linux-kernel - [PATCH 4/4] mm/mmap/vma_merge: be explicit about the non-mergeable case

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Message-Id: <d5fae254dc394e96860f41327f24b6ffd20cdf22.1679137163.git.lstoakes@gmail.com>
Date:   Sat, 18 Mar 2023 11:13:21 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        maple-tree@...ts.infradead.org,
        Lorenzo Stoakes <lstoakes@...il.com>
Subject: [PATCH 4/4] mm/mmap/vma_merge: be explicit about the non-mergeable case

Rather than setting err = -1 and only resetting if we hit merge cases,
explicitly check the non-mergeable case to make it abundantly clear that we
only proceed with the rest if something is mergeable, default err to 0 and
only update if an error might occur.

Additionally set merge_next directly as there is no need for an if() {}
statement assigning a boolean.

This has no functional impact.

Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
---
 mm/mmap.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index e60c637f4e49..2ac43b2b9a00 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -913,7 +913,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	pgoff_t vma_pgoff;
 	struct vm_area_struct *curr, *next, *res;
 	struct vm_area_struct *vma, *adjust, *remove, *remove2;
-	int err = -1;
+	int err = 0;
 	bool merge_prev = false;
 	bool merge_next = false;
 	bool vma_expanded = false;
@@ -1002,12 +1002,15 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	}
 
 	/* Can we merge the successor? */
-	if (next && 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;
-	}
+	merge_next = next &&
+		mpol_equal(policy, vma_policy(next)) &&
+		can_vma_merge_before(next, vm_flags,
+				     anon_vma, file, pgoff+pglen,
+				     vm_userfaultfd_ctx, anon_name);
+
+	/* Not mergeable. */
+	if (!merge_prev && !merge_next)
+		return NULL;
 
 	remove = remove2 = adjust = NULL;
 	/* Can we merge both the predecessor and the successor? */
@@ -1023,7 +1026,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 				err = dup_anon_vma(prev, curr);
 		}
 	} else if (merge_prev) {
-		err = 0;				/* case 2 */
+							/* case 2 */
 		if (curr) {
 			err = dup_anon_vma(prev, curr);
 			if (end == curr->vm_end) {	/* case 7 */
@@ -1033,7 +1036,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 				adj_start = (end - curr->vm_start);
 			}
 		}
-	} else if (merge_next) {
+	} else { /* merge_next */
 		res = next;
 		if (prev && addr < prev->vm_end) {	/* case 4 */
 			vma_end = addr;
@@ -1049,7 +1052,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			vma_start = addr;
 			vma_end = next->vm_end;
 			vma_pgoff = next->vm_pgoff;
-			err = 0;
+
 			if (curr) {			/* case 8 */
 				vma_pgoff = curr->vm_pgoff;
 				remove = curr;
@@ -1058,7 +1061,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 		}
 	}
 
-	/* Cannot merge or error in anon_vma clone */
+	/* Error in anon_vma clone. */
 	if (err)
 		return NULL;
 
-- 
2.39.2