Re: [PATCH 1/2] btrfs: Do per-chunk degrade mode check at mount time

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

To Anand Jain,

Any feedback on this method to allow single chunk still be degraded 
mountable?

It should be much better than allowing degraded mount for any missing 
device case.

Thanks,
Qu

Qu Wenruo wrote on 2015/09/16 11:43 +0800:
> Btrfs supports different raid profile for meta/data/sys, and as
> different profile support different tolerated missing device, it's
> better to check if it can be mounted degraded at a per-chunk base.
>
> So this patch will add check for read_one_chunk() against its profile,
> other than checking it against with the lowest duplication profile.
>
> Reported-by: Zhao Lei <zhaolei@cn.fujitsu.com>
> Reported-by: Anand Jain <anand.jain@oracle.com>
> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
> ---
>   fs/btrfs/volumes.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 644e070..3272187 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -6164,12 +6164,15 @@ static int read_one_chunk(struct btrfs_root *root, struct btrfs_key *key,
>   			  struct btrfs_chunk *chunk)
>   {
>   	struct btrfs_mapping_tree *map_tree = &root->fs_info->mapping_tree;
> +	struct super_block *sb = root->fs_info->sb;
>   	struct map_lookup *map;
>   	struct extent_map *em;
>   	u64 logical;
>   	u64 length;
>   	u64 devid;
>   	u8 uuid[BTRFS_UUID_SIZE];
> +	int missing = 0;
> +	int max_tolerated;
>   	int num_stripes;
>   	int ret;
>   	int i;
> @@ -6238,7 +6241,21 @@ static int read_one_chunk(struct btrfs_root *root, struct btrfs_key *key,
>   			btrfs_warn(root->fs_info, "devid %llu uuid %pU is missing",
>   						devid, uuid);
>   		}
> +		if (map->stripes[i].dev->missing)
> +			missing++;
>   		map->stripes[i].dev->in_fs_metadata = 1;
> +
> +	}
> +
> +	/* XXX: Why the function name is SO LOOOOOOOOOOOOOOOOONG?! */
> +	max_tolerated =
> +		btrfs_get_num_tolerated_disk_barrier_failures(map->type);
> +	if (missing > max_tolerated && !(sb->s_flags & MS_RDONLY)) {
> +		free_extent_map(em);
> +		btrfs_error(root->fs_info, -EIO,
> +			"missing device(%d) exceeds the limit(%d), writeable mount is not allowed\n",
> +			missing, max_tolerated);
> +		return -EIO;
>   	}
>
>   	write_lock(&map_tree->map_tree.lock);
>