[PATCH mm-unstable 2/4] mm/khugepaged: consistently order cc->is_khugepaged and pte_* checks

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

cc->is_khugepaged is used to predicate the khugepaged-only behavior
of enforcing khugepaged heuristics limited by the sysfs knobs
khugepaged_max_ptes_[none|swap|shared].

In branches where khugepaged_max_ptes_* is checked, consistently check
cc->is_khugepaged first.  Also, local counters (for comparison vs
khugepaged_max_ptes_* limits) were previously incremented in the
comparison expression.  Some of these counters (unmapped) are
additionally used outside of khugepaged_max_ptes_* enforcement, and
all counters are communicated in tracepoints.  Move the correct
accounting of these counters before branching statements to avoid future
errors due to C's short-circuiting evaluation.

Fixes: 9fab4752a181 ("mm/khugepaged: add flag to predicate khugepaged-only behavior")
Link: https://lore.kernel.org/linux-mm/Ys2qJm6FaOQcxkha@xxxxxxxxxx/
Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>
---
 mm/khugepaged.c | 49 +++++++++++++++++++++++++++++--------------------
 1 file changed, 29 insertions(+), 20 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ecd28bfeab60..290422577172 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -574,9 +574,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		pte_t pteval = *_pte;
 		if (pte_none(pteval) || (pte_present(pteval) &&
 				is_zero_pfn(pte_pfn(pteval)))) {
+			++none_or_zero;
 			if (!userfaultfd_armed(vma) &&
-			    (++none_or_zero <= khugepaged_max_ptes_none ||
-			     !cc->is_khugepaged)) {
+			    (!cc->is_khugepaged ||
+			     none_or_zero <= khugepaged_max_ptes_none)) {
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -596,11 +597,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 		VM_BUG_ON_PAGE(!PageAnon(page), page);
 
-		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;
+		if (page_mapcount(page) > 1) {
+			++shared;
+			if (cc->is_khugepaged &&
+			    shared > khugepaged_max_ptes_shared) {
+				result = SCAN_EXCEED_SHARED_PTE;
+				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				goto out;
+			}
 		}
 
 		if (PageCompound(page)) {
@@ -1170,8 +1174,9 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 	     _pte++, _address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
 		if (is_swap_pte(pteval)) {
-			if (++unmapped <= khugepaged_max_ptes_swap ||
-			    !cc->is_khugepaged) {
+			++unmapped;
+			if (!cc->is_khugepaged ||
+			    unmapped <= khugepaged_max_ptes_swap) {
 				/*
 				 * Always be strict with uffd-wp
 				 * enabled swap entries.  Please see
@@ -1189,9 +1194,10 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 			}
 		}
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+			++none_or_zero;
 			if (!userfaultfd_armed(vma) &&
-			    (++none_or_zero <= khugepaged_max_ptes_none ||
-			     !cc->is_khugepaged)) {
+			    (!cc->is_khugepaged ||
+			     none_or_zero <= khugepaged_max_ptes_none)) {
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -1221,12 +1227,14 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 			goto out_unmap;
 		}
 
-		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;
+		if (page_mapcount(page) > 1) {
+			++shared;
+			if (cc->is_khugepaged &&
+			    shared > khugepaged_max_ptes_shared) {
+				result = SCAN_EXCEED_SHARED_PTE;
+				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				goto out_unmap;
+			}
 		}
 
 		page = compound_head(page);
@@ -1961,8 +1969,9 @@ static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
 			continue;
 
 		if (xa_is_value(page)) {
+			++swap;
 			if (cc->is_khugepaged &&
-			    ++swap > khugepaged_max_ptes_swap) {
+			    swap > khugepaged_max_ptes_swap) {
 				result = SCAN_EXCEED_SWAP_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
 				break;
@@ -2013,8 +2022,8 @@ 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 &&
-		    cc->is_khugepaged) {
+		if (cc->is_khugepaged &&
+		    present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
 			result = SCAN_EXCEED_NONE_PTE;
 			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 		} else {
-- 
2.37.0.170.g444d1eabd0-goog