[RFC 4/7] zram: add free space management in backing device

[Minchan Kim <minchan@kernel.org>]

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@kernel.org>
---
 drivers/block/zram/Kconfig    | 13 ++++++++++++
 drivers/block/zram/zram_drv.c | 48 ++++++++++++++++++++++++++++++++++++++++++-
 drivers/block/zram/zram_drv.h |  3 +++
 3 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
index 2f3dd1f..f2ca2b5 100644
--- a/drivers/block/zram/Kconfig
+++ b/drivers/block/zram/Kconfig
@@ -27,3 +27,16 @@ config ZRAM_DEDUP
 	  computation time trade-off. Please check the benefit before
 	  enabling this option. Experiment shows the positive effect when
 	  the zram is used as blockdev and is used to store build output.
+
+config ZRAM_WRITEBACK
+	bool "Write back incompressible page to backing device"
+	depends on ZRAM
+	default n
+	help
+	  With incompressible page, there is no memory saving to keep it
+	  in memory. Instead, write it out to backing device.
+	  For this feature, admin should set up backing device via
+	  /sys/block/zramX/backing_dev.
+
+	  See zram.txt for more infomration.
+
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index dcb6f83..d82914e 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -298,6 +298,9 @@ 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,
@@ -337,7 +340,8 @@ 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;
 	size_t sz;
@@ -388,16 +392,27 @@ 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);
@@ -407,6 +422,9 @@ 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);
 
@@ -422,6 +440,34 @@ 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 void reset_bdev(struct zram *zram) {};
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 5193bcb..8ae3b3f 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -140,6 +140,9 @@ 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
 };
 
-- 
2.7.4