zram: use zram_read_from_zspool() in writeback

We only can read pages from zspool in writeback, zram_read_page() is not
really right in that context not only because it's a more generic function
that handles ZRAM_WB pages, but also because it requires us to unlock slot
between slot flag check and actual page read.  Use zram_read_from_zspool()
instead and do slot flags check and page read under the same slot lock.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Sergey Senozhatsky <[email protected]>
Cc: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: sergey-senozhatsky

drivers/block/zram/zram_drv.c

@@ -55,8 +55,8 @@ static size_t huge_class_size;
 static const struct block_device_operations zram_devops;
 
 static void zram_free_page(struct zram *zram, size_t index);
-static int zram_read_page(struct zram *zram, struct page *page, u32 index,
-			  struct bio *parent);
+static int zram_read_from_zspool(struct zram *zram, struct page *page,
+				 u32 index);
 
 static int zram_slot_trylock(struct zram *zram, u32 index)
 {
@@ -831,13 +831,10 @@ static ssize_t writeback_store(struct device *dev,
 		 */
 		if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
 			goto next;
+		if (zram_read_from_zspool(zram, page, index))
+			goto next;
 		zram_slot_unlock(zram, index);
 
-		if (zram_read_page(zram, page, index, NULL)) {
-			release_pp_slot(zram, pps);
-			continue;
-		}
-
 		bio_init(&bio, zram->bdev, &bio_vec, 1,
 			 REQ_OP_WRITE | REQ_SYNC);
 		bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);