Re: [PATCH][v2] btrfs: introduce rescue=onlyfs

Qu Wenruo
2020-07-21T23:05:20Z

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On 2020/7/21 下午11:10, Josef Bacik wrote:
> One of the things that came up consistently in talking with Fedora about
> switching to btrfs as default is that btrfs is particularly vulnerable
> to metadata corruption. If any of the core global roots are corrupted,
> the fs is unmountable and fsck can't usually do anything for you without
> some special options.
>
> Qu addressed this sort of with rescue=skipbg, but that's poorly named as
> what it really does is just allow you to operate without an extent root.
> However there are a lot of other roots, and I'd rather not have to do
>
> mount -o rescue=skipbg,rescue=nocsum,rescue=nofreespacetree,rescue=blah
>
> Instead take his original idea and modify it so it just works for
> everything. Turn it into rescue=onlyfs, and then any major root we fail
> to read just gets left empty and we carry on.
>
> Obviously if the fs roots are screwed then the user is in trouble, but
> otherwise this makes it much easier to pull stuff off the disk without
> needing our special rescue tools. I tested this with my TEST_DEV that
> had a bunch of data on it by corrupting the csum tree and then reading
> files off the disk.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> v1->v2:
> - Rebase onto recent misc-next.
> - Add in the fill_dummy_bgs since skipbg is no longer in this tree.
>
> fs/btrfs/block-group.c | 49 +++++++++++++++++++++++++++++
> fs/btrfs/ctree.h | 1 +
> fs/btrfs/disk-io.c | 71 +++++++++++++++++++++++++++++-------------
> fs/btrfs/inode.c | 6 +++-
> fs/btrfs/super.c | 29 +++++++++++++++--
> fs/btrfs/volumes.c | 7 +++++
> 6 files changed, 138 insertions(+), 25 deletions(-)
>
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 884de28a41e4..416fc419fd95 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -1997,6 +1997,52 @@ static int read_one_block_group(struct btrfs_fs_info *info,
> return ret;
> }
>
> +static int fill_dummy_bgs(struct btrfs_fs_info *fs_info)
> +{
> + struct extent_map_tree *em_tree = &fs_info->mapping_tree;
> + struct extent_map *em;
> + struct map_lookup *map;
> + struct btrfs_block_group *bg;
> + struct btrfs_space_info *space_info;
> + struct rb_node *node;
> + int ret = 0;
> +
> + read_lock(&em_tree->lock);
> + for (node = rb_first_cached(&em_tree->map); node;
> + node = rb_next(node)) {
> + em = rb_entry(node, struct extent_map, rb_node);
> + map = em->map_lookup;
> + bg = btrfs_create_block_group_cache(fs_info, em->start);
> + if (!bg) {
> + ret = -ENOMEM;
> + goto out;
> + }
> +
> + /* Fill dummy cache as FULL */
> + bg->length = em->len;
> + bg->flags = map->type;
> + bg->last_byte_to_unpin = (u64)-1;
> + bg->cached = BTRFS_CACHE_FINISHED;
> + bg->used = em->len;
> + bg->flags = map->type;
> + ret = btrfs_add_block_group_cache(fs_info, bg);
> + if (ret) {
> + btrfs_remove_free_space_cache(bg);
> + btrfs_put_block_group(bg);
> + goto out;
> + }
> + btrfs_update_space_info(fs_info, bg->flags, em->len, em->len,
> + 0, &space_info);
> + bg->space_info = space_info;
> + link_block_group(bg);
> +
> + set_avail_alloc_bits(fs_info, bg->flags);
> + }
> +out:
> + read_unlock(&em_tree->lock);
> + return ret;
> +}
> +
> int btrfs_read_block_groups(struct btrfs_fs_info *info)
> {
> struct btrfs_path *path;
> @@ -2007,6 +2053,9 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
> int need_clear = 0;
> u64 cache_gen;
>
> + if (btrfs_test_opt(info, ONLYFS))
> + return fill_dummy_bgs(info);
> +
> key.objectid = 0;
> key.offset = 0;
> key.type = BTRFS_BLOCK_GROUP_ITEM_KEY;
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index b70c2024296f..0be7d9461443 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1266,6 +1266,7 @@ static inline u32 BTRFS_MAX_XATTR_SIZE(const struct btrfs_fs_info *info)
> #define BTRFS_MOUNT_NOLOGREPLAY (1 << 27)
> #define BTRFS_MOUNT_REF_VERIFY (1 << 28)
> #define BTRFS_MOUNT_DISCARD_ASYNC (1 << 29)
> +#define BTRFS_MOUNT_ONLYFS (1 << 30)
>
> #define BTRFS_DEFAULT_COMMIT_INTERVAL (30)
> #define BTRFS_DEFAULT_MAX_INLINE (2048)
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index c850d7f44fbe..0dffa4c12d8c 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -2326,8 +2326,13 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
>
> root = btrfs_read_tree_root(tree_root, &location);
> if (IS_ERR(root)) {
> - ret = PTR_ERR(root);
> - goto out;
> + if (!btrfs_test_opt(fs_info, ONLYFS)) {
> + ret = PTR_ERR(root);
> + goto out;
> + }
> + } else {
> + set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> + fs_info->extent_root = root;
> }
> set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> fs_info->extent_root = root;
> @@ -2335,21 +2340,27 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
> location.objectid = BTRFS_DEV_TREE_OBJECTID;
> root = btrfs_read_tree_root(tree_root, &location);
> if (IS_ERR(root)) {
> - ret = PTR_ERR(root);
> - goto out;
> + if (!btrfs_test_opt(fs_info, ONLYFS)) {
> + ret = PTR_ERR(root);
> + goto out;
> + }
> + } else {
> + set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> + fs_info->dev_root = root;
> + btrfs_init_devices_late(fs_info);
> }
> - set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> - fs_info->dev_root = root;
> - btrfs_init_devices_late(fs_info);
>
> location.objectid = BTRFS_CSUM_TREE_OBJECTID;
> root = btrfs_read_tree_root(tree_root, &location);
> if (IS_ERR(root)) {
> - ret = PTR_ERR(root);
> - goto out;
> + if (!btrfs_test_opt(fs_info, ONLYFS)) {
> + ret = PTR_ERR(root);
> + goto out;
> + }
> + } else {
> + set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> + fs_info->csum_root = root;
> }
> - set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> - fs_info->csum_root = root;
>
> /*
> * This tree can share blocks with some other fs tree during relocation
> @@ -2358,11 +2369,14 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
> root = btrfs_get_fs_root(tree_root->fs_info,
> BTRFS_DATA_RELOC_TREE_OBJECTID, true);
> if (IS_ERR(root)) {
> - ret = PTR_ERR(root);
> - goto out;
> + if (!btrfs_test_opt(fs_info, ONLYFS)) {
> + ret = PTR_ERR(root);
> + goto out;
> + }
> + } else {
> + set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> + fs_info->data_reloc_root = root;
> }
> - set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> - fs_info->data_reloc_root = root;
>
> location.objectid = BTRFS_QUOTA_TREE_OBJECTID;
> root = btrfs_read_tree_root(tree_root, &location);
> @@ -2375,9 +2389,11 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
> location.objectid = BTRFS_UUID_TREE_OBJECTID;
> root = btrfs_read_tree_root(tree_root, &location);
> if (IS_ERR(root)) {
> - ret = PTR_ERR(root);
> - if (ret != -ENOENT)
> - goto out;
> + if (!btrfs_test_opt(fs_info, ONLYFS)) {
> + ret = PTR_ERR(root);
> + if (ret != -ENOENT)
> + goto out;
> + }
> } else {
> set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> fs_info->uuid_root = root;
> @@ -2387,11 +2403,14 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
> location.objectid = BTRFS_FREE_SPACE_TREE_OBJECTID;
> root = btrfs_read_tree_root(tree_root, &location);
> if (IS_ERR(root)) {
> - ret = PTR_ERR(root);
> - goto out;
> + if (!btrfs_test_opt(fs_info, ONLYFS)) {
> + ret = PTR_ERR(root);
> + goto out;
> + }
> + } else {
> + set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> + fs_info->free_space_root = root;
> }
> - set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
> - fs_info->free_space_root = root;
> }
>
> return 0;
> @@ -3106,6 +3125,14 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
> goto fail_alloc;
> }
>
> + /* Skip bg needs RO and no tree-log to replay */

Comment is still the skip-bg version.

> + if (btrfs_test_opt(fs_info, ONLYFS) && !sb_rdonly(sb)) {
> + btrfs_err(fs_info,
> + "rescue=onlyfs can only be used on read-only mount");
> + err = -EINVAL;
> + goto fail_alloc;
> + }
> +
> ret = btrfs_init_workqueues(fs_info, fs_devices);
> if (ret) {
> err = ret;
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 611b3412fbfd..49cd3eba651e 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -2191,7 +2191,8 @@ static blk_status_t btrfs_submit_bio_hook(struct inode *inode, struct bio *bio,
> int skip_sum;
> int async = !atomic_read(&BTRFS_I(inode)->sync_writers);
>
> - skip_sum = BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM;
> + skip_sum = (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM) ||
> + btrfs_test_opt(fs_info, ONLYFS);

This means, if onlyfs is spcified, csum is completely skipped even it
matches.

I'm not yet determined whether it's preferred.

On one hand, even at recovery, user may want csum verification to detect
bad copy if possible, but on the other hand, since user is doing data
salvage, bothering csum could only lead to extra error.

Despite that, the patch looks pretty good and indeed an enhancement to
skipbg.

Thanks,
Qu
>
> if (btrfs_is_free_space_inode(BTRFS_I(inode)))
> metadata = BTRFS_WQ_ENDIO_FREE_SPACE;
> @@ -2846,6 +2847,9 @@ static int btrfs_readpage_end_io_hook(struct btrfs_io_bio *io_bio,
> if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)
> return 0;
>
> + if (btrfs_test_opt(root->fs_info, ONLYFS))
> + return 0;
> +
> if (root->root_key.objectid == BTRFS_DATA_RELOC_TREE_OBJECTID &&
> test_range_bit(io_tree, start, end, EXTENT_NODATASUM, 1, NULL)) {
> clear_extent_bits(io_tree, start, end, EXTENT_NODATASUM);
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 58f890f73650..4dc3f35ca7e3 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -345,6 +345,7 @@ enum {
> Opt_rescue,
> Opt_usebackuproot,
> Opt_nologreplay,
> + Opt_rescue_onlyfs,
>
> /* Deprecated options */
> Opt_recovery,
> @@ -440,6 +441,7 @@ static const match_table_t tokens = {
> static const match_table_t rescue_tokens = {
> {Opt_usebackuproot, "usebackuproot"},
> {Opt_nologreplay, "nologreplay"},
> + {Opt_rescue_onlyfs, "onlyfs"},
> {Opt_err, NULL},
> };
>
> @@ -472,6 +474,11 @@ static int parse_rescue_options(struct btrfs_fs_info *info, const char *options)
> btrfs_set_and_info(info, NOLOGREPLAY,
> "disabling log replay at mount time");
> break;
> + case Opt_rescue_onlyfs:
> + btrfs_set_and_info(info, ONLYFS,
> + "only reading fs roots, also setting nologreplay");
> + btrfs_set_opt(info->mount_opt, NOLOGREPLAY);
> + break;
> case Opt_err:
> btrfs_info(info, "unrecognized rescue option '%s'", p);
> ret = -EINVAL;
> @@ -1400,6 +1407,8 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
> seq_puts(seq, ",notreelog");
> if (btrfs_test_opt(info, NOLOGREPLAY))
> seq_puts(seq, ",rescue=nologreplay");
> + if (btrfs_test_opt(info, ONLYFS))
> + seq_puts(seq, ",rescue=onlyfs");
> if (btrfs_test_opt(info, FLUSHONCOMMIT))
> seq_puts(seq, ",flushoncommit");
> if (btrfs_test_opt(info, DISCARD_SYNC))
> @@ -1839,6 +1848,14 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
> if (ret)
> goto restore;
>
> + if (btrfs_test_opt(fs_info, ONLYFS) !=
> + (old_opts & BTRFS_MOUNT_ONLYFS)) {
> + btrfs_err(fs_info,
> + "rescue=onlyfs mount option can't be changed during remount");
> + ret = -EINVAL;
> + goto restore;
> + }
> +
> btrfs_remount_begin(fs_info, old_opts, *flags);
> btrfs_resize_thread_pool(fs_info,
> fs_info->thread_pool_size, old_thread_pool_size);
> @@ -1904,6 +1921,13 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
> goto restore;
> }
>
> + if (btrfs_test_opt(fs_info, ONLYFS)) {
> + btrfs_err(fs_info,
> + "remounting read-write with rescue=onlyfs is not allowed");
> + ret = -EINVAL;
> + goto restore;
> + }
> +
> ret = btrfs_cleanup_fs_roots(fs_info);
> if (ret)
> goto restore;
> @@ -2208,8 +2232,9 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
> * still can allocate chunks and thus are fine using the currently
> * calculated f_bavail.
> */
> - if (!mixed && block_rsv->space_info->full &&
> - total_free_meta - thresh < block_rsv->size)
> + if (btrfs_test_opt(fs_info, ONLYFS) ||
> + (!mixed && block_rsv->space_info->full &&
> + total_free_meta - thresh < block_rsv->size))
> buf->f_bavail = 0;
>
> buf->f_type = BTRFS_SUPER_MAGIC;
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 537ccf66ee20..e86f78579884 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -7628,6 +7628,13 @@ int btrfs_verify_dev_extents(struct btrfs_fs_info *fs_info)
> u64 prev_dev_ext_end = 0;
> int ret = 0;
>
> + /*
> + * For rescue=onlyfs mount option, we're already RO and are salvaging
> + * data, no need for such strict check.
> + */
> + if (btrfs_test_opt(fs_info, ONLYFS))
> + return 0;
> +
> key.objectid = 1;
> key.type = BTRFS_DEV_EXTENT_KEY;
> key.offset = 0;
>

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