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    Subject[PATCH v2 27/35] mm: implement and enable speculative fault handling in handle_pte_fault()
    Date
    In handle_pte_fault(), allow speculative execution to proceed.

    Use pte_spinlock() to validate the mmap sequence count when locking
    the page table.

    If speculative execution proceeds through do_wp_page(), ensure that we
    end up in the wp_page_reuse() or wp_page_copy() paths, rather than
    wp_pfn_shared() or wp_page_shared() (both unreachable as we only
    handle anon vmas so far) or handle_userfault() (needs an explicit
    abort to handle non-speculatively).

    Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
    ---
    mm/memory.c | 12 ++++++------
    1 file changed, 6 insertions(+), 6 deletions(-)

    diff --git a/mm/memory.c b/mm/memory.c
    index fd8984d89109..7f8dbd729dce 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -3293,6 +3293,7 @@ static vm_fault_t wp_pfn_shared(struct vm_fault *vmf)
    {
    struct vm_area_struct *vma = vmf->vma;

    + VM_BUG_ON(vmf->flags & FAULT_FLAG_SPECULATIVE);
    if (vma->vm_ops && vma->vm_ops->pfn_mkwrite) {
    vm_fault_t ret;

    @@ -3313,6 +3314,8 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
    struct vm_area_struct *vma = vmf->vma;
    vm_fault_t ret = VM_FAULT_WRITE;

    + VM_BUG_ON(vmf->flags & FAULT_FLAG_SPECULATIVE);
    +
    get_page(vmf->page);

    if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
    @@ -3366,6 +3369,8 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)

    if (userfaultfd_pte_wp(vma, *vmf->pte)) {
    pte_unmap_unlock(vmf->pte, vmf->ptl);
    + if (vmf->flags & FAULT_FLAG_SPECULATIVE)
    + return VM_FAULT_RETRY;
    return handle_userfault(vmf, VM_UFFD_WP);
    }

    @@ -4646,13 +4651,8 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
    if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
    return do_numa_page(vmf);

    - if (vmf->flags & FAULT_FLAG_SPECULATIVE) {
    - pte_unmap(vmf->pte);
    + if (!pte_spinlock(vmf))
    return VM_FAULT_RETRY;
    - }
    -
    - vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
    - spin_lock(vmf->ptl);
    entry = vmf->orig_pte;
    if (unlikely(!pte_same(*vmf->pte, entry))) {
    update_mmu_tlb(vmf->vma, vmf->address, vmf->pte);
    --
    2.20.1
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