memremap: provide an optional internal refcount in struct dev_pagemap

Provide an internal refcounting logic if no ->ref field is provided
in the pagemap passed into devm_memremap_pages so that callers don't
have to reinvent it poorly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

kernel/memremap.c

@@ -29,7 +29,7 @@ static void devmap_managed_enable_put(void *data)
 
 static int devmap_managed_enable_get(struct device *dev, struct dev_pagemap *pgmap)
 {
-	if (!pgmap->ops->page_free) {
+	if (!pgmap->ops || !pgmap->ops->page_free) {
 		WARN(1, "Missing page_free method\n");
 		return -EINVAL;
 	}
@@ -75,6 +75,24 @@ static unsigned long pfn_next(unsigned long pfn)
 #define for_each_device_pfn(pfn, map) \
 	for (pfn = pfn_first(map); pfn < pfn_end(map); pfn = pfn_next(pfn))
 
+static void dev_pagemap_kill(struct dev_pagemap *pgmap)
+{
+	if (pgmap->ops && pgmap->ops->kill)
+		pgmap->ops->kill(pgmap);
+	else
+		percpu_ref_kill(pgmap->ref);
+}
+
+static void dev_pagemap_cleanup(struct dev_pagemap *pgmap)
+{
+	if (pgmap->ops && pgmap->ops->cleanup) {
+		pgmap->ops->cleanup(pgmap);
+	} else {
+		wait_for_completion(&pgmap->done);
+		percpu_ref_exit(pgmap->ref);
+	}
+}
+
 static void devm_memremap_pages_release(void *data)
 {
 	struct dev_pagemap *pgmap = data;
@@ -84,10 +102,10 @@ static void devm_memremap_pages_release(void *data)
 	unsigned long pfn;
 	int nid;
 
-	pgmap->ops->kill(pgmap);
+	dev_pagemap_kill(pgmap);
 	for_each_device_pfn(pfn, pgmap)
 		put_page(pfn_to_page(pfn));
-	pgmap->ops->cleanup(pgmap);
+	dev_pagemap_cleanup(pgmap);
 
 	/* pages are dead and unused, undo the arch mapping */
 	align_start = res->start & ~(SECTION_SIZE - 1);
@@ -114,20 +132,29 @@ static void devm_memremap_pages_release(void *data)
 		      "%s: failed to free all reserved pages\n", __func__);
 }
 
+static void dev_pagemap_percpu_release(struct percpu_ref *ref)
+{
+	struct dev_pagemap *pgmap =
+		container_of(ref, struct dev_pagemap, internal_ref);
+
+	complete(&pgmap->done);
+}
+
 /**
  * devm_memremap_pages - remap and provide memmap backing for the given resource
  * @dev: hosting device for @res
  * @pgmap: pointer to a struct dev_pagemap
  *
  * Notes:
- * 1/ At a minimum the res, ref and type and ops members of @pgmap must be
- *    initialized by the caller before passing it to this function
+ * 1/ At a minimum the res and type members of @pgmap must be initialized
+ *    by the caller before passing it to this function
  *
  * 2/ The altmap field may optionally be initialized, in which case
  *    PGMAP_ALTMAP_VALID must be set in pgmap->flags.
  *
- * 3/ pgmap->ref must be 'live' on entry and will be killed and reaped
- *    at devm_memremap_pages_release() time, or if this routine fails.
+ * 3/ The ref field may optionally be provided, in which pgmap->ref must be
+ *    'live' on entry and will be killed and reaped at
+ *    devm_memremap_pages_release() time, or if this routine fails.
  *
  * 4/ res is expected to be a host memory range that could feasibly be
  *    treated as a "System RAM" range, i.e. not a device mmio range, but
@@ -175,10 +202,21 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap)
 		break;
 	}
 
-	if (!pgmap->ref || !pgmap->ops || !pgmap->ops->kill ||
-	    !pgmap->ops->cleanup) {
-		WARN(1, "Missing reference count teardown definition\n");
-		return ERR_PTR(-EINVAL);
+	if (!pgmap->ref) {
+		if (pgmap->ops && (pgmap->ops->kill || pgmap->ops->cleanup))
+			return ERR_PTR(-EINVAL);
+
+		init_completion(&pgmap->done);
+		error = percpu_ref_init(&pgmap->internal_ref,
+				dev_pagemap_percpu_release, 0, GFP_KERNEL);
+		if (error)
+			return ERR_PTR(error);
+		pgmap->ref = &pgmap->internal_ref;
+	} else {
+		if (!pgmap->ops || !pgmap->ops->kill || !pgmap->ops->cleanup) {
+			WARN(1, "Missing reference count teardown definition\n");
+			return ERR_PTR(-EINVAL);
+		}
 	}
 
 	if (need_devmap_managed) {
@@ -296,8 +334,8 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap)
  err_pfn_remap:
 	pgmap_array_delete(res);
  err_array:
-	pgmap->ops->kill(pgmap);
-	pgmap->ops->cleanup(pgmap);
+	dev_pagemap_kill(pgmap);
+	dev_pagemap_cleanup(pgmap);
 	return ERR_PTR(error);
 }
 EXPORT_SYMBOL_GPL(devm_memremap_pages);