[PATCH v1 5/9] zram: add free space management in backing device

[Minchan Kim]

With backing device, zram needs management of free space of
backing device.
This patch adds bitmap logic to manage free space which is
very naive. However, it would be simple enough as considering
uncompressible pages's frequenty in zram.

Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 drivers/block/zram/zram_drv.c | 48 ++++++++++++++++++++++++++++++++++++++++++-
 drivers/block/zram/zram_drv.h |  3 +++
 2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index eb20655..e31fef7 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -292,6 +292,9 @@ static void reset_bdev(struct zram *zram)
 	zram->backing_dev = NULL;
 	zram->old_block_size = 0;
 	zram->bdev = NULL;
+
+	kvfree(zram->bitmap);
+	zram->bitmap = NULL;
 }
 
 static ssize_t backing_dev_show(struct device *dev,
@@ -330,7 +333,8 @@ static ssize_t backing_dev_store(struct device *dev,
 	struct file *backing_dev = NULL;
 	struct inode *inode;
 	struct address_space *mapping;
-	unsigned int old_block_size = 0;
+	unsigned int bitmap_sz, old_block_size = 0;
+	unsigned long nr_pages, *bitmap = NULL;
 	struct block_device *bdev = NULL;
 	int err;
 	struct zram *zram = dev_to_zram(dev);
@@ -369,16 +373,27 @@ static ssize_t backing_dev_store(struct device *dev,
 	if (err < 0)
 		goto out;
 
+	nr_pages = i_size_read(inode) >> PAGE_SHIFT;
+	bitmap_sz = BITS_TO_LONGS(nr_pages) * sizeof(long);
+	bitmap = kvzalloc(bitmap_sz, GFP_KERNEL);
+	if (!bitmap) {
+		err = -ENOMEM;
+		goto out;
+	}
+
 	old_block_size = block_size(bdev);
 	err = set_blocksize(bdev, PAGE_SIZE);
 	if (err)
 		goto out;
 
 	reset_bdev(zram);
+	spin_lock_init(&zram->bitmap_lock);
 
 	zram->old_block_size = old_block_size;
 	zram->bdev = bdev;
 	zram->backing_dev = backing_dev;
+	zram->bitmap = bitmap;
+	zram->nr_pages = nr_pages;
 	up_write(&zram->init_lock);
 
 	pr_info("setup backing device %s\n", file_name);
@@ -386,6 +401,9 @@ static ssize_t backing_dev_store(struct device *dev,
 
 	return len;
 out:
+	if (bitmap)
+		kvfree(bitmap);
+
 	if (bdev)
 		blkdev_put(bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);
 
@@ -399,6 +417,34 @@ static ssize_t backing_dev_store(struct device *dev,
 	return err;
 }
 
+static unsigned long get_entry_bdev(struct zram *zram)
+{
+	unsigned long entry;
+
+	spin_lock(&zram->bitmap_lock);
+	/* skip 0 bit to confuse zram.handle = 0 */
+	entry = find_next_zero_bit(zram->bitmap, zram->nr_pages, 1);
+	if (entry == zram->nr_pages) {
+		spin_unlock(&zram->bitmap_lock);
+		return 0;
+	}
+
+	set_bit(entry, zram->bitmap);
+	spin_unlock(&zram->bitmap_lock);
+
+	return entry;
+}
+
+static void put_entry_bdev(struct zram *zram, unsigned long entry)
+{
+	int was_set;
+
+	spin_lock(&zram->bitmap_lock);
+	was_set = test_and_clear_bit(entry, zram->bitmap);
+	spin_unlock(&zram->bitmap_lock);
+	WARN_ON_ONCE(!was_set);
+}
+
 #else
 static bool zram_wb_enabled(struct zram *zram) { return false; }
 static inline void reset_bdev(struct zram *zram) {};
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 113a411..707aec0 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -119,6 +119,9 @@ struct zram {
 	struct file *backing_dev;
 	struct block_device *bdev;
 	unsigned int old_block_size;
+	unsigned long *bitmap;
+	unsigned long nr_pages;
+	spinlock_t bitmap_lock;
 #endif
 };
 #endif
-- 
2.7.4