[PATCH 6/7] dax: Implement dax_pfn_mkwrite()

[Jan Kara <jack@suse.cz>]

Implement a function that marks existing page table entry (PTE or PMD)
as writeable and takes care of marking it dirty in the radix tree. This
function will be used to finish synchronous page fault where the page
table entry is first inserted as read-only and then needs to be marked
as read-write.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/dax.c            | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/dax.h |  1 +
 2 files changed, 49 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index 8a6cf158c691..90b763c86dc2 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1485,3 +1485,51 @@ int dax_iomap_fault(struct vm_fault *vmf, enum page_entry_size pe_size,
 	}
 }
 EXPORT_SYMBOL_GPL(dax_iomap_fault);
+
+/**
+ * dax_pfn_mkwrite - make page table entry writeable on a DAX file
+ * @vmf: The description of the fault
+ * @pe_size: size of entry to be marked writeable
+ *
+ * This function mark PTE or PMD entry as writeable in page tables for mmaped
+ * DAX file. It takes care of marking corresponding radix tree entry as dirty
+ * as well.
+ */
+int dax_pfn_mkwrite(struct vm_fault *vmf, enum page_entry_size pe_size)
+{
+	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
+	void *entry, **slot;
+	pgoff_t index = vmf->pgoff;
+	pfn_t pfn = pfn_to_pfn_t(pte_pfn(vmf->orig_pte));
+	int vmf_ret, error;
+
+	spin_lock_irq(&mapping->tree_lock);
+	entry = get_unlocked_mapping_entry(mapping, index, &slot);
+	/* Did we race with someone splitting entry or so? */
+	if (!entry || (pe_size == PE_SIZE_PTE && !dax_is_pte_entry(entry)) ||
+	    (pe_size == PE_SIZE_PMD && !dax_is_pmd_entry(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);
+	entry = lock_slot(mapping, slot);
+	spin_unlock_irq(&mapping->tree_lock);
+	switch (pe_size) {
+	case PE_SIZE_PTE:
+		error = vm_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn);
+		vmf_ret = dax_fault_return(error);
+		break;
+#ifdef CONFIG_FS_DAX_PMD
+	case PE_SIZE_PMD:
+		vmf_ret = vmf_insert_pfn_pmd(vmf->vma, vmf->address, vmf->pmd,
+			pfn, true);
+		break;
+#endif
+	default:
+		vmf_ret = VM_FAULT_FALLBACK;
+	}
+	put_locked_mapping_entry(mapping, index);
+	return vmf_ret;
+}
+EXPORT_SYMBOL_GPL(dax_pfn_mkwrite);
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 98950f4d127e..6ce5912e4516 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -92,6 +92,7 @@ ssize_t dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
 		const struct iomap_ops *ops);
 int dax_iomap_fault(struct vm_fault *vmf, enum page_entry_size pe_size,
 		    bool sync, const struct iomap_ops *ops);
+int dax_pfn_mkwrite(struct vm_fault *vmf, enum page_entry_size pe_size);
 int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index);
 int dax_invalidate_mapping_entry_sync(struct address_space *mapping,
 				      pgoff_t index);
-- 
2.12.3