[mm-unstable v7 06/18] mm/khugepaged: add flag to predicate khugepaged-only behavior

["Zach O'Keefe" <zokeefe@google.com>]

Add .is_khugepaged flag to struct collapse_control so
khugepaged-specific behavior can be elided by MADV_COLLAPSE context.

Start by protecting khugepaged-specific heuristics by this flag. In
MADV_COLLAPSE, the user presumably has reason to believe the collapse
will be beneficial and khugepaged heuristics shouldn't prevent the user
from doing so:

1) sysfs-controlled knobs khugepaged_max_ptes_[none|swap|shared]

2) requirement that some pages in region being collapsed be young or
   referenced

Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---

v6 -> v7: There is no functional change here from v6, just a renaming of
	  flags to explicitly be predicated on khugepaged.
---
 mm/khugepaged.c | 62 ++++++++++++++++++++++++++++++++++---------------
 1 file changed, 43 insertions(+), 19 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 147f5828f052..d89056d8cbad 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -73,6 +73,8 @@ static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
  * default collapse hugepages if there is at least one pte mapped like
  * it would have happened if the vma was large enough during page
  * fault.
+ *
+ * Note that these are only respected if collapse was initiated by khugepaged.
  */
 static unsigned int khugepaged_max_ptes_none __read_mostly;
 static unsigned int khugepaged_max_ptes_swap __read_mostly;
@@ -86,6 +88,8 @@ static struct kmem_cache *mm_slot_cache __read_mostly;
 #define MAX_PTE_MAPPED_THP 8
 
 struct collapse_control {
+	bool is_khugepaged;
+
 	/* Num pages scanned per node */
 	int node_load[MAX_NUMNODES];
 
@@ -554,6 +558,7 @@ 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;
@@ -567,7 +572,8 @@ 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->is_khugepaged)) {
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -587,8 +593,8 @@ 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->is_khugepaged && page_mapcount(page) > 1 &&
+		    ++shared > khugepaged_max_ptes_shared) {
 			result = SCAN_EXCEED_SHARED_PTE;
 			count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 			goto out;
@@ -654,10 +660,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		if (PageCompound(page))
 			list_add_tail(&page->lru, compound_pagelist);
 next:
-		/* There should be enough young pte to collapse the page */
-		if (pte_young(pteval) ||
-		    page_is_young(page) || PageReferenced(page) ||
-		    mmu_notifier_test_young(vma->vm_mm, address))
+		/*
+		 * If collapse was initiated by khugepaged, check that there is
+		 * enough young pte to justify collapsing the page
+		 */
+		if (cc->is_khugepaged &&
+		    (pte_young(pteval) || page_is_young(page) ||
+		     PageReferenced(page) || mmu_notifier_test_young(vma->vm_mm,
+								     address)))
 			referenced++;
 
 		if (pte_write(pteval))
@@ -666,7 +676,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 	if (unlikely(!writable)) {
 		result = SCAN_PAGE_RO;
-	} else if (unlikely(!referenced)) {
+	} else if (unlikely(cc->is_khugepaged && !referenced)) {
 		result = SCAN_LACK_REFERENCED_PAGE;
 	} else {
 		result = SCAN_SUCCEED;
@@ -745,6 +755,7 @@ static void khugepaged_alloc_sleep(void)
 
 
 struct collapse_control khugepaged_collapse_control = {
+	.is_khugepaged = true,
 	.last_target_node = NUMA_NO_NODE,
 };
 
@@ -1023,7 +1034,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	mmu_notifier_invalidate_range_end(&range);
 
 	spin_lock(pte_ptl);
-	result =  __collapse_huge_page_isolate(vma, address, pte,
+	result =  __collapse_huge_page_isolate(vma, address, pte, cc,
 					       &compound_pagelist);
 	spin_unlock(pte_ptl);
 
@@ -1114,7 +1125,8 @@ static int khugepaged_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->is_khugepaged) {
 				/*
 				 * Always be strict with uffd-wp
 				 * enabled swap entries.  Please see
@@ -1133,7 +1145,8 @@ static int khugepaged_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->is_khugepaged)) {
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -1163,8 +1176,9 @@ static int khugepaged_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->is_khugepaged &&
+		    page_mapcount(page) > 1 &&
+		    ++shared > khugepaged_max_ptes_shared) {
 			result = SCAN_EXCEED_SHARED_PTE;
 			count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 			goto out_unmap;
@@ -1218,14 +1232,22 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
 			result = SCAN_PAGE_COUNT;
 			goto out_unmap;
 		}
-		if (pte_young(pteval) ||
-		    page_is_young(page) || PageReferenced(page) ||
-		    mmu_notifier_test_young(vma->vm_mm, address))
+
+		/*
+		 * If collapse was initiated by khugepaged, check that there is
+		 * enough young pte to justify collapsing the page
+		 */
+		if (cc->is_khugepaged &&
+		    (pte_young(pteval) || page_is_young(page) ||
+		     PageReferenced(page) || mmu_notifier_test_young(vma->vm_mm,
+								     address)))
 			referenced++;
 	}
 	if (!writable) {
 		result = SCAN_PAGE_RO;
-	} else if (!referenced || (unmapped && referenced < HPAGE_PMD_NR/2)) {
+	} else if (cc->is_khugepaged &&
+		   (!referenced ||
+		    (unmapped && referenced < HPAGE_PMD_NR / 2))) {
 		result = SCAN_LACK_REFERENCED_PAGE;
 	} else {
 		result = SCAN_SUCCEED;
@@ -1894,7 +1916,8 @@ static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
 			continue;
 
 		if (xa_is_value(page)) {
-			if (++swap > khugepaged_max_ptes_swap) {
+			if (cc->is_khugepaged &&
+			    ++swap > khugepaged_max_ptes_swap) {
 				result = SCAN_EXCEED_SWAP_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
 				break;
@@ -1945,7 +1968,8 @@ static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
 	rcu_read_unlock();
 
 	if (result == SCAN_SUCCEED) {
-		if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
+		if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none &&
+		    cc->is_khugepaged) {
 			result = SCAN_EXCEED_NONE_PTE;
 			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 		} else {
-- 
2.37.0.rc0.161.g10f37bed90-goog