linux-kernel - [RFC 04/11] mm/mmap/vma_merge: use the proper vma pointers in cases 1 and 6

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Date:   Fri, 24 Feb 2023 11:00:15 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     linux-mm@...ck.org, "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        maple-tree@...ts.infradead.org, Vlastimil Babka <vbabka@...e.cz>
Subject: [RFC 04/11] mm/mmap/vma_merge: use the proper vma pointers in cases 1 and 6

Case 1 is now shown in the comment as next vma being merged with prev,
so use 'next' instead of 'mid'. In case 1 they both point to the same
vma.

As a consequence, in case 6, the dup_anon_vma() is now tried first on
'next' and then on 'mid', before it was the opposite order. This is not
a functional change, as those two vma's cannnot have a different
anon_vma, as that would have prevented the merging in the first place.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/mmap.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 1af4c9bc2c87..704afa278a98 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -605,7 +605,7 @@ static inline void vma_complete(struct vma_prepare *vp,
 
 		/*
 		 * In mprotect's case 6 (see comments on vma_merge),
-		 * we must remove the one after next as well.
+		 * we are removing both mid and next vma's
 		 */
 		if (vp->remove2) {
 			vp->remove = vp->remove2;
@@ -948,13 +948,14 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	/* 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)) {
-		remove = mid;				/* case 1 */
+		remove = next;				/* case 1 */
 		vma_end = next->vm_end;
-		err = dup_anon_vma(prev, mid);
+		err = dup_anon_vma(prev, next);
 		if (mid != next) {			/* case 6 */
+			remove = mid;
 			remove2 = next;
-			if (!mid->anon_vma)
-				err = dup_anon_vma(prev, next);
+			if (!next->anon_vma)
+				err = dup_anon_vma(prev, mid);
 		}
 	} else if (merge_prev) {
 		err = 0;				/* case 2 */
-- 
2.39.2