[PATCH 3/5] zram: add alloc_block_bdev_range() and free_block_bdev_range()

[Pankaj Raghav <kernel@pankajraghav.com>]

From: Pankaj Raghav <p.raghav@samsung.com>

Add [alloc|free]_block_bdev_range() which accepts number of blocks to
allocate or free from the block bitmap.

alloc_block_bdev_range() tries to allocate a range of bitmap based in
the input nr_of_blocks whenever possible, or else it will retry with a
smaller value. This is done so that we don't unnecessarily return EIO
when the underlying device is fragmented.

alloc_block_bdev_range() is not an atomic operation as this function can
be called only from writeback_store() and init_lock is anyway taken
making sure there cannot be two processes allocating from bdev bitmap.

free_block_bdev_range() is just a simple loop that calls the atomic
free_block_bdev() function. As bdev bitmap free can be called from
two different process simulataneously without a lock, atomicity needs
to be maintained.

This is useful when we want to send larger IOs to the backing dev.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
 drivers/block/zram/zram_drv.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index bd93ed653b99..0b8f814e11dd 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -576,6 +576,39 @@ static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
 	atomic64_dec(&zram->stats.bd_count);
 }
 
+static unsigned long alloc_block_bdev_range(struct zram *zram,
+					    unsigned int *nr_of_blocksp)
+{
+	unsigned long blk_idx;
+	unsigned int nr_of_blocks = *nr_of_blocksp;
+retry:
+	/* skip 0 bit to confuse zram.handle = 0 */
+	blk_idx = 1;
+	blk_idx = bitmap_find_next_zero_area(zram->bitmap, zram->nr_pages,
+					     blk_idx, nr_of_blocks, 0);
+
+	if ((blk_idx + nr_of_blocks) > zram->nr_pages) {
+		if (nr_of_blocks == 1)
+			return 0;
+
+		nr_of_blocks = nr_of_blocks / 2;
+		goto retry;
+	}
+
+	bitmap_set(zram->bitmap, blk_idx, nr_of_blocks);
+	atomic64_add(nr_of_blocks, &zram->stats.bd_count);
+	*nr_of_blocksp = nr_of_blocks;
+
+	return blk_idx;
+}
+
+static void free_block_bdev_range(struct zram *zram, unsigned long blk_idx,
+				  unsigned int nr_of_blocks)
+{
+	for (unsigned int i = 0; i < nr_of_blocks; i++)
+		free_block_bdev(zram, blk_idx + i);
+}
+
 static void read_from_bdev_async(struct zram *zram, struct page *page,
 			unsigned long entry, struct bio *parent)
 {
-- 
2.40.1