btrfs: access superblock via pagecache in scan_one_device

btrfs_scan_one_device is calling set_blocksize() which can race
with a concurrent process making dirty page cache pages.  It can end up
dropping dirty page cache pages on the floor, which isn't very nice when
someone is just running btrfs dev scan to find filesystems on the
box.

Now that udev is registering btrfs devices as it discovers them, we can
actually end up racing with our own mkfs program too.  When this
happens, we drop some of the important blocks written by mkfs.

This commit changes scan_one_device to read the super out of the page
cache instead of trying to use bread.  This way we don't have to care
about the blocksize of the device.

This also drops the invalidate_bdev() call.  It wasn't very polite to
invalidate during the scan either.  mkfs is putting the super into the
page cache, there's no reason to invalidate at this point.
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/volumes.c

@@ -792,26 +792,77 @@ int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
 	return ret;
 }
 
+/*
+ * Look for a btrfs signature on a device. This may be called out of the mount path
+ * and we are not allowed to call set_blocksize during the scan. The superblock
+ * is read via pagecache
+ */
 int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 			  struct btrfs_fs_devices **fs_devices_ret)
 {
 	struct btrfs_super_block *disk_super;
 	struct block_device *bdev;
-	struct buffer_head *bh;
-	int ret;
+	struct page *page;
+	void *p;
+	int ret = -EINVAL;
 	u64 devid;
 	u64 transid;
 	u64 total_devices;
+	u64 bytenr;
+	pgoff_t index;
 
+	/*
+	 * we would like to check all the supers, but that would make
+	 * a btrfs mount succeed after a mkfs from a different FS.
+	 * So, we need to add a special mount option to scan for
+	 * later supers, using BTRFS_SUPER_MIRROR_MAX instead
+	 */
+	bytenr = btrfs_sb_offset(0);
 	flags |= FMODE_EXCL;
 	mutex_lock(&uuid_mutex);
-	ret = btrfs_get_bdev_and_sb(path, flags, holder, 0, &bdev, &bh);
-	if (ret)
+
+	bdev = blkdev_get_by_path(path, flags, holder);
+
+	if (IS_ERR(bdev)) {
+		ret = PTR_ERR(bdev);
+		printk(KERN_INFO "btrfs: open %s failed\n", path);
 		goto error;
-	disk_super = (struct btrfs_super_block *)bh->b_data;
+	}
+
+	/* make sure our super fits in the device */
+	if (bytenr + PAGE_CACHE_SIZE >= i_size_read(bdev->bd_inode))
+		goto error_bdev_put;
+
+	/* make sure our super fits in the page */
+	if (sizeof(*disk_super) > PAGE_CACHE_SIZE)
+		goto error_bdev_put;
+
+	/* make sure our super doesn't straddle pages on disk */
+	index = bytenr >> PAGE_CACHE_SHIFT;
+	if ((bytenr + sizeof(*disk_super) - 1) >> PAGE_CACHE_SHIFT != index)
+		goto error_bdev_put;
+
+	/* pull in the page with our super */
+	page = read_cache_page_gfp(bdev->bd_inode->i_mapping,
+				   index, GFP_NOFS);
+
+	if (IS_ERR_OR_NULL(page))
+		goto error_bdev_put;
+
+	p = kmap(page);
+
+	/* align our pointer to the offset of the super block */
+	disk_super = p + (bytenr & ~PAGE_CACHE_MASK);
+
+	if (btrfs_super_bytenr(disk_super) != bytenr ||
+	    strncmp((char *)(&disk_super->magic), BTRFS_MAGIC,
+		    sizeof(disk_super->magic)))
+		goto error_unmap;
+
 	devid = btrfs_stack_device_id(&disk_super->dev_item);
 	transid = btrfs_super_generation(disk_super);
 	total_devices = btrfs_super_num_devices(disk_super);
+
 	if (disk_super->label[0]) {
 		if (disk_super->label[BTRFS_LABEL_SIZE - 1])
 			disk_super->label[BTRFS_LABEL_SIZE - 1] = '\0';
@@ -819,12 +870,19 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 	} else {
 		printk(KERN_INFO "device fsid %pU ", disk_super->fsid);
 	}
+
 	printk(KERN_CONT "devid %llu transid %llu %s\n",
 	       (unsigned long long)devid, (unsigned long long)transid, path);
+
 	ret = device_list_add(path, disk_super, devid, fs_devices_ret);
 	if (!ret && fs_devices_ret)
 		(*fs_devices_ret)->total_devices = total_devices;
-	brelse(bh);
+
+error_unmap:
+	kunmap(page);
+	page_cache_release(page);
+
+error_bdev_put:
 	blkdev_put(bdev, flags);
 error:
 	mutex_unlock(&uuid_mutex);