_pte_scan_limits && page_mapcount(page) > 1 &&
+		    ++shared > khugepaged_max_ptes_shared) {
 			result = SCAN_EXCEED_SHARED_PTE;
 			count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 			goto out;
@@ -1207,7 +1212,7 @@ static void collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	mmu_notifier_invalidate_range_end(&range);
 
 	spin_lock(pte_ptl);
-	cr->result =  __collapse_huge_page_isolate(vma, address, pte,
+	cr->result =  __collapse_huge_page_isolate(vma, address, pte, cc,
 						   &compound_pagelist);
 	spin_unlock(pte_ptl);
 
@@ -1296,7 +1301,8 @@ static void scan_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
 	     _pte++, _address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
 		if (is_swap_pte(pteval)) {
-			if (++unmapped <= khugepaged_max_ptes_swap) {
+			if (++unmapped <= khugepaged_max_ptes_swap ||
+			    !cc->enforce_pte_scan_limits) {
 				/*
 				 * Always be strict with uffd-wp
 				 * enabled swap entries.  Please see
@@ -1315,7 +1321,8 @@ static void scan_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
 		}
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
 			if (!userfaultfd_armed(vma) &&
-			    ++none_or_zero <= khugepaged_max_ptes_none) {
+			    (++none_or_zero <= khugepaged_max_ptes_none ||
+			     !cc->enforce_pte_scan_limits)) {
 				continue;
 			} else {
 				cr->result = SCAN_EXCEED_NONE_PTE;
@@ -1345,8 +1352,9 @@ static void scan_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
 			goto out_unmap;
 		}
 
-		if (page_mapcount(page) > 1 &&
-				++shared > khugepaged_max_ptes_shared) {
+		if (cc->enforce_pte_scan_limits &&
+		    page_mapcount(page) > 1 &&
+		    ++shared > khugepaged_max_ptes_shared) {
 			cr->result = SCAN_EXCEED_SHARED_PTE;
 			count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 			goto out_unmap;
@@ -2086,7 +2094,8 @@ static void khugepaged_scan_file(struct mm_struct *mm,
 			continue;
 
 		if (xa_is_value(page)) {
-			if (++swap > khugepaged_max_ptes_swap) {
+			if (cc->enforce_pte_scan_limits &&
+			    ++swap > khugepaged_max_ptes_swap) {
 				cr->result = SCAN_EXCEED_SWAP_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
 				break;
@@ -2137,7 +2146,8 @@ static void khugepaged_scan_file(struct mm_struct *mm,
 	rcu_read_unlock();
 
 	if (cr->result == SCAN_SUCCEED) {
-		if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
+		if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none &&
+		    cc->enforce_pte_scan_limits) {
 			cr->result = SCAN_EXCEED_NONE_PTE;
 			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 		} else {
@@ -2364,6 +2374,7 @@ static int khugepaged(void *none)
 {
 	struct mm_slot *mm_slot;
 	struct collapse_control cc = {
+		.enforce_pte_scan_limits = true,
 		.last_target_node = NUMA_NO_NODE,
 		.alloc_hpage = &khugepaged_alloc_page,
 	};
@@ -2505,6 +2516,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
 		     unsigned long start, unsigned long end)
 {
 	struct collapse_control cc = {
+		.enforce_pte_scan_limits = false,
 		.last_target_node = NUMA_NO_NODE,
 		.hpage = NULL,
 		.alloc_hpage = &alloc_hpage,
-- 
2.36.0.rc0.470.gd361397f0d-goog