On 2017年12月08日 15:57, Anand Jain wrote:
> -EXPERIMENTAL-
> As of now when primary SB fails we won't self heal and would fail mount,
> this is an experimental patch which thinks why not go and read backup
> copy.

Just curious about in which real world case that backup super block can
help.
At least from what I see in mail list, only few cases where backup super
helps.

Despite that self super heal seems good, although I agree with David, we
need a weaker but necessary check (magic and fsid from primary super?)
to ensure it's a valid btrfs before we use the backup supers.

Thanks,
Qu


> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>  fs/btrfs/disk-io.c |  8 +++++++-
>  fs/btrfs/volumes.c | 10 +++++++---
>  2 files changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 9b20c1f3563b..a791b8dfe8a8 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -3190,7 +3190,7 @@ struct buffer_head *btrfs_read_dev_super(struct block_device *bdev)
>  	 * So, we need to add a special mount option to scan for
>  	 * later supers, using BTRFS_SUPER_MIRROR_MAX instead
>  	 */
> -	for (i = 0; i < 1; i++) {
> +	for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
>  		ret = btrfs_read_dev_one_super(bdev, i, &bh);
>  		if (ret)
>  			continue;
> @@ -4015,11 +4015,17 @@ static int btrfs_check_super_valid(struct btrfs_fs_info *fs_info)
>  		ret = -EINVAL;
>  	}
>  
> +#if 0
> +	/*
> +	 * Need a way to check for any copy of SB, as its not a
> +	 * strong check, just ignore this for now.
> +	 */
>  	if (btrfs_super_bytenr(sb) != BTRFS_SUPER_INFO_OFFSET) {
>  		btrfs_err(fs_info, "super offset mismatch %llu != %u",
>  			  btrfs_super_bytenr(sb), BTRFS_SUPER_INFO_OFFSET);
>  		ret = -EINVAL;
>  	}
> +#endif
>  
>  	/*
>  	 * Obvious sys_chunk_array corruptions, it must hold at least one key
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 9fa2539a8493..f368db94d62b 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1369,7 +1369,7 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
>  {
>  	struct btrfs_super_block *disk_super;
>  	struct block_device *bdev;
> -	struct page *page;
> +	struct buffer_head *sb_bh;
>  	int ret = -EINVAL;
>  	u64 devid;
>  	u64 transid;
> @@ -1392,8 +1392,12 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
>  		goto error;
>  	}
>  
> -	if (btrfs_read_disk_super(bdev, bytenr, &page, &disk_super))
> +	sb_bh = btrfs_read_dev_super(bdev);
> +	if (IS_ERR(sb_bh)) {
> +		ret = PTR_ERR(sb_bh);
>  		goto error_bdev_put;
> +	}
> +	disk_super = (struct btrfs_super_block *) sb_bh->b_data;
>  
>  	devid = btrfs_stack_device_id(&disk_super->dev_item);
>  	transid = btrfs_super_generation(disk_super);
> @@ -1413,7 +1417,7 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
>  	if (!ret && fs_devices_ret)
>  		(*fs_devices_ret)->total_devices = total_devices;
>  
> -	btrfs_release_disk_super(page);
> +	brelse(sb_bh);
>  
>  error_bdev_put:
>  	blkdev_put(bdev, flags);
>