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    Subject[PATCH 5.8 001/255] khugepaged: khugepaged_test_exit() check mmget_still_valid()
    Date
    From: Hugh Dickins <hughd@google.com>

    [ 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>
    ---
    mm/khugepaged.c | 5 +----
    1 file changed, 1 insertion(+), 4 deletions(-)

    diff --git a/mm/khugepaged.c b/mm/khugepaged.c
    index 1d6a9b0b6a9fd..dd592ea9a4a06 100644
    --- a/mm/khugepaged.c
    +++ b/mm/khugepaged.c
    @@ -431,7 +431,7 @@ 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);
    }

    static bool hugepage_vma_check(struct vm_area_struct *vma,
    @@ -1100,9 +1100,6 @@ static void collapse_huge_page(struct mm_struct *mm,
    * handled by the anon_vma lock + PG_lock.
    */
    mmap_write_lock(mm);
    - result = SCAN_ANY_PROCESS;
    - if (!mmget_still_valid(mm))
    - goto out;
    result = hugepage_vma_revalidate(mm, address, &vma);
    if (result)
    goto out;
    --
    2.25.1


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