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    Subject[ 72/95] mm: hugetlbfs: correctly populate shared pmd
    3.2-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Michal Hocko <mhocko@suse.cz>

    commit eb48c071464757414538c68a6033c8f8c15196f8 upstream.

    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: Larry Woodman <lwoodman@redhat.com>
    Tested-by: Larry Woodman <lwoodman@redhat.com>
    Reviewed-by: Mel Gorman <mgorman@suse.de>
    Signed-off-by: Michal Hocko <mhocko@suse.cz>
    Reviewed-by: Rik 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>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/x86/mm/hugetlbpage.c | 21 ++++++++++++++++-----
    1 file changed, 16 insertions(+), 5 deletions(-)

    diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
    index f6679a7..b91e485 100644
    --- a/arch/x86/mm/hugetlbpage.c
    +++ b/arch/x86/mm/hugetlbpage.c
    @@ -56,9 +56,16 @@ static int vma_shareable(struct vm_area_struct *vma, unsigned long addr)
    }

    /*
    - * search for a shareable pmd page for hugetlb.
    + * Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc()
    + * and returns the corresponding pte. While this is not necessary for the
    + * !shared pmd case because we can allocate the pmd later as well, it makes the
    + * code much cleaner. pmd allocation is essential for the shared case because
    + * pud has to be populated inside the same i_mmap_mutex section - otherwise
    + * racing tasks could either miss the sharing (see huge_pte_offset) or select a
    + * bad pmd for sharing.
    */
    -static void huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
    +static pte_t *
    +huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
    {
    struct vm_area_struct *vma = find_vma(mm, addr);
    struct address_space *mapping = vma->vm_file->f_mapping;
    @@ -68,9 +75,10 @@ static void huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
    struct vm_area_struct *svma;
    unsigned long saddr;
    pte_t *spte = NULL;
    + pte_t *pte;

    if (!vma_shareable(vma, addr))
    - return;
    + return (pte_t *)pmd_alloc(mm, pud, addr);

    mutex_lock(&mapping->i_mmap_mutex);
    vma_prio_tree_foreach(svma, &iter, &mapping->i_mmap, idx, idx) {
    @@ -97,7 +105,9 @@ static void huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
    put_page(virt_to_page(spte));
    spin_unlock(&mm->page_table_lock);
    out:
    + pte = (pte_t *)pmd_alloc(mm, pud, addr);
    mutex_unlock(&mapping->i_mmap_mutex);
    + return pte;
    }

    /*
    @@ -142,8 +152,9 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
    } else {
    BUG_ON(sz != PMD_SIZE);
    if (pud_none(*pud))
    - huge_pmd_share(mm, addr, pud);
    - pte = (pte_t *) pmd_alloc(mm, pud, addr);
    + pte = huge_pmd_share(mm, addr, pud);
    + else
    + pte = (pte_t *)pmd_alloc(mm, pud, addr);
    }
    }
    BUG_ON(pte && !pte_none(*pte) && !pte_huge(*pte));



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