linux-kernel - Re: [PATCH v2 5/5] mm/madvise: eliminate very confusing manipulation of prev VMA

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Message-ID: <dca94cde-8afb-4eab-8e57-3f508624d670@lucifer.local>
Date: Tue, 24 Jun 2025 14:16:37 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Lance Yang <ioworker0@...il.com>, SeongJae Park <sj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v2 5/5] mm/madvise: eliminate very confusing manipulation
 of prev VMA

Hi Andrew,

stress-ng picked up a very subtle bug here. Please apply this fix-patch to
correct it!

Cheers, Lorenzo

----8<----
>From a04de12e1deb50e708392b0e716612f26c6b386a Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Date: Tue, 24 Jun 2025 14:14:19 +0100
Subject: [PATCH] mm/madvise: fix very subtle bug

With thanks to stress-ng --madvise :)

vma may have been updated (in the modify call in madvise_update_vma()), so
we can't assign prev = vma, we must first reassign vma to
madv_behavior->vma to account for this, before setting prev = vma.
---
 mm/madvise.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 4491bf080f55..c467ee42596f 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1665,8 +1665,8 @@ int madvise_walk_vmas(struct madvise_behavior *madv_behavior)
 			vma = NULL;
 			madv_behavior->lock_dropped = false;
 		} else {
-			prev = vma;
 			vma = madv_behavior->vma;
+			prev = vma;
 		}

 		if (vma && range->end < vma->vm_end)
--
2.50.0