linux-kernel - [PATCH] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged

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Date:   Fri, 14 Aug 2020 15:13:18 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Hugh Dickins <hughd@...gle.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Song Liu <songliubraving@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged_enter()

syzbot crashes on the VM_BUG_ON_MM(khugepaged_test_exit(mm), mm) in
__khugepaged_enter(): yes, when one thread is about to dump core, has set
core_state, and is waiting for others, another might do something calling
__khugepaged_enter(), which now crashes because I lumped the core_state
test (known as "mmget_still_valid") into khugepaged_test_exit().  I still
think it's best to lump them together, so just in this exceptional case,
check mm->mm_users directly instead of khugepaged_test_exit().

Reported-by: syzbot <syzkaller@...glegroups.com>
Fixes: bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()")
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Cc: stable@...r.kernel.org # v4.8+
---

 mm/khugepaged.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- v5.9-rc/mm/khugepaged.c	2020-08-12 19:46:50.867196579 -0700
+++ linux/mm/khugepaged.c	2020-08-14 14:24:32.739457309 -0700
@@ -466,7 +466,7 @@ int __khugepaged_enter(struct mm_struct
 		return -ENOMEM;
 
 	/* __khugepaged_exit() must not run from under us */
-	VM_BUG_ON_MM(khugepaged_test_exit(mm), mm);
+	VM_BUG_ON_MM(atomic_read(&mm->mm_users) == 0, mm);
 	if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags))) {
 		free_mm_slot(mm_slot);
 		return 0;