khugepaged: khugepaged_test_exit() check mmget_still_valid()

[ Upstream commit bbe98f9cadff58cdd6a4acaeba0efa8565dabe65 ]

Move collapse_huge_page()'s mmget_still_valid() check into
khugepaged_test_exit() itself.  collapse_huge_page() is used for anon THP
only, and earned its mmget_still_valid() check because it inserts a huge
pmd entry in place of the page table's pmd entry; whereas
collapse_file()'s retract_page_tables() or collapse_pte_mapped_thp()
merely clears the page table's pmd entry.  But core dumping without mmap
lock must have been as open to mistaking a racily cleared pmd entry for a
page table at physical page 0, as exit_mmap() was.  And we certainly have
no interest in mapping as a THP once dumping core.

Fixes: 59ea6d06cfa9 ("coredump: fix race condition between collapse_huge_page() and core dumping")
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: <stable@vger.kernel.org>	[4.8+]
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008021217020.27773@eggly.anvils
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c5628ebc0fc295d321a4376ee1eff282119c4bfc..1c4d7d2f53d22e413a889bd8144f3b019b907c22 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2136,7 +2136,7 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm,
 
 static inline int khugepaged_test_exit(struct mm_struct *mm)
 {
-       return atomic_read(&mm->mm_users) == 0;
+       return atomic_read(&mm->mm_users) == 0 || !mmget_still_valid(mm);
 }
 
 int __khugepaged_enter(struct mm_struct *mm)
@@ -2587,9 +2587,6 @@ static void collapse_huge_page(struct mm_struct *mm,
         * handled by the anon_vma lock + PG_lock.
         */
        down_write(&mm->mmap_sem);
-       result = SCAN_ANY_PROCESS;
-       if (!mmget_still_valid(mm))
-               goto out;
        if (unlikely(khugepaged_test_exit(mm)))
                goto out;