[RFC PATCH 26/37] mm: implement speculative fault handling in finish_fault()

[Michel Lespinasse]

In the speculative case, we want to avoid direct pmd checks (which
would require some extra synchronization to be safe), and rely on
pte_map_lock which will both lock the page table and verify that the
pmd has not changed from its initial value.

Signed-off-by: Michel Lespinasse <mic...@lespinasse.org>
---
 mm/memory.c | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7139004c624d..13e2aaf900e5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3915,23 +3915,25 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
                        return ret;
        }
 
-       if (pmd_none(*vmf->pmd)) {
-               if (PageTransCompound(page)) {
-                       ret = do_set_pmd(vmf, page);
-                       if (ret != VM_FAULT_FALLBACK)
-                               return ret;
+       if (!(vmf->flags & FAULT_FLAG_SPECULATIVE)) {
+               if (pmd_none(*vmf->pmd)) {
+                       if (PageTransCompound(page)) {
+                               ret = do_set_pmd(vmf, page);
+                               if (ret != VM_FAULT_FALLBACK)
+                                       return ret;
+                       }
+
+                       if (unlikely(pte_alloc(vma->vm_mm, vmf->pmd)))
+                               return VM_FAULT_OOM;
                }
 
-               if (unlikely(pte_alloc(vma->vm_mm, vmf->pmd)))
-                       return VM_FAULT_OOM;
+               /* See comment in __handle_mm_fault() */
+               if (pmd_devmap_trans_unstable(vmf->pmd))
+                       return 0;
        }
 
-       /* See comment in __handle_mm_fault() */
-       if (pmd_devmap_trans_unstable(vmf->pmd))
-               return 0;
-
-       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
-                                     vmf->address, &vmf->ptl);
+       if (!pte_map_lock(vmf))
+               return VM_FAULT_RETRY;
        ret = 0;
        /* Re-check under ptl */
        if (likely(pte_none(*vmf->pte)))
-- 
2.20.1