btrfs: don't kmap() pages from block devices

Block device mappings are never in highmem so kmap() / kunmap() calls for
pages from block devices are unneeded. Use page_address() instead of
kmap() to get to the virtual addreses.

While we're at it, read_cache_page_gfp() doesn't return NULL on error,
only an ERR_PTR, so use IS_ERR() to check for errors.

Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 89be36a0fed62b69068236db2fb0cef166890784..00baa806662a03dbe3a34e72c66561369027c0f8 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1249,7 +1249,6 @@ int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
 
 void btrfs_release_disk_super(struct page *page)
 {
-       kunmap(page);
        put_page(page);
 }
 
@@ -1277,10 +1276,10 @@ static int btrfs_read_disk_super(struct block_device *bdev, u64 bytenr,
        *page = read_cache_page_gfp(bdev->bd_inode->i_mapping,
                                   index, GFP_KERNEL);
 
-       if (IS_ERR_OR_NULL(*page))
+       if (IS_ERR(*page))
                return 1;
 
-       p = kmap(*page);
+       p = page_address(*page);
 
        /* align our pointer to the offset of the super block */
        *disk_super = p + offset_in_page(bytenr);