+ mm-rmap-fix-missing-barrier-between-anon_vma-init-and-vma-anon_vma-publish.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/rmap: fix missing barrier between anon_vma init and vma->anon_vma publish
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-rmap-fix-missing-barrier-between-anon_vma-init-and-vma-anon_vma-publish.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-rmap-fix-missing-barrier-between-anon_vma-init-and-vma-anon_vma-publish.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Jinjiang Tu <tujinjiang@huawei.com>
Subject: mm/rmap: fix missing barrier between anon_vma init and vma->anon_vma publish
Date: Sat, 5 Sep 2026 14:18:19 +0800

On arm64 server, we found __anon_vma_prepare() reuses anon_vma and
anon_vma->root is stale due to missing memory barrier, leading to lock and
unlock two different anon_vma->root, thus leading to a anon_vma will never
be unlocked, and another anon_vma couldn't be locked anymore.

The race is as follows:

    THREAD A                             THREAD B
__anon_vma_prepare                __anon_vma_prepare
 anon_vma = anon_vma_alloc();
 // writes may out of order here
 vma->anon_vma = anon_vma;
                                   anon_vma = find_mergeable_anon_vma(vma);
                                   anon_vma_lock_write(anon_vma);
                                     // may still see the old root
                                     down_write(&anon_vma->root->rwsem);
                                   anon_vma_unlock_write(anon_vma);
                                     // see the new root, never unlock old
                                     up_write(&anon_vma->root->rwsem);

thread A triggers page fault and calls __anon_vma_prepare() to prepare
anon_vma for the faulting vma.  __anon_vma_prepare() allocates and
initializes a new anon_vma, and then publishes it to the vma with a plain
store.  anon_vma_prepare() only requires the mmap_lock to be held for
reading, so two threads can fault on adjacent VMAs at the same time. 
While thread A publishes a new anon_vma, thread B could finds the anon_vma
via find_mergeable_anon_vma() and then locks anon_vma->root->rwsem.

However, due to missing barrier, thread B can observe the published
pointer but a stale anon_vma->root because the stores from
anon_vma_alloc() aren't yet visible.  What's the value of the stale
anon_vma->root?  __put_anon_vma() doesn't clear anon_vma->root, so the
root of the new allocated anon_vma may point to a valid anon_vma.

As a result, thread B can call anon_vma_lock_write() with the old root,
and call anon_vma_unlock_write() with the new root, leading to a anon_vma
will never be unlocked, and another anon_vma couldn't be locked anymore
(it's count is dropped from 0 to -1 due to wrong unlock).

To fix it, change the plain store `vma->anon_vma = anon_vma` to store
release, so that the fields of anon_vma are visible before anon_vma is
published to vma->anon_vma.

We don't need a read barrier at read side for thread B.  The load of
anon_vma and anon_vma->root have address-dependency.  According to
Documentation/memory-barriers.txt and some investigations, only Alpha
needs address-dependency barriers and it has been handled by READ_ONCE().

This issue needs two adjacent VMAs aren't merged but are compatible for
anon_vma.  We reproduced this issue in v5.10 with KSM enabled.  The kernel
doesn't merge commit cf7e7a3503df ("mm: prevent KSM from breaking VMA
merging for new VMAs"), so there are many adjacent VMAs that aren't merged
but are compatible for anon_vma.

Without this fix, our production environment could reproduce this issue
about 2-5 times each month.  After adding a smp_mb() before
anon_vma_lock_write(anon_vma) in __anon_vma_prepare(), which is different
to this patch, this issue hasn't be reproduced for one month.

Link: https://lore.kernel.org/20260905061820.642437-1-tujinjiang@huawei.com
Fixes: 5c341ee1dfc8 ("mm: track the root (oldest) anon_vma")
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Harry Yoo <harry@kernel.org>
Cc: Hiroyouki Kamezawa <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/rmap.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/mm/rmap.c~mm-rmap-fix-missing-barrier-between-anon_vma-init-and-vma-anon_vma-publish
+++ a/mm/rmap.c
@@ -209,7 +209,11 @@ int __anon_vma_prepare(struct vm_area_st
 	/* page_table_lock to protect against threads */
 	spin_lock(&mm->page_table_lock);
 	if (likely(!vma->anon_vma)) {
-		vma->anon_vma = anon_vma;
+		/*
+		 * The fields of anon_vma must be visible before anon_vma
+		 * is published to vma->anon_vma.
+		 */
+		smp_store_release(&vma->anon_vma, anon_vma);
 		anon_vma_chain_assign(vma, avc, anon_vma);
 		anon_rmap_tree_insert(avc, anon_vma);
 		anon_vma->num_active_vmas++;
_

Patches currently in -mm which might be from tujinjiang@huawei.com are

mm-rmap-fix-missing-barrier-between-anon_vma-init-and-vma-anon_vma-publish.patch
docs-ksm-fix-typos-in-sysfs-knob-names.patch
mm-ksm-fix-advisor_min_pages_to_scan-description.patch