On 13.02.2018 03:13, Qu Wenruo wrote:
> There are reports in mail list, even with latest mainline kernel, btrfs
> can't survive a power loss.
>
> Unlike journal based filesystem, btrfs doesn't use journal for such
> work. (log tree is an optimization for fsync, not to keep fs healthy)
> In btrfs we use metadata CoW to ensure all tree blocks are as atomic as
> superblock.
>
> This leads to an obvious assumption, some code breaks such metadata CoW
> makes btrfs no longer bullet-proof against power loss.
>
> This patch adds extra runtime selftest to find_free_extent(), which
> will check the range in commit root of extent tree to ensure there is no
> overlap at all.
>
> And hopes this could help us to catch the cause of the problem.
>
> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
> ---
> Unfortunately, no new problem exposed yet.
> ---
> fs/btrfs/extent-tree.c | 118 +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 118 insertions(+)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 2f4328511ac8..3b3cd82bce3a 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -7458,6 +7458,114 @@ btrfs_release_block_group(struct btrfs_block_group_cache *cache,
> btrfs_put_block_group(cache);
> }
>
> +#ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
> +/*
> + * Verify if the given extent range [@start, @start + @len) conflicts any
> + * existing extent in commit root.
> + *
> + * Btrfs doesn't use journal, but depends on metadata (and data) CoW to keep
> + * the whole filesystem consistent against powerloss.
> + * If we have overwritten any extent used by previous trans (commit root),
> + * and powerloss happen we will corrupt our filesystem.
Currently do we know if the corruption is caused due to extent overlap
or some other subtle bug? I.e is there a guarnteee that this self-check
should trigger?
> + *
> + * Return 0 if nothing wrong.
> + * Return <0 (including ENOMEM) means we have something wrong.
> + * Except NOEMEM, this normally means we have extent conflicts with previous
> + * transaction.
> + */
> +static int check_extent_conflicts(struct btrfs_fs_info *fs_info,
> + u64 start, u64 len)
> +{
> + struct btrfs_key key;
> + struct btrfs_path *path;
> + struct btrfs_root *extent_root = fs_info->extent_root;
> + u64 extent_start;
> + u64 extent_len;
> + int ret;
> +
> + path = btrfs_alloc_path();
> + if (!path)
> + return -ENOMEM;
> +
> +
> + key.objectid = start + len;
> + key.type = 0;
> + key.offset = 0;
> +
> + /*
> + * Here we use extent commit root to search for any conflicts.
> + * If extent commit tree is already overwritten, we will get transid
> + * error and error out any way.
> + * If extent commit tree is OK, but other extent conflicts, we will
> + * find it.
> + * So anyway, such search should be OK to find the conflicts.
> + */
> + path->search_commit_root = true;
> + ret = btrfs_search_slot(NULL, extent_root, &key, path, 0, 0);
> +
> + if (ret < 0)
> + goto out;
> + /* Impossible as no type/offset should be (u64)-1 */
> + if (ret == 0) {
> + ret = -EUCLEAN;
> + goto out;
> + }
> + ret = btrfs_previous_extent_item(extent_root, path, start);
> + if (ret < 0)
> + goto out;
> + if (ret == 0) {
> + btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
> + extent_start = key.objectid;
> + if (key.type == BTRFS_EXTENT_ITEM_KEY)
> + extent_len = key.offset;
> + else
> + extent_len = fs_info->nodesize;
> + goto report;
> + }
> + /*
> + * Even we didn't found extent starts after @start, we still need to
> + * ensure previous extent doesn't overlap with [@start, @start + @len)
> + */
> + while (1) {
> + extent_len = 0;
> + btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
> + if (key.type == BTRFS_EXTENT_ITEM_KEY)
> + extent_len = key.offset;
> + else if (key.type == BTRFS_METADATA_ITEM_KEY)
> + extent_len = fs_info->nodesize;
> +
> + if (extent_len) {
> + if (extent_len + key.objectid <= start) {
> + ret = 0;
> + goto out;
> + }
> + extent_start = key.objectid;
> + goto report;
> + }
> + if (path->slots[0] == 0) {
> + ret = btrfs_prev_leaf(extent_root, path);
> + if (ret > 0) {
> + ret = 0;
> + goto out;
> + }
> + if (ret < 0)
> + goto out;
> + } else {
> + path->slots[0]--;
> + }
> + }
> +out:
> + btrfs_free_path(path);
> + return ret;
> +report:
> + WARN(1,
> +"broken CoW detected: old extent [%llu, %llu) new extent [%llu, %llu)\n",
> + extent_start, extent_start + extent_len, start, start + len);
> + btrfs_free_path(path);
> + return -EEXIST;
> +}
> +#endif
> +
> /*
> * walks the btree of allocated extents and find a hole of a given size.
> * The key ins is changed to record the hole:
> @@ -7949,6 +8057,16 @@ static noinline int find_free_extent(struct btrfs_fs_info *fs_info,
> spin_unlock(&space_info->lock);
> ins->offset = max_extent_size;
> }
> +#ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
> + if (!ret) {
> + /*
> + * Any extent allocated must not conflict with any extent in
> + * commit root
> + */
> + ret = check_extent_conflicts(fs_info, ins->objectid,
> + ins->offset);
> + }
> +#endif
> return ret;
> }
>
>
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