zram: directly call zram_read_page in writeback_store

writeback_store always reads a full page, so just call zram_read_page
directly and bypass the boune buffer handling.

Link: https://lkml.kernel.org/r/20230411171459.567614-11-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -54,9 +54,8 @@ 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_bvec_read(struct zram *zram, struct bio_vec *bvec,
-				u32 index, int offset, struct bio *bio);
-
+static int zram_read_page(struct zram *zram, struct page *page, u32 index,
+			  struct bio *bio, bool partial_io);
 
 static int zram_slot_trylock(struct zram *zram, u32 index)
 {
@@ -672,10 +671,6 @@ static ssize_t writeback_store(struct device *dev,
 	}
 
 	for (; nr_pages != 0; index++, nr_pages--) {
-		struct bio_vec bvec;
-
-		bvec_set_page(&bvec, page, PAGE_SIZE, 0);
-
 		spin_lock(&zram->wb_limit_lock);
 		if (zram->wb_limit_enable && !zram->bd_wb_limit) {
 			spin_unlock(&zram->wb_limit_lock);
@@ -719,7 +714,7 @@ static ssize_t writeback_store(struct device *dev,
 		/* Need for hugepage writeback racing */
 		zram_set_flag(zram, index, ZRAM_IDLE);
 		zram_slot_unlock(zram, index);
-		if (zram_bvec_read(zram, &bvec, index, 0, NULL)) {
+		if (zram_read_page(zram, page, index, NULL, false)) {
 			zram_slot_lock(zram, index);
 			zram_clear_flag(zram, index, ZRAM_UNDER_WB);
 			zram_clear_flag(zram, index, ZRAM_IDLE);
@@ -730,9 +725,8 @@ static ssize_t writeback_store(struct device *dev,
 		bio_init(&bio, zram->bdev, &bio_vec, 1,
 			 REQ_OP_WRITE | REQ_SYNC);
 		bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
+		bio_add_page(&bio, page, PAGE_SIZE, 0);
 
-		bio_add_page(&bio, bvec.bv_page, bvec.bv_len,
-				bvec.bv_offset);
 		/*
 		 * XXX: A single page IO would be inefficient for write
 		 * but it would be not bad as starter.