[RFC PATCHv5 5/6] zram: rework bdev block allocation

[Sergey Senozhatsky <senozhatsky@chromium.org>]

First, writeback bdev ->bitmap bits are set only from one
context, as we can have only one single task performing
writeback, so we cannot race with anything else.  Remove
retry path.

Second, we always check ZRAM_WB flag to distinguish writtenback
slots, so we should not confuse 0 bdev block index and 0 handle.
We can use first bdev block (0 bit) for writeback as well.

While at it, give functions slightly more accurate names, as
we don't alloc/free anything there, we reserve a block for
async writeback or release the block.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/block/zram/zram_drv.c | 37 +++++++++++++++++------------------
 1 file changed, 18 insertions(+), 19 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 671ef2ec9b11..ecbd9b25dfed 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -500,6 +500,8 @@ static ssize_t idle_store(struct device *dev,
 }
 
 #ifdef CONFIG_ZRAM_WRITEBACK
+#define INVALID_BDEV_BLOCK		(~0UL)
+
 struct zram_wb_ctl {
 	struct list_head idle_reqs;
 	struct list_head done_reqs;
@@ -746,23 +748,20 @@ static ssize_t backing_dev_store(struct device *dev,
 	return err;
 }
 
-static unsigned long alloc_block_bdev(struct zram *zram)
+static unsigned long zram_reserve_bdev_block(struct zram *zram)
 {
-	unsigned long blk_idx = 1;
-retry:
-	/* skip 0 bit to confuse zram.handle = 0 */
-	blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, blk_idx);
-	if (blk_idx == zram->nr_pages)
-		return 0;
+	unsigned long blk_idx;
 
-	if (test_and_set_bit(blk_idx, zram->bitmap))
-		goto retry;
+	blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, 0);
+	if (blk_idx == zram->nr_pages)
+		return INVALID_BDEV_BLOCK;
 
+	set_bit(blk_idx, zram->bitmap);
 	atomic64_inc(&zram->stats.bd_count);
 	return blk_idx;
 }
 
-static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
+static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
 {
 	int was_set;
 
@@ -887,7 +886,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
 		 * (if enabled).
 		 */
 		zram_account_writeback_rollback(zram);
-		free_block_bdev(zram, req->blk_idx);
+		zram_release_bdev_block(zram, req->blk_idx);
 		return err;
 	}
 
@@ -901,7 +900,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
 	 * finishes.
 	 */
 	if (!zram_test_flag(zram, index, ZRAM_PP_SLOT)) {
-		free_block_bdev(zram, req->blk_idx);
+		zram_release_bdev_block(zram, req->blk_idx);
 		goto out;
 	}
 
@@ -989,8 +988,8 @@ static int zram_writeback_slots(struct zram *zram,
 				struct zram_pp_ctl *ctl,
 				struct zram_wb_ctl *wb_ctl)
 {
+	unsigned long blk_idx = INVALID_BDEV_BLOCK;
 	struct zram_wb_req *req = NULL;
-	unsigned long blk_idx = 0;
 	struct zram_pp_slot *pps;
 	int ret = 0, err = 0;
 	u32 index = 0;
@@ -1022,9 +1021,9 @@ static int zram_writeback_slots(struct zram *zram,
 				ret = err;
 		}
 
-		if (!blk_idx) {
-			blk_idx = alloc_block_bdev(zram);
-			if (!blk_idx) {
+		if (blk_idx == INVALID_BDEV_BLOCK) {
+			blk_idx = zram_reserve_bdev_block(zram);
+			if (blk_idx == INVALID_BDEV_BLOCK) {
 				ret = -ENOSPC;
 				break;
 			}
@@ -1058,7 +1057,7 @@ static int zram_writeback_slots(struct zram *zram,
 		__bio_add_page(&req->bio, req->page, PAGE_SIZE, 0);
 
 		zram_submit_wb_request(zram, wb_ctl, req);
-		blk_idx = 0;
+		blk_idx = INVALID_BDEV_BLOCK;
 		req = NULL;
 		cond_resched();
 		continue;
@@ -1365,7 +1364,7 @@ static int read_from_bdev(struct zram *zram, struct page *page,
 	return -EIO;
 }
 
-static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
+static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
 {
 }
 #endif
@@ -1889,7 +1888,7 @@ static void zram_free_page(struct zram *zram, size_t index)
 
 	if (zram_test_flag(zram, index, ZRAM_WB)) {
 		zram_clear_flag(zram, index, ZRAM_WB);
-		free_block_bdev(zram, zram_get_handle(zram, index));
+		zram_release_bdev_block(zram, zram_get_handle(zram, index));
 		goto out;
 	}
 
-- 
2.52.0.rc1.455.g30608eb744-goog