• Michal Hocko's avatar
    mm: hugetlbfs: correctly populate shared pmd · eb48c071
    Michal Hocko authored
    
    
    Each page mapped in a process's address space must be correctly
    accounted for in _mapcount.  Normally the rules for this are
    straightforward but hugetlbfs page table sharing is different.  The page
    table pages at the PMD level are reference counted while the mapcount
    remains the same.
    
    If this accounting is wrong, it causes bugs like this one reported by
    Larry Woodman:
    
      kernel BUG at mm/filemap.c:135!
      invalid opcode: 0000 [#1] SMP
      CPU 22
      Modules linked in: bridge stp llc sunrpc binfmt_misc dcdbas microcode pcspkr acpi_pad acpi]
      Pid: 18001, comm: mpitest Tainted: G        W    3.3.0+ #4 Dell Inc. PowerEdge R620/07NDJ2
      RIP: 0010:[<ffffffff8112cfed>]  [<ffffffff8112cfed>] __delete_from_page_cache+0x15d/0x170
      Process mpitest (pid: 18001, threadinfo ffff880428972000, task ffff880428b5cc20)
      Call Trace:
        delete_from_page_cache+0x40/0x80
        truncate_hugepages+0x115/0x1f0
        hugetlbfs_evict_inode+0x18/0x30
        evict+0x9f/0x1b0
        iput_final+0xe3/0x1e0
        iput+0x3e/0x50
        d_kill+0xf8/0x110
        dput+0xe2/0x1b0
        __fput+0x162/0x240
    
    During fork(), copy_hugetlb_page_range() detects if huge_pte_alloc()
    shared page tables with the check dst_pte == src_pte.  The logic is if
    the PMD page is the same, they must be shared.  This assumes that the
    sharing is between the parent and child.  However, if the sharing is
    with a different process entirely then this check fails as in this
    diagram:
    
      parent
        |
        ------------>pmd
                     src_pte----------> data page
                                            ^
      other--------->pmd--------------------|
                      ^
      child-----------|
                     dst_pte
    
    For this situation to occur, it must be possible for Parent and Other to
    have faulted and failed to share page tables with each other.  This is
    possible due to the following style of race.
    
      PROC A                                          PROC B
      copy_hugetlb_page_range                         copy_hugetlb_page_range
        src_pte == huge_pte_offset                      src_pte == huge_pte_offset
        !src_pte so no sharing                          !src_pte so no sharing
    
      (time passes)
    
      hugetlb_fault                                   hugetlb_fault
        huge_pte_alloc                                  huge_pte_alloc
          huge_pmd_share                                 huge_pmd_share
            LOCK(i_mmap_mutex)
            find nothing, no sharing
            UNLOCK(i_mmap_mutex)
                                                          LOCK(i_mmap_mutex)
                                                          find nothing, no sharing
                                                          UNLOCK(i_mmap_mutex)
          pmd_alloc                                       pmd_alloc
          LOCK(instantiation_mutex)
          fault
          UNLOCK(instantiation_mutex)
                                                      LOCK(instantiation_mutex)
                                                      fault
                                                      UNLOCK(instantiation_mutex)
    
    These two processes are not poing to the same data page but are not
    sharing page tables because the opportunity was missed.  When either
    process later forks, the src_pte == dst pte is potentially insufficient.
    As the check falls through, the wrong PTE information is copied in
    (harmless but wrong) and the mapcount is bumped for a page mapped by a
    shared page table leading to the BUG_ON.
    
    This patch addresses the issue by moving pmd_alloc into huge_pmd_share
    which guarantees that the shared pud is populated in the same critical
    section as pmd.  This also means that huge_pte_offset test in
    huge_pmd_share is serialized correctly now which in turn means that the
    success of the sharing will be higher as the racing tasks see the pud
    and pmd populated together.
    
    Race identified and changelog written mostly by Mel Gorman.
    
    {akpm@linux-foundation.org: attempt to make the huge_pmd_share() comment comprehensible, clean up coding style]
    Reported-by: default avatarLarry Woodman <lwoodman@redhat.com>
    Tested-by: default avatarLarry Woodman <lwoodman@redhat.com>
    Reviewed-by: default avatarMel Gorman <mgorman@suse.de>
    Signed-off-by: default avatarMichal Hocko <mhocko@suse.cz>
    Reviewed-by: default avatarRik van Riel <riel@redhat.com>
    Cc: David Gibson <david@gibson.dropbear.id.au>
    Cc: Ken Chen <kenchen@google.com>
    Cc: Cong Wang <xiyou.wangcong@gmail.com>
    Cc: Hillf Danton <dhillf@gmail.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    eb48c071
hugetlbpage.c 10.9 KB