[RFC,v6,5/7] dma-buf: heaps: Add deferred-free-helper library code

diff --git a/drivers/dma-buf/heaps/Kconfig b/drivers/dma-buf/heaps/Kconfig
index a5eef06c4226..f7aef8bc7119 100644
--- a/drivers/dma-buf/heaps/Kconfig
+++ b/drivers/dma-buf/heaps/Kconfig
@@ -1,3 +1,6 @@ 
+config DMABUF_HEAPS_DEFERRED_FREE
+	tristate
+
 config DMABUF_HEAPS_SYSTEM
 	bool "DMA-BUF System Heap"
 	depends on DMABUF_HEAPS
diff --git a/drivers/dma-buf/heaps/Makefile b/drivers/dma-buf/heaps/Makefile
index 974467791032..4e7839875615 100644
--- a/drivers/dma-buf/heaps/Makefile
+++ b/drivers/dma-buf/heaps/Makefile
@@ -1,3 +1,4 @@ 
 # SPDX-License-Identifier: GPL-2.0
+obj-$(CONFIG_DMABUF_HEAPS_DEFERRED_FREE) += deferred-free-helper.o
 obj-$(CONFIG_DMABUF_HEAPS_SYSTEM)	+= system_heap.o
 obj-$(CONFIG_DMABUF_HEAPS_CMA)		+= cma_heap.o
diff --git a/drivers/dma-buf/heaps/deferred-free-helper.c b/drivers/dma-buf/heaps/deferred-free-helper.c
new file mode 100644
index 000000000000..672c3d5872e9
--- /dev/null
+++ b/drivers/dma-buf/heaps/deferred-free-helper.c
@@ -0,0 +1,145 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Deferred dmabuf freeing helper
+ *
+ * Copyright (C) 2020 Linaro, Ltd.
+ *
+ * Based on the ION page pool code
+ * Copyright (C) 2011 Google, Inc.
+ */
+
+#include <linux/freezer.h>
+#include <linux/list.h>
+#include <linux/slab.h>
+#include <linux/swap.h>
+#include <linux/sched/signal.h>
+
+#include "deferred-free-helper.h"
+
+static LIST_HEAD(free_list);
+static size_t list_size_pages;
+wait_queue_head_t freelist_waitqueue;
+struct task_struct *freelist_task;
+static DEFINE_SPINLOCK(free_list_lock);
+
+static inline size_t size_to_pages(size_t size)
+{
+	if (!size)
+		return 0;
+	return ((size - 1) >> PAGE_SHIFT) + 1;
+}
+
+void deferred_free(struct deferred_freelist_item *item,
+		   void (*free)(struct deferred_freelist_item*,
+				enum df_reason),
+		   size_t size)
+{
+	unsigned long flags;
+
+	INIT_LIST_HEAD(&item->list);
+	item->size = size;
+	item->free = free;
+
+	spin_lock_irqsave(&free_list_lock, flags);
+	list_add(&item->list, &free_list);
+	list_size_pages += size_to_pages(size);
+	spin_unlock_irqrestore(&free_list_lock, flags);
+	wake_up(&freelist_waitqueue);
+}
+EXPORT_SYMBOL_GPL(deferred_free);
+
+static size_t free_one_item(enum df_reason reason)
+{
+	unsigned long flags;
+	size_t nr_pages;
+	struct deferred_freelist_item *item;
+
+	spin_lock_irqsave(&free_list_lock, flags);
+	if (list_empty(&free_list)) {
+		spin_unlock_irqrestore(&free_list_lock, flags);
+		return 0;
+	}
+	item = list_first_entry(&free_list, struct deferred_freelist_item, list);
+	list_del(&item->list);
+	nr_pages = size_to_pages(item->size);
+	list_size_pages -= nr_pages;
+	spin_unlock_irqrestore(&free_list_lock, flags);
+
+	item->free(item, reason);
+	return nr_pages;
+}
+
+static unsigned long get_freelist_size_pages(void)
+{
+	unsigned long size;
+	unsigned long flags;
+
+	spin_lock_irqsave(&free_list_lock, flags);
+	size = list_size_pages;
+	spin_unlock_irqrestore(&free_list_lock, flags);
+	return size;
+}
+
+static unsigned long freelist_shrink_count(struct shrinker *shrinker,
+					   struct shrink_control *sc)
+{
+	return get_freelist_size_pages();
+}
+
+static unsigned long freelist_shrink_scan(struct shrinker *shrinker,
+					  struct shrink_control *sc)
+{
+	unsigned long total_freed = 0;
+
+	if (sc->nr_to_scan == 0)
+		return 0;
+
+	while (total_freed < sc->nr_to_scan) {
+		size_t pages_freed = free_one_item(DF_UNDER_PRESSURE);
+
+		if (!pages_freed)
+			break;
+
+		total_freed += pages_freed;
+	}
+
+	return total_freed;
+}
+
+static struct shrinker freelist_shrinker = {
+	.count_objects = freelist_shrink_count,
+	.scan_objects = freelist_shrink_scan,
+	.seeks = DEFAULT_SEEKS,
+	.batch = 0,
+};
+
+static int deferred_free_thread(void *data)
+{
+	while (true) {
+		wait_event_freezable(freelist_waitqueue,
+				     get_freelist_size_pages() > 0);
+
+		free_one_item(DF_NORMAL);
+	}
+
+	return 0;
+}
+
+static int deferred_freelist_init(void)
+{
+	list_size_pages = 0;
+
+	init_waitqueue_head(&freelist_waitqueue);
+	freelist_task = kthread_run(deferred_free_thread, NULL,
+				    "%s", "dmabuf-deferred-free-worker");
+	if (IS_ERR(freelist_task)) {
+		pr_err("Creating thread for deferred free failed\n");
+		return -1;
+	}
+	sched_set_normal(freelist_task, 19);
+
+	return register_shrinker(&freelist_shrinker);
+}
+module_init(deferred_freelist_init);
+MODULE_LICENSE("GPL v2");
+
diff --git a/drivers/dma-buf/heaps/deferred-free-helper.h b/drivers/dma-buf/heaps/deferred-free-helper.h
new file mode 100644
index 000000000000..18b44ac86ef6
--- /dev/null
+++ b/drivers/dma-buf/heaps/deferred-free-helper.h
@@ -0,0 +1,55 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef DEFERRED_FREE_HELPER_H
+#define DEFERRED_FREE_HELPER_H
+
+/**
+ * df_reason - enum for reason why item was freed
+ *
+ * This provides a reason for why the free function was called
+ * on the item. This is useful when deferred_free is used in
+ * combination with a pagepool, so under pressure the page can
+ * be immediately freed.
+ *
+ * DF_NORMAL:         Normal deferred free
+ *
+ * DF_UNDER_PRESSURE: Free was called because the system
+ *                    is under memory pressure. Usually
+ *                    from a shrinker. Avoid allocating
+ *                    memory in the free call, as it may
+ *                    fail.
+ */
+enum df_reason {
+	DF_NORMAL,
+	DF_UNDER_PRESSURE,
+};
+
+/**
+ * deferred_freelist_item - item structure for deferred freelist
+ *
+ * This is to be added to the structure for whatever you want to
+ * defer freeing on.
+ *
+ * @size: size of the item to be freed
+ * @free: function pointer to be called when freeing the item
+ * @list: list entry for the deferred list
+ */
+struct deferred_freelist_item {
+	size_t size;
+	void (*free)(struct deferred_freelist_item *i,
+		     enum df_reason reason);
+	struct list_head list;
+};
+
+/**
+ * deferred_free - call to add item to the deferred free list
+ *
+ * @item: Pointer to deferred_freelist_item field of a structure
+ * @free: Function pointer to the free call
+ * @size: Size of the item to be freed
+ */
+void deferred_free(struct deferred_freelist_item *item,
+		   void (*free)(struct deferred_freelist_item *i,
+				enum df_reason reason),
+		   size_t size);
+#endif