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>

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 5797c03..3dea1d5 100644 (file)
--- a/drivers/md/bcache/super.c
+++ b/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;
 }
 

diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
index 1d8b3a9..9a1965c 100644 (file)
--- a/include/uapi/linux/bcache.h
+++ b/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