[RFC PATCH v2 1/4] [PATCH 1/4] mm: refactor of vma_merge()

["Jakub Matěna" <matenajakub@gmail.com>]

Refactor vma_merge() to make it shorter, more understandable and
suitable for tracing of successful merges that are made possible by
following patches in the series. Main change is the elimination of code
duplicity in the case of merge next check. This is done by first doing
checks and caching the results before executing the merge itself. Exit
paths are also unified.

Signed-off-by: Jakub Matěna <matenajakub@gmail.com>
---
 mm/mmap.c | 81 +++++++++++++++++++++++++------------------------------
 1 file changed, 36 insertions(+), 45 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 1e8fdb0b51ed..8d817b11c656 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1171,7 +1171,9 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 {
 	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
 	struct vm_area_struct *area, *next;
-	int err;
+	int err = -1;
+	bool merge_prev = false;
+	bool merge_next = false;
 
 	/*
 	 * We later require that vma->vm_flags == vm_flags,
@@ -1190,66 +1192,55 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 	VM_WARN_ON(area && end > area->vm_end);
 	VM_WARN_ON(addr >= end);
 
-	/*
-	 * Can it merge with the predecessor?
-	 */
+	/* Can we merge the predecessor? */
 	if (prev && prev->vm_end == addr &&
 			mpol_equal(vma_policy(prev), policy) &&
 			can_vma_merge_after(prev, vm_flags,
 					    anon_vma, file, pgoff,
 					    vm_userfaultfd_ctx, anon_name)) {
-		/*
-		 * OK, it can.  Can we now merge in the successor as well?
-		 */
-		if (next && end == next->vm_start &&
-				mpol_equal(policy, vma_policy(next)) &&
-				can_vma_merge_before(next, vm_flags,
-						     anon_vma, file,
-						     pgoff+pglen,
-						     vm_userfaultfd_ctx, anon_name) &&
-				is_mergeable_anon_vma(prev->anon_vma,
-						      next->anon_vma, NULL)) {
-							/* cases 1, 6 */
-			err = __vma_adjust(prev, prev->vm_start,
-					 next->vm_end, prev->vm_pgoff, NULL,
-					 prev);
-		} else					/* cases 2, 5, 7 */
-			err = __vma_adjust(prev, prev->vm_start,
-					 end, prev->vm_pgoff, NULL, prev);
-		if (err)
-			return NULL;
-		khugepaged_enter_vma_merge(prev, vm_flags);
-		return prev;
+		merge_prev = true;
+		area = prev;
 	}
-
-	/*
-	 * Can this new request be merged in front of next?
-	 */
+	/* Can we merge the successor? */
 	if (next && end == next->vm_start &&
 			mpol_equal(policy, vma_policy(next)) &&
 			can_vma_merge_before(next, vm_flags,
 					     anon_vma, file, pgoff+pglen,
 					     vm_userfaultfd_ctx, anon_name)) {
+		merge_next = true;
+	}
+	/* Can we merge both the predecessor and the successor? */
+	if (merge_prev && merge_next &&
+			is_mergeable_anon_vma(prev->anon_vma,
+				next->anon_vma, NULL)) {	 /* cases 1, 6 */
+		err = __vma_adjust(prev, prev->vm_start,
+					next->vm_end, prev->vm_pgoff, NULL,
+					prev);
+	} else if (merge_prev) {			/* cases 2, 5, 7 */
+		err = __vma_adjust(prev, prev->vm_start,
+					end, prev->vm_pgoff, NULL, prev);
+	} else if (merge_next) {
 		if (prev && addr < prev->vm_end)	/* case 4 */
 			err = __vma_adjust(prev, prev->vm_start,
-					 addr, prev->vm_pgoff, NULL, next);
-		else {					/* cases 3, 8 */
+					addr, prev->vm_pgoff, NULL, next);
+		else					/* cases 3, 8 */
 			err = __vma_adjust(area, addr, next->vm_end,
-					 next->vm_pgoff - pglen, NULL, next);
-			/*
-			 * In case 3 area is already equal to next and
-			 * this is a noop, but in case 8 "area" has
-			 * been removed and next was expanded over it.
-			 */
-			area = next;
-		}
-		if (err)
-			return NULL;
-		khugepaged_enter_vma_merge(area, vm_flags);
-		return area;
+					next->vm_pgoff - pglen, NULL, next);
+		/*
+		 * In case 3 and 4 area is already equal to next and
+		 * this is a noop, but in case 8 "area" has
+		 * been removed and next was expanded over it.
+		 */
+		area = next;
 	}
 
-	return NULL;
+	/*
+	 * Cannot merge with predecessor or successor or error in __vma_adjust?
+	 */
+	if (err)
+		return NULL;
+	khugepaged_enter_vma_merge(area, vm_flags);
+	return area;
 }
 
 /*
-- 
2.34.1