From f3f99d63a8156c7a4a6b20aac22b53c5579c7dc1 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Thu, 20 Aug 2020 17:42:02 -0700 Subject: 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(). Fixes: bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()") Reported-by: syzbot Signed-off-by: Hugh Dickins Signed-off-by: Andrew Morton Acked-by: Yang Shi Cc: "Kirill A. Shutemov" Cc: Andrea Arcangeli Cc: Song Liu Cc: Mike Kravetz Cc: Eric Dumazet Cc: [4.8+] Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008141503370.18085@eggly.anvils Signed-off-by: Linus Torvalds --- mm/khugepaged.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'mm/khugepaged.c') diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 15a9af791014..e749e568e1ea 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -466,7 +466,7 @@ int __khugepaged_enter(struct mm_struct *mm) 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; -- cgit