linux-kernel - [PATCH v2 2/4] mm/mmap/vma_merge: set next to NULL if not applicable

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Message-Id: <3506997a8146a287ec13ee7a53cf916707537bf8.1679431180.git.lstoakes@gmail.com>
Date:   Tue, 21 Mar 2023 20:45:56 +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, Vernon Yang <vernon2gm@...il.com>,
        Lorenzo Stoakes <lstoakes@...il.com>
Subject: [PATCH v2 2/4] mm/mmap/vma_merge: set next to NULL if not applicable

We are only interested in next if end == next->vm_start (in which case we
check to see if we can set merge_next), so perform this check alongside
checking whether curr should be set.

This groups all of the simple range checks together and establishes the
invariant that, if prev, curr or next are non-NULL then their positions are
as expected.

This has no functional impact.

Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
---
 mm/mmap.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index c9834364ac98..6361baf75601 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -930,15 +930,15 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	if (vm_flags & VM_SPECIAL)
 		return NULL;
 
-	curr = find_vma(mm, prev ? prev->vm_end : 0);
-	if (curr && curr->vm_end == end)		/* cases 6, 7, 8 */
-		next = find_vma(mm, curr->vm_end);
-	else
-		next = curr;
+	/* Does the input range span an existing VMA? (cases 5 - 8) */
+	curr = find_vma_intersection(mm, prev ? prev->vm_end : 0, end);
 
-	/* In cases 1 - 4 there's no CCCC vma */
-	if (curr && end <= curr->vm_start)
-		curr = NULL;
+	if (curr && end == curr->vm_end)
+		/* Is there is a VMA immediately adjacent (cases 6 - 8)? */
+		next = vma_lookup(mm, curr->vm_end);
+	else if (!curr)
+		/* Is there a VMA next to a hole (case 1 - 3) or prev (4)? */
+		next = vma_lookup(mm, end);
 
 	/* verify some invariant that must be enforced by the caller */
 	VM_WARN_ON(prev && addr <= prev->vm_start);
@@ -959,11 +959,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 		}
 	}
 	/* 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)) {
+	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;
 	}
 
-- 
2.39.2