bcache: use read_cache_page_gfp to read the superblock

Avoid a pointless dependency on buffer heads in bcache by simply open
coding reading a single page.  Also add a SB_OFFSET define for the
byte offset of the superblock instead of using magic numbers.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -15,7 +15,6 @@
 #include "writeback.h"
 
 #include <linux/blkdev.h>
-#include <linux/buffer_head.h>
 #include <linux/debugfs.h>
 #include <linux/genhd.h>
 #include <linux/idr.h>
@@ -64,13 +63,14 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
 {
 	const char *err;
 	struct cache_sb_disk *s;
-	struct buffer_head *bh = __bread(bdev, 1, SB_SIZE);
+	struct page *page;
 	unsigned int i;
 
-	if (!bh)
+	page = read_cache_page_gfp(bdev->bd_inode->i_mapping,
+				   SB_OFFSET >> PAGE_SHIFT, GFP_KERNEL);
+	if (IS_ERR(page))
 		return "IO error";
-
-	s = (struct cache_sb_disk *)bh->b_data;
+	s = page_address(page) + offset_in_page(SB_OFFSET);
 
 	sb->offset		= le64_to_cpu(s->offset);
 	sb->version		= le64_to_cpu(s->version);
@@ -188,12 +188,10 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
 	}
 
 	sb->last_mount = (u32)ktime_get_real_seconds();
-	err = NULL;
-
-	get_page(bh->b_page);
 	*res = s;
+	return NULL;
 err:
-	put_bh(bh);
+	put_page(page);
 	return err;
 }
 

include/uapi/linux/bcache.h

@@ -148,6 +148,7 @@ static inline struct bkey *bkey_idx(const struct bkey *k, unsigned int nr_keys)
 #define BCACHE_SB_MAX_VERSION		4
 
 #define SB_SECTOR			8
+#define SB_OFFSET			(SB_SECTOR << SECTOR_SHIFT)
 #define SB_SIZE				4096
 #define SB_LABEL_SIZE			32
 #define SB_JOURNAL_BUCKETS		256U