mm: provide helper for finishing mkwrite faults

Provide a helper function for finishing write faults due to PTE being
read-only.  The helper will be used by DAX to avoid the need of
complicating generic MM code with DAX locking specifics.

Link: http://lkml.kernel.org/r/1479460644-25076-16-git-send-email-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 40 insertions, 27 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 82e7689e3059..bbc25da48a18 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2269,6 +2269,38 @@ oom:
         return VM_FAULT_OOM;
 }
 
+/**
+ * finish_mkwrite_fault - finish page fault for a shared mapping, making PTE
+ *                        writeable once the page is prepared
+ *
+ * @vmf: structure describing the fault
+ *
+ * This function handles all that is needed to finish a write page fault in a
+ * shared mapping due to PTE being read-only once the mapped page is prepared.
+ * It handles locking of PTE and modifying it. The function returns
+ * VM_FAULT_WRITE on success, 0 when PTE got changed before we acquired PTE
+ * lock.
+ *
+ * The function expects the page to be locked or other protection against
+ * concurrent faults / writeback (such as DAX radix tree locks).
+ */
+int finish_mkwrite_fault(struct vm_fault *vmf)
+{
+        WARN_ON_ONCE(!(vmf->vma->vm_flags & VM_SHARED));
+        vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, vmf->address,
+                                       &vmf->ptl);
+        /*
+         * We might have raced with another page fault while we released the
+         * pte_offset_map_lock.
+         */
+        if (!pte_same(*vmf->pte, vmf->orig_pte)) {
+                pte_unmap_unlock(vmf->pte, vmf->ptl);
+                return 0;
+        }
+        wp_page_reuse(vmf);
+        return VM_FAULT_WRITE;
+}
+
 /*
  * Handle write page faults for VM_MIXEDMAP or VM_PFNMAP for a VM_SHARED
  * mapping
@@ -2285,16 +2317,7 @@ static int wp_pfn_shared(struct vm_fault *vmf)
                 ret = vma->vm_ops->pfn_mkwrite(vma, vmf);
                 if (ret & VM_FAULT_ERROR)
                         return ret;
-                vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
-                                vmf->address, &vmf->ptl);
-                /*
-                 * We might have raced with another page fault while we
-                 * released the pte_offset_map_lock.
-                 */
-                if (!pte_same(*vmf->pte, vmf->orig_pte)) {
-                        pte_unmap_unlock(vmf->pte, vmf->ptl);
-                        return 0;
-                }
+                return finish_mkwrite_fault(vmf);
         }
         wp_page_reuse(vmf);
         return VM_FAULT_WRITE;
@@ -2304,7 +2327,6 @@ static int wp_page_shared(struct vm_fault *vmf)
         __releases(vmf->ptl)
 {
         struct vm_area_struct *vma = vmf->vma;
-        int page_mkwrite = 0;
 
         get_page(vmf->page);
 
@@ -2318,26 +2340,17 @@ static int wp_page_shared(struct vm_fault *vmf)
                         put_page(vmf->page);
                         return tmp;
                 }
-                /*
-                 * Since we dropped the lock we need to revalidate
-                 * the PTE as someone else may have changed it.  If
-                 * they did, we just return, as we can count on the
-                 * MMU to tell us if they didn't also make it writable.
-                 */
-                vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
-                                                vmf->address, &vmf->ptl);
-                if (!pte_same(*vmf->pte, vmf->orig_pte)) {
+                tmp = finish_mkwrite_fault(vmf);
+                if (unlikely(!tmp || (tmp &
+                                      (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
                         unlock_page(vmf->page);
-                        pte_unmap_unlock(vmf->pte, vmf->ptl);
                         put_page(vmf->page);
-                        return 0;
+                        return tmp;
                 }
-                page_mkwrite = 1;
-        }
-
-        wp_page_reuse(vmf);
-        if (!page_mkwrite)
+        } else {
+                wp_page_reuse(vmf);
                 lock_page(vmf->page);
+        }
         fault_dirty_shared_page(vma, vmf->page);
         put_page(vmf->page);