linux-kernel - [PATCH 3/3] mmap: Add clarifying comment to vma

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Date:   Wed, 27 Sep 2023 12:07:46 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>
Subject: [PATCH 3/3] mmap: Add clarifying comment to vma_merge() code

When tracing through the code in vma_merge(), it was not completely
clear why the error return to a dup_anon_vma() call would not overwrite
a previous attempt to the same function.  This commit adds a comment
specifying why it is safe.

Suggested-by: Jann Horn <jannh@...gle.com>
Link: https://lore.kernel.org/linux-mm/CAG48ez3iDwFPR=Ed1BfrNuyUJPMK_=StjxhUsCkL6po1s7bONg@mail.gmail.com/
Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
---
 mm/mmap.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 2f0ee489db8a..3c78afb707cf 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -937,6 +937,11 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			vma_start_write(curr);
 			remove = curr;
 			remove2 = next;
+			/*
+			 * Note that the dup_anon_vma below cannot overwrite err
+			 * since the first caller would do nothing unless next
+			 * has an anon_vma.
+			 */
 			if (!next->anon_vma)
 				err = dup_anon_vma(prev, curr, &anon_dup);
 		}
-- 
2.40.1