Re: [PATCH 12/15] mm, dax: enable filesystems to trigger page-idle callbacks

+DEFINE_MUTEX(devmap_lock);

static?

+#if IS_ENABLED(CONFIG_FS_DAX)
+static void generic_dax_pagefree(struct page *page, void *data)
+{
+}
+
+struct dax_device *fs_dax_claim_bdev(struct block_device *bdev, void *owner)
+{
+	struct dax_device *dax_dev;
+	struct dev_pagemap *pgmap;
+
+	if (!blk_queue_dax(bdev->bd_queue))
+		return NULL;
+	dax_dev = fs_dax_get_by_host(bdev->bd_disk->disk_name);
+	if (!dax_dev->pgmap)
+		return dax_dev;
+	pgmap = dax_dev->pgmap;

+	mutex_lock(&devmap_lock);
+	if ((pgmap->data && pgmap->data != owner) || pgmap->page_free
+			|| pgmap->page_fault
+			|| pgmap->type != MEMORY_DEVICE_HOST) {
+		put_dax(dax_dev);
+		mutex_unlock(&devmap_lock);
+		return NULL;
+	}
+
+	pgmap->type = MEMORY_DEVICE_FS_DAX;
+	pgmap->page_free = generic_dax_pagefree;
+	pgmap->data = owner;
+	mutex_unlock(&devmap_lock);

All this deep magic will need some explanation.  So far I don't understand
it at all, but maybe the later patches will help..

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