[19/20] dax: Protect PTE modification on WP fault by radix tree entry lock

Commit Message

Jan Kara Nov. 1, 2016, 10:36 p.m. UTC

Currently PTE gets updated in wp_pfn_shared() after dax_pfn_mkwrite()
has released corresponding radix tree entry lock. When we want to
writeprotect PTE on cache flush, we need PTE modification to happen
under radix tree entry lock to ensure consisten updates of PTE and radix
tree (standard faults use page lock to ensure this consistency). So move
update of PTE bit into dax_pfn_mkwrite().

Signed-off-by: Jan Kara <[e-mail]>
---
 fs/dax.c    | 22 ++++++++++++++++------
 mm/memory.c |  2 +-
 2 files changed, 17 insertions(+), 7 deletions(-)

Patch

diff --git a/fs/dax.c b/fs/dax.c
index c6cadf8413a3..a2d3781c9f4e 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1163,17 +1163,27 @@  int dax_pfn_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
 	struct file *file = vma->vm_file;
 	struct address_space *mapping = file->f_mapping;
-	void *entry;
+	void *entry, **slot;
 	pgoff_t index = vmf->pgoff;
 
 	spin_lock_irq(&mapping->tree_lock);
-	entry = get_unlocked_mapping_entry(mapping, index, NULL);
-	if (!entry || !radix_tree_exceptional_entry(entry))
-		goto out;
+	entry = get_unlocked_mapping_entry(mapping, index, &slot);
+	if (!entry || !radix_tree_exceptional_entry(entry)) {
+		if (entry)
+			put_unlocked_mapping_entry(mapping, index, entry);
+		spin_unlock_irq(&mapping->tree_lock);
+		return VM_FAULT_NOPAGE;
+	}
 	radix_tree_tag_set(&mapping->page_tree, index, PAGECACHE_TAG_DIRTY);
-	put_unlocked_mapping_entry(mapping, index, entry);
-out:
+	entry = lock_slot(mapping, slot);
 	spin_unlock_irq(&mapping->tree_lock);
+	/*
+	 * If we race with somebody updating the PTE and finish_mkwrite_fault()
+	 * fails, we don't care. We need to return VM_FAULT_NOPAGE and retry
+	 * the fault in either case.
+	 */
+	finish_mkwrite_fault(vmf);
+	put_locked_mapping_entry(mapping, index, entry);
 	return VM_FAULT_NOPAGE;
 }
 EXPORT_SYMBOL_GPL(dax_pfn_mkwrite);
diff --git a/mm/memory.c b/mm/memory.c
index e7a4a30a5e88..5fa3d0c5196e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2310,7 +2310,7 @@  static int wp_pfn_shared(struct vm_fault *vmf)
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
 		vmf->flags |= FAULT_FLAG_MKWRITE;
 		ret = vma->vm_ops->pfn_mkwrite(vma, vmf);
-		if (ret & VM_FAULT_ERROR)
+		if (ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))
 			return ret;
 		return finish_mkwrite_fault(vmf);
 	}