[PATCH v2 0/3] btrfs: Introduce new rescue= mount options

[PATCH v2 0/3] btrfs: Introduce new rescue= mount options

[Qu Wenruo]

This patchset can be fetched from github:
https://github.com/adam900710/linux/tree/rescue_options

There are quite some btrfs extent tree corruption report in the mail
list.
Since btrfs will do mount time block group item search, one corrupted
leaf containing block group item will prevent the whole fs to be
mounted.

This patchset will try to address the problem by introducing a new mount
option, "rescue=skip_bg", as a last-resort rescue.
Of course this option will have a lot of restriction to prevent further
screwing up the fs, including:
- Permanent RO
  No remount rw is allowed

- No dirty log
  Either clean the log or use rescue=no_log_replay mount option

This "rescue=skip_bg" has some advantage compared to user space tool
like "btrfs-restore":
- Unified recovery tool
  User can use any tool they're familiar with, as long as the kernel
  doesn't panic.

- More info for subvolume.
  "btrfs subv list" can word now!


Although, move the following mount options to "rescue=" mount option:
- nologreplay
  to rescue=no_log_replay

- usebackuproot
  to rescue=use_backup_root

Old options are still available for compatibility purpose, but they are
deprecated in favor of new 'rescue=' super option.

Also different rescue sub options can be separated by ':', like:
"rescue=no_log_replay:skip_bg:use_backup_root".

The separation character is chosen by:
- No conflicts with existing character
  Especially no conflict with ','.

- No extra escaping/quota
  Original plan is ';', but since it'll be interpreted by bash, it's
  changed to current ':'.

Changelog:
v2:
- Introduce 'rescue=' super option.
- Rename original 'usebackuproot' and 'nologreplay'.
  It at least makes my vim spell check happier.
- Remove 'recovery' mount option.
  As its successor is now deprecated, not need to keep the predecessor.

Qu Wenruo (3):
  btrfs: Remove "recovery" mount option
  btrfs: Introduce "rescue=" mount option
  btrfs: Introduce new mount option to skip block group items scan

 fs/btrfs/ctree.h       |  1 +
 fs/btrfs/disk-io.c     | 29 +++++++++++--
 fs/btrfs/extent-tree.c | 59 ++++++++++++++++++++++++++
 fs/btrfs/super.c       | 96 ++++++++++++++++++++++++++++++++++++------
 fs/btrfs/volumes.c     |  7 +++
 5 files changed, 176 insertions(+), 16 deletions(-)

-- 
2.20.1

[PATCH v2 1/3] btrfs: Remove "recovery" mount option

[Qu Wenruo]

Commit 8dcddfa048de ("btrfs: Introduce new mount option usebackuproot to
replace recovery") deprecates "recovery" mount option in 2016, and it
has been 3 years, it should be OK to remove "recovery" mount option.

As we're even going to deprecate the successor, "usebackuproot" mount
option, there isn't really much need to keep the original option.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/super.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index c5586ffd1426..6a08f23e5600 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -311,7 +311,6 @@ enum {
 	Opt_defrag, Opt_nodefrag,
 	Opt_discard, Opt_nodiscard,
 	Opt_nologreplay,
-	Opt_norecovery,
 	Opt_ratio,
 	Opt_rescan_uuid_tree,
 	Opt_skip_balance,
@@ -329,7 +328,6 @@ enum {
 
 	/* Deprecated options */
 	Opt_alloc_start,
-	Opt_recovery,
 	Opt_subvolrootid,
 
 	/* Debugging options */
@@ -374,7 +372,6 @@ static const match_table_t tokens = {
 	{Opt_discard, "discard"},
 	{Opt_nodiscard, "nodiscard"},
 	{Opt_nologreplay, "nologreplay"},
-	{Opt_norecovery, "norecovery"},
 	{Opt_ratio, "metadata_ratio=%u"},
 	{Opt_rescan_uuid_tree, "rescan_uuid_tree"},
 	{Opt_skip_balance, "skip_balance"},
@@ -396,7 +393,6 @@ static const match_table_t tokens = {
 
 	/* Deprecated options */
 	{Opt_alloc_start, "alloc_start=%s"},
-	{Opt_recovery, "recovery"},
 	{Opt_subvolrootid, "subvolrootid=%d"},
 
 	/* Debugging options */
@@ -664,7 +660,6 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 			btrfs_clear_and_info(info, NOTREELOG,
 					     "enabling tree log");
 			break;
-		case Opt_norecovery:
 		case Opt_nologreplay:
 			btrfs_set_and_info(info, NOLOGREPLAY,
 					   "disabling log replay at mount time");
@@ -753,10 +748,6 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 			btrfs_clear_and_info(info, AUTO_DEFRAG,
 					     "disabling auto defrag");
 			break;
-		case Opt_recovery:
-			btrfs_warn(info,
-				   "'recovery' is deprecated, use 'usebackuproot' instead");
-			/* fall through */
 		case Opt_usebackuproot:
 			btrfs_info(info,
 				   "trying to use backup root at mount time");
-- 
2.20.1

[PATCH v2 2/3] btrfs: Introduce "rescue=" mount option

[Qu Wenruo]

This patch introduces a new "rescue=" mount option for all those mount
options for data recovery.

Different rescue sub options are seperated by ':'.
(The original plan is to use ';', but ';' needs to be escaped/quoted,
or it will be interpreted by bash)

The following mount options are converted to "rescue=", old mount
options are deprecated but still available for compatibility purpose:

- usebackuproot
  Now it's "rescue=use_backup_root"

- nologreplay
  Now it's "rescue=no_log_replay"

The new underscore is here to make the option more readable and make
spell check happier.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/super.c | 65 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 62 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 6a08f23e5600..0fb251e7bf5b 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -310,7 +310,6 @@ enum {
 	Opt_datasum, Opt_nodatasum,
 	Opt_defrag, Opt_nodefrag,
 	Opt_discard, Opt_nodiscard,
-	Opt_nologreplay,
 	Opt_ratio,
 	Opt_rescan_uuid_tree,
 	Opt_skip_balance,
@@ -323,7 +322,6 @@ enum {
 	Opt_subvolid,
 	Opt_thread_pool,
 	Opt_treelog, Opt_notreelog,
-	Opt_usebackuproot,
 	Opt_user_subvol_rm_allowed,
 
 	/* Deprecated options */
@@ -341,6 +339,8 @@ enum {
 #ifdef CONFIG_BTRFS_FS_REF_VERIFY
 	Opt_ref_verify,
 #endif
+	/* Rescue options */
+	Opt_rescue, Opt_usebackuproot, Opt_nologreplay,
 	Opt_err,
 };
 
@@ -401,6 +401,7 @@ static const match_table_t tokens = {
 	{Opt_check_integrity_print_mask, "check_int_print_mask=%u"},
 	{Opt_enospc_debug, "enospc_debug"},
 	{Opt_noenospc_debug, "noenospc_debug"},
+	{Opt_rescue, "rescue=%s"},
 #ifdef CONFIG_BTRFS_DEBUG
 	{Opt_fragment_data, "fragment=data"},
 	{Opt_fragment_metadata, "fragment=metadata"},
@@ -412,6 +413,55 @@ static const match_table_t tokens = {
 	{Opt_err, NULL},
 };
 
+static const match_table_t rescue_tokens = {
+	{Opt_usebackuproot, "use_backup_root"},
+	{Opt_nologreplay, "no_log_replay"},
+	{Opt_err, NULL},
+};
+
+static int parse_rescue_options(struct btrfs_fs_info *info, const char *options)
+{
+	char *opts;
+	char *orig;
+	char *p;
+	substring_t args[MAX_OPT_ARGS];
+	int ret = 0;
+
+	opts = kstrdup(options, GFP_KERNEL);
+	if (!opts)
+		return -ENOMEM;
+	orig = opts;
+
+	while ((p = strsep(&opts, ":")) != NULL) {
+		int token;
+
+		if (!*p)
+			continue;
+		token = match_token(p, rescue_tokens, args);
+		switch (token){
+		case Opt_usebackuproot:
+			btrfs_info(info,
+				   "trying to use backup root at mount time");
+			btrfs_set_opt(info->mount_opt, USEBACKUPROOT);
+			break;
+		case Opt_nologreplay:
+			btrfs_set_and_info(info, NOLOGREPLAY,
+					   "disabling log replay at mount time");
+			break;
+		case Opt_err:
+			btrfs_info(info, "unrecognized rescue option '%s'", p);
+			ret = -EINVAL;
+			goto out;
+		default:
+			break;
+		}
+
+	}
+out:
+	kfree(orig);
+	return ret;
+}
+
 /*
  * Regular mount options parser.  Everything that is needed only when
  * reading in a new superblock is parsed here.
@@ -661,6 +711,8 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 					     "enabling tree log");
 			break;
 		case Opt_nologreplay:
+			btrfs_warn(info,
+	"'nologreplay' is deprecated, use 'rescue=no_log_replay' instead");
 			btrfs_set_and_info(info, NOLOGREPLAY,
 					   "disabling log replay at mount time");
 			break;
@@ -749,6 +801,8 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 					     "disabling auto defrag");
 			break;
 		case Opt_usebackuproot:
+			btrfs_warn(info,
+	"'usebackuproot' is deprecated, use 'rescue=use_backup_root' instead");
 			btrfs_info(info,
 				   "trying to use backup root at mount time");
 			btrfs_set_opt(info->mount_opt, USEBACKUPROOT);
@@ -835,6 +889,11 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 			btrfs_set_opt(info->mount_opt, REF_VERIFY);
 			break;
 #endif
+		case Opt_rescue:
+			ret = parse_rescue_options(info, args[0].from);
+			if (ret < 0)
+				goto out;
+			break;
 		case Opt_err:
 			btrfs_info(info, "unrecognized mount option '%s'", p);
 			ret = -EINVAL;
@@ -1301,7 +1360,7 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
 	if (btrfs_test_opt(info, NOTREELOG))
 		seq_puts(seq, ",notreelog");
 	if (btrfs_test_opt(info, NOLOGREPLAY))
-		seq_puts(seq, ",nologreplay");
+		seq_puts(seq, ",rescue=no_log_replay");
 	if (btrfs_test_opt(info, FLUSHONCOMMIT))
 		seq_puts(seq, ",flushoncommit");
 	if (btrfs_test_opt(info, DISCARD))
-- 
2.20.1

[PATCH v2 3/3] btrfs: Introduce new mount option to skip block group items scan

[Qu Wenruo]

[PROBLEM]
There are some reports of corrupted fs which can't be mounted due to
corrupted extent tree.

However under such situation, it's more likely the fs/subvolume trees
are still fine.

For such case we normally go btrfs-restore and salvage as much as we
can. However btrfs-restore can't list subvolumes as "btrfs subv list",
making it harder to restore a fs.

[ENHANCEMENT]
This patch will introduce a new mount option "rescue=skip_bg" to skip
the mount time block group scan, and use chunk info purely to populate
fake block group cache.

The mount option has the following dependency:
- RO mount
  Obviously.

- No log tree or use nologreplay mount option

- No way to remoutn RW
  Similar to nologreplay option.

This should allow kernel to accept most extent tree corruption, and
salvage data and subvolume info.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/ctree.h       |  1 +
 fs/btrfs/disk-io.c     | 29 ++++++++++++++++++---
 fs/btrfs/extent-tree.c | 59 ++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/super.c       | 24 ++++++++++++++++-
 fs/btrfs/volumes.c     |  7 +++++
 5 files changed, 115 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 0a68cf7032f5..00ae6d72b790 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1403,6 +1403,7 @@ static inline u32 BTRFS_MAX_XATTR_SIZE(const struct btrfs_fs_info *info)
 #define BTRFS_MOUNT_FREE_SPACE_TREE	(1 << 26)
 #define BTRFS_MOUNT_NOLOGREPLAY		(1 << 27)
 #define BTRFS_MOUNT_REF_VERIFY		(1 << 28)
+#define BTRFS_MOUNT_SKIP_BG		(1 << 29)
 
 #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 8da2f380d3c0..a44ad0815478 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2315,11 +2315,15 @@ 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, SKIP_BG)) {
+			ret = PTR_ERR(root);
+			goto out;
+		}
+		fs_info->extent_root = NULL;
+	} 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;
 
 	location.objectid = BTRFS_DEV_TREE_OBJECTID;
 	root = btrfs_read_tree_root(tree_root, &location);
@@ -2936,6 +2940,23 @@ int open_ctree(struct super_block *sb,
 		goto fail_alloc;
 	}
 
+	/* Skip bg needs RO and no log tree replay */
+	if (btrfs_test_opt(fs_info, SKIP_BG)) {
+		if (!sb_rdonly(sb)) {
+			btrfs_err(fs_info,
+		"skip_bg mount option can only be used with read-only mount");
+			err = -EINVAL;
+			goto fail_alloc;
+		}
+		if (btrfs_super_log_root(disk_super) &&
+		    !btrfs_test_opt(fs_info, NOTREELOG)) {
+			btrfs_err(fs_info,
+	"skip_bg must be used with notreelog mount option for dirty log");
+			err = -EINVAL;
+			goto fail_alloc;
+		}
+	}
+
 	ret = btrfs_init_workqueues(fs_info, fs_devices);
 	if (ret) {
 		err = ret;
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index b15afeae16df..e2bd6aafe28e 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -10289,6 +10289,62 @@ static int check_chunk_block_group_mappings(struct btrfs_fs_info *fs_info)
 	return ret;
 }
 
+static int fill_dummy_bgs(struct btrfs_fs_info *fs_info)
+{
+	struct extent_map_tree *em_tree = &fs_info->mapping_tree.map_tree;
+	struct extent_map *em;
+	struct map_lookup *map;
+	struct btrfs_block_group_cache *cache;
+	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;
+		cache = btrfs_create_block_group_cache(fs_info, em->start,
+						       em->len);
+		if (!cache) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		/* Fill dummy cache as FULL */
+		cache->flags = map->type;
+		cache->last_byte_to_unpin = (u64)-1;
+		cache->cached = BTRFS_CACHE_FINISHED;
+		btrfs_set_block_group_used(&cache->item, em->len);
+		btrfs_set_block_group_chunk_objectid(&cache->item, em->start);
+		btrfs_set_block_group_flags(&cache->item, map->type);
+		ret = exclude_super_stripes(cache);
+		if (ret) {
+			free_excluded_extents(cache);
+			btrfs_put_block_group(cache);
+			goto out;
+		}
+		free_excluded_extents(cache);
+		ret = btrfs_add_block_group_cache(fs_info, cache);
+		if (ret) {
+			btrfs_remove_free_space_cache(cache);
+			btrfs_put_block_group(cache);
+			goto out;
+		}
+		update_space_info(fs_info, cache->flags, em->start, em->len,
+				  cache->bytes_super, &space_info);
+		cache->space_info = space_info;
+		link_block_group(cache);
+
+		set_avail_alloc_bits(fs_info, cache->flags);
+		if (btrfs_chunk_readonly(fs_info, em->start))
+			inc_block_group_ro(cache, 1);
+	}
+out:
+	read_unlock(&em_tree->lock);
+	return ret;
+}
+
 int btrfs_read_block_groups(struct btrfs_fs_info *info)
 {
 	struct btrfs_path *path;
@@ -10303,6 +10359,9 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
 	u64 feature;
 	int mixed;
 
+	if (btrfs_test_opt(info, SKIP_BG))
+		return fill_dummy_bgs(info);
+
 	feature = btrfs_super_incompat_flags(info->super_copy);
 	mixed = !!(feature & BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS);
 
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 0fb251e7bf5b..6544df00aba1 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -340,7 +340,7 @@ enum {
 	Opt_ref_verify,
 #endif
 	/* Rescue options */
-	Opt_rescue, Opt_usebackuproot, Opt_nologreplay,
+	Opt_rescue, Opt_usebackuproot, Opt_nologreplay, Opt_rescue_skip_bg,
 	Opt_err,
 };
 
@@ -416,6 +416,7 @@ static const match_table_t tokens = {
 static const match_table_t rescue_tokens = {
 	{Opt_usebackuproot, "use_backup_root"},
 	{Opt_nologreplay, "no_log_replay"},
+	{Opt_rescue_skip_bg, "skip_bg"},
 	{Opt_err, NULL},
 };
 
@@ -448,6 +449,10 @@ 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_skip_bg:
+			btrfs_set_and_info(info, SKIP_BG,
+				"skip mount time block group searching");
+			break;
 		case Opt_err:
 			btrfs_info(info, "unrecognized rescue option '%s'", p);
 			ret = -EINVAL;
@@ -1361,6 +1366,8 @@ 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=no_log_replay");
+	if (btrfs_test_opt(info, SKIP_BG))
+		seq_puts(seq, ",rescue=skip_bg");
 	if (btrfs_test_opt(info, FLUSHONCOMMIT))
 		seq_puts(seq, ",flushoncommit");
 	if (btrfs_test_opt(info, DISCARD))
@@ -1787,6 +1794,14 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 	if (ret)
 		goto restore;
 
+	if (btrfs_test_opt(fs_info, SKIP_BG) !=
+	    (old_opts & BTRFS_MOUNT_SKIP_BG)) {
+		btrfs_err(fs_info,
+		"rescue=skip_bg 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);
@@ -1848,6 +1863,13 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 			goto restore;
 		}
 
+		if (btrfs_test_opt(fs_info, SKIP_BG)) {
+			btrfs_err(fs_info,
+		"remounting read-write with rescue=skip_bg is not allowed");
+			ret = -EINVAL;
+			goto restore;
+		}
+
 		ret = btrfs_cleanup_fs_roots(fs_info);
 		if (ret)
 			goto restore;
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 2576b1a379c9..4b2a59a652da 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7879,6 +7879,13 @@ int btrfs_verify_dev_extents(struct btrfs_fs_info *fs_info)
 	u64 prev_dev_ext_end = 0;
 	int ret = 0;
 
+	/*
+	 * For rescue=skip_bg mount option, we're already RO and are salvaging
+	 * data, no need for such strict check.
+	 */
+	if (btrfs_test_opt(fs_info, SKIP_BG))
+		return 0;
+
 	key.objectid = 1;
 	key.type = BTRFS_DEV_EXTENT_KEY;
 	key.offset = 0;
-- 
2.20.1

[PATCH v2 0/3] btrfs: Introduce new rescue= mount options

[Qu Wenruo]

This patchset can be fetched from github:
https://github.com/adam900710/linux/tree/rescue_options

There are quite some btrfs extent tree corruption report in the mail
list.
Since btrfs will do mount time block group item search, one corrupted
leaf containing block group item will prevent the whole fs to be
mounted.

This patchset will try to address the problem by introducing a new mount
option, "rescue=skip_bg", as a last-resort rescue.
Of course this option will have a lot of restriction to prevent further
screwing up the fs, including:
- Permanent RO
  No remount rw is allowed

- No dirty log
  Either clean the log or use rescue=no_log_replay mount option

This "rescue=skip_bg" has some advantage compared to user space tool
like "btrfs-restore":
- Unified recovery tool
  User can use any tool they're familiar with, as long as the kernel
  doesn't panic.

- More info for subvolume.
  "btrfs subv list" can word now!


Although, move the following mount options to "rescue=" mount option:
- nologreplay
  to rescue=no_log_replay

- usebackuproot
  to rescue=use_backup_root

Old options are still available for compatibility purpose, but they are
deprecated in favor of new 'rescue=' super option.

Also different rescue sub options can be separated by ':', like:
"rescue=no_log_replay:skip_bg:use_backup_root".

The separation character is chosen by:
- No conflicts with existing character
  Especially no conflict with ','.

- No extra escaping/quota
  Original plan is ';', but since it'll be interpreted by bash, it's
  changed to current ':'.

Changelog:
v2:
- Introduce 'rescue=' super option.
- Rename original 'usebackuproot' and 'nologreplay'.
  It at least makes my vim spell check happier.
- Remove 'recovery' mount option.
  As its successor is now deprecated, not need to keep the predecessor.

Qu Wenruo (3):
  btrfs: Remove "recovery" mount option
  btrfs: Introduce "rescue=" mount option
  btrfs: Introduce new mount option to skip block group items scan

 fs/btrfs/ctree.h       |  1 +
 fs/btrfs/disk-io.c     | 29 +++++++++++--
 fs/btrfs/extent-tree.c | 59 ++++++++++++++++++++++++++
 fs/btrfs/super.c       | 96 ++++++++++++++++++++++++++++++++++++------
 fs/btrfs/volumes.c     |  7 +++
 5 files changed, 176 insertions(+), 16 deletions(-)

-- 
2.20.1

[PATCH v2 3/3] btrfs: Introduce new mount option to skip block group items scan

[Qu Wenruo]

[PROBLEM]
There are some reports of corrupted fs which can't be mounted due to
corrupted extent tree.

However under such situation, it's more likely the fs/subvolume trees
are still fine.

For such case we normally go btrfs-restore and salvage as much as we
can. However btrfs-restore can't list subvolumes as "btrfs subv list",
making it harder to restore a fs.

[ENHANCEMENT]
This patch will introduce a new mount option "rescue=skip_bg" to skip
the mount time block group scan, and use chunk info purely to populate
fake block group cache.

The mount option has the following dependency:
- RO mount
  Obviously.

- No log tree or use nologreplay mount option

- No way to remoutn RW
  Similar to nologreplay option.

This should allow kernel to accept most extent tree corruption, and
salvage data and subvolume info.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/ctree.h       |  1 +
 fs/btrfs/disk-io.c     | 29 ++++++++++++++++++---
 fs/btrfs/extent-tree.c | 59 ++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/super.c       | 24 ++++++++++++++++-
 fs/btrfs/volumes.c     |  7 +++++
 5 files changed, 115 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 0a68cf7032f5..00ae6d72b790 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1403,6 +1403,7 @@ static inline u32 BTRFS_MAX_XATTR_SIZE(const struct btrfs_fs_info *info)
 #define BTRFS_MOUNT_FREE_SPACE_TREE	(1 << 26)
 #define BTRFS_MOUNT_NOLOGREPLAY		(1 << 27)
 #define BTRFS_MOUNT_REF_VERIFY		(1 << 28)
+#define BTRFS_MOUNT_SKIP_BG		(1 << 29)
 
 #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 8da2f380d3c0..a44ad0815478 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2315,11 +2315,15 @@ 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, SKIP_BG)) {
+			ret = PTR_ERR(root);
+			goto out;
+		}
+		fs_info->extent_root = NULL;
+	} 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;
 
 	location.objectid = BTRFS_DEV_TREE_OBJECTID;
 	root = btrfs_read_tree_root(tree_root, &location);
@@ -2936,6 +2940,23 @@ int open_ctree(struct super_block *sb,
 		goto fail_alloc;
 	}
 
+	/* Skip bg needs RO and no log tree replay */
+	if (btrfs_test_opt(fs_info, SKIP_BG)) {
+		if (!sb_rdonly(sb)) {
+			btrfs_err(fs_info,
+		"skip_bg mount option can only be used with read-only mount");
+			err = -EINVAL;
+			goto fail_alloc;
+		}
+		if (btrfs_super_log_root(disk_super) &&
+		    !btrfs_test_opt(fs_info, NOTREELOG)) {
+			btrfs_err(fs_info,
+	"skip_bg must be used with notreelog mount option for dirty log");
+			err = -EINVAL;
+			goto fail_alloc;
+		}
+	}
+
 	ret = btrfs_init_workqueues(fs_info, fs_devices);
 	if (ret) {
 		err = ret;
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index b15afeae16df..e2bd6aafe28e 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -10289,6 +10289,62 @@ static int check_chunk_block_group_mappings(struct btrfs_fs_info *fs_info)
 	return ret;
 }
 
+static int fill_dummy_bgs(struct btrfs_fs_info *fs_info)
+{
+	struct extent_map_tree *em_tree = &fs_info->mapping_tree.map_tree;
+	struct extent_map *em;
+	struct map_lookup *map;
+	struct btrfs_block_group_cache *cache;
+	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;
+		cache = btrfs_create_block_group_cache(fs_info, em->start,
+						       em->len);
+		if (!cache) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		/* Fill dummy cache as FULL */
+		cache->flags = map->type;
+		cache->last_byte_to_unpin = (u64)-1;
+		cache->cached = BTRFS_CACHE_FINISHED;
+		btrfs_set_block_group_used(&cache->item, em->len);
+		btrfs_set_block_group_chunk_objectid(&cache->item, em->start);
+		btrfs_set_block_group_flags(&cache->item, map->type);
+		ret = exclude_super_stripes(cache);
+		if (ret) {
+			free_excluded_extents(cache);
+			btrfs_put_block_group(cache);
+			goto out;
+		}
+		free_excluded_extents(cache);
+		ret = btrfs_add_block_group_cache(fs_info, cache);
+		if (ret) {
+			btrfs_remove_free_space_cache(cache);
+			btrfs_put_block_group(cache);
+			goto out;
+		}
+		update_space_info(fs_info, cache->flags, em->start, em->len,
+				  cache->bytes_super, &space_info);
+		cache->space_info = space_info;
+		link_block_group(cache);
+
+		set_avail_alloc_bits(fs_info, cache->flags);
+		if (btrfs_chunk_readonly(fs_info, em->start))
+			inc_block_group_ro(cache, 1);
+	}
+out:
+	read_unlock(&em_tree->lock);
+	return ret;
+}
+
 int btrfs_read_block_groups(struct btrfs_fs_info *info)
 {
 	struct btrfs_path *path;
@@ -10303,6 +10359,9 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
 	u64 feature;
 	int mixed;
 
+	if (btrfs_test_opt(info, SKIP_BG))
+		return fill_dummy_bgs(info);
+
 	feature = btrfs_super_incompat_flags(info->super_copy);
 	mixed = !!(feature & BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS);
 
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 0fb251e7bf5b..6544df00aba1 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -340,7 +340,7 @@ enum {
 	Opt_ref_verify,
 #endif
 	/* Rescue options */
-	Opt_rescue, Opt_usebackuproot, Opt_nologreplay,
+	Opt_rescue, Opt_usebackuproot, Opt_nologreplay, Opt_rescue_skip_bg,
 	Opt_err,
 };
 
@@ -416,6 +416,7 @@ static const match_table_t tokens = {
 static const match_table_t rescue_tokens = {
 	{Opt_usebackuproot, "use_backup_root"},
 	{Opt_nologreplay, "no_log_replay"},
+	{Opt_rescue_skip_bg, "skip_bg"},
 	{Opt_err, NULL},
 };
 
@@ -448,6 +449,10 @@ 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_skip_bg:
+			btrfs_set_and_info(info, SKIP_BG,
+				"skip mount time block group searching");
+			break;
 		case Opt_err:
 			btrfs_info(info, "unrecognized rescue option '%s'", p);
 			ret = -EINVAL;
@@ -1361,6 +1366,8 @@ 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=no_log_replay");
+	if (btrfs_test_opt(info, SKIP_BG))
+		seq_puts(seq, ",rescue=skip_bg");
 	if (btrfs_test_opt(info, FLUSHONCOMMIT))
 		seq_puts(seq, ",flushoncommit");
 	if (btrfs_test_opt(info, DISCARD))
@@ -1787,6 +1794,14 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 	if (ret)
 		goto restore;
 
+	if (btrfs_test_opt(fs_info, SKIP_BG) !=
+	    (old_opts & BTRFS_MOUNT_SKIP_BG)) {
+		btrfs_err(fs_info,
+		"rescue=skip_bg 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);
@@ -1848,6 +1863,13 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 			goto restore;
 		}
 
+		if (btrfs_test_opt(fs_info, SKIP_BG)) {
+			btrfs_err(fs_info,
+		"remounting read-write with rescue=skip_bg is not allowed");
+			ret = -EINVAL;
+			goto restore;
+		}
+
 		ret = btrfs_cleanup_fs_roots(fs_info);
 		if (ret)
 			goto restore;
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 2576b1a379c9..4b2a59a652da 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7879,6 +7879,13 @@ int btrfs_verify_dev_extents(struct btrfs_fs_info *fs_info)
 	u64 prev_dev_ext_end = 0;
 	int ret = 0;
 
+	/*
+	 * For rescue=skip_bg mount option, we're already RO and are salvaging
+	 * data, no need for such strict check.
+	 */
+	if (btrfs_test_opt(fs_info, SKIP_BG))
+		return 0;
+
 	key.objectid = 1;
 	key.type = BTRFS_DEV_EXTENT_KEY;
 	key.offset = 0;
-- 
2.20.1