The khugepaged daemon and madvise_collapse have two different
implementations that do almost the same thing.

Create collapse_single_pmd to increase code reuse and create an entry
point to these two users.

Refactor madvise_collapse and collapse_scan_mm_slot to use the new
collapse_single_pmd function. This introduces a minor behavioral change
that is most likely an undiscovered bug. The current implementation of
khugepaged tests collapse_test_exit_or_disable before calling
collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse
case. By unifying these two callers madvise_collapse now also performs
this check. We also modify the return value to be SCAN_ANY_PROCESS which
properly indicates that this process is no longer valid to operate on.

We also guard the khugepaged_pages_collapsed variable to ensure its only
incremented for khugepaged.

Reviewed-by: Wei Yang <redacted>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Lorenzo Stoakes <redacted>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <redacted>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 106 +++++++++++++++++++++++++++---------------------
 1 file changed, 60 insertions(+), 46 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index fefcbdca4510..59e5a5588d85 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c

@@ -2394,6 +2394,54 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, unsigned long a
 	return result;
 }
 
+/*
+ * Try to collapse a single PMD starting at a PMD aligned addr, and return
+ * the results.
+ */
+static enum scan_result collapse_single_pmd(unsigned long addr,
+		struct vm_area_struct *vma, bool *mmap_locked,
+		struct collapse_control *cc)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	enum scan_result result;
+	struct file *file;
+	pgoff_t pgoff;
+
+	if (vma_is_anonymous(vma)) {
+		result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc);
+		goto end;
+	}
+
+	file = get_file(vma->vm_file);
+	pgoff = linear_page_index(vma, addr);
+
+	mmap_read_unlock(mm);
+	*mmap_locked = false;
+	result = collapse_scan_file(mm, addr, file, pgoff, cc);
+	fput(file);
+
+	if (result != SCAN_PTE_MAPPED_HUGEPAGE)
+		goto end;
+
+	mmap_read_lock(mm);
+	*mmap_locked = true;
+	if (collapse_test_exit_or_disable(mm)) {
+		mmap_read_unlock(mm);
+		*mmap_locked = false;
+		return SCAN_ANY_PROCESS;
+	}
+	result = try_collapse_pte_mapped_thp(mm, addr, !cc->is_khugepaged);
+	if (result == SCAN_PMD_MAPPED)
+		result = SCAN_SUCCEED;
+	mmap_read_unlock(mm);
+	*mmap_locked = false;
+
+end:
+	if (cc->is_khugepaged && result == SCAN_SUCCEED)
+		++khugepaged_pages_collapsed;
+	return result;
+}
+
 static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_result *result,
 					    struct collapse_control *cc)
 	__releases(&khugepaged_mm_lock)

@@ -2466,34 +2514,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_result *
 			VM_BUG_ON(khugepaged_scan.address < hstart ||
 				  khugepaged_scan.address + HPAGE_PMD_SIZE >
 				  hend);
-			if (!vma_is_anonymous(vma)) {
-				struct file *file = get_file(vma->vm_file);
-				pgoff_t pgoff = linear_page_index(vma,
-						khugepaged_scan.address);
-
-				mmap_read_unlock(mm);
-				mmap_locked = false;
-				*result = collapse_scan_file(mm,
-					khugepaged_scan.address, file, pgoff, cc);
-				fput(file);
-				if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
-					mmap_read_lock(mm);
-					if (collapse_test_exit_or_disable(mm))
-						goto breakouterloop;
-					*result = try_collapse_pte_mapped_thp(mm,
-						khugepaged_scan.address, false);
-					if (*result == SCAN_PMD_MAPPED)
-						*result = SCAN_SUCCEED;
-					mmap_read_unlock(mm);
-				}
-			} else {
-				*result = collapse_scan_pmd(mm, vma,
-					khugepaged_scan.address, &mmap_locked, cc);
-			}
-
-			if (*result == SCAN_SUCCEED)
-				++khugepaged_pages_collapsed;
 
+			*result = collapse_single_pmd(khugepaged_scan.address,
+						      vma, &mmap_locked, cc);
 			/* move to next address */
 			khugepaged_scan.address += HPAGE_PMD_SIZE;
 			progress += HPAGE_PMD_NR;

@@ -2799,6 +2822,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 			cond_resched();
 			mmap_read_lock(mm);
 			mmap_locked = true;
+			*lock_dropped = true;
 			result = hugepage_vma_revalidate(mm, addr, false, &vma,
 							 cc);
 			if (result  != SCAN_SUCCEED) {

@@ -2809,17 +2833,17 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
 		}
 		mmap_assert_locked(mm);
-		if (!vma_is_anonymous(vma)) {
-			struct file *file = get_file(vma->vm_file);
-			pgoff_t pgoff = linear_page_index(vma, addr);
 
-			mmap_read_unlock(mm);
-			mmap_locked = false;
+		result = collapse_single_pmd(addr, vma, &mmap_locked, cc);
+
+		if (!mmap_locked)
 			*lock_dropped = true;
-			result = collapse_scan_file(mm, addr, file, pgoff, cc);
 
-			if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb &&
-			    mapping_can_writeback(file->f_mapping)) {
+		if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb) {
+			struct file *file = get_file(vma->vm_file);
+			pgoff_t pgoff = linear_page_index(vma, addr);
+
+			if (mapping_can_writeback(file->f_mapping)) {
 				loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
 				loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
 

@@ -2829,26 +2853,16 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 				goto retry;
 			}
 			fput(file);
-		} else {
-			result = collapse_scan_pmd(mm, vma, addr, &mmap_locked, cc);
 		}
-		if (!mmap_locked)
-			*lock_dropped = true;
 
-handle_result:
 		switch (result) {
 		case SCAN_SUCCEED:
 		case SCAN_PMD_MAPPED:
 			++thps;
 			break;
-		case SCAN_PTE_MAPPED_HUGEPAGE:
-			BUG_ON(mmap_locked);
-			mmap_read_lock(mm);
-			result = try_collapse_pte_mapped_thp(mm, addr, true);
-			mmap_read_unlock(mm);
-			goto handle_result;
 		/* Whitelisted set of results where continuing OK */
 		case SCAN_NO_PTE_TABLE:
+		case SCAN_PTE_MAPPED_HUGEPAGE:
 		case SCAN_PTE_NON_PRESENT:
 		case SCAN_PTE_UFFD_WP:
 		case SCAN_LACK_REFERENCED_PAGE: