[RFC PATCH 20/37] mm: implement and enable speculative fault handling in handle_pte_fault()

[Michel Lespinasse]

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 <mic...@lespinasse.org>
---
 mm/memory.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 547d9d0ee962..fc555fae0844 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3103,6 +3103,7 @@ 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;
 
@@ -3123,6 +3124,8 @@ 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) {
@@ -3176,6 +3179,8 @@ 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);
        }
 
@@ -4366,13 +4371,8 @@ 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