btrfs-progs: add zero-log to rescue command

Copy the functionality of standalone btrfs-zero-log to the main tool.
Delete man page for btrfs-zero-log and copy the relevant parts into
btrfs-rescue(8).  The standalone utility will be removed later.

Signed-off-by: David Sterba <dsterba@suse.cz>

Documentation/Makefile.in

@@ -10,7 +10,6 @@ MAN8_TXT += btrfs-image.asciidoc
 MAN8_TXT += btrfs-map-logical.asciidoc
 MAN8_TXT += btrfs-show-super.asciidoc
 MAN8_TXT += btrfstune.asciidoc
-MAN8_TXT += btrfs-zero-log.asciidoc
 MAN8_TXT += fsck.btrfs.asciidoc
 MAN8_TXT += mkfs.btrfs.asciidoc
 
@@ -93,6 +92,7 @@ install-man: man
 	$(MV) $(DESTDIR)$(man5dir)/btrfs-mount.5.gz $(DESTDIR)$(man5dir)/btrfs.5.gz
 	$(INSTALL) -m 644 $(GZ_MAN8) $(DESTDIR)$(man8dir)
 	$(LN_S) -f btrfs-check.8.gz $(DESTDIR)$(man8dir)/btrfsck.8.gz
+	$(LN_S) -f btrfs-rescue.8.gz $(DESTDIR)$(man8dir)/btrfs-zero-log.8.gz
 
 uninstall:
 	cd $(DESTDIR)$(man8dir); rm -f btrfs-check.8.gz $(GZ_MAN8)

Documentation/btrfs-rescue.asciidoc

@@ -40,6 +40,30 @@ assume an answer of 'yes' to all questions.
 -v::::
 verbose mode.
 
+*zero-log* <device>::
+clear out log tree
+
+*btrfs rescue zero-log* will remove the log tree if log tree is corrupt, which
+will allow you to mount the filesystem again.
+
+The common case where this happens has been fixed a long time ago,
+so it is unlikely that you will see this particular problem.
+
+One can determine whether *btrfs-zero-log* is needed according to the kernel
+backtrace:
+----
+? replay_one_dir_item+0xb5/0xb5 [btrfs]
+? walk_log_tree+0x9c/0x19d [btrfs]
+? btrfs_read_fs_root_no_radix+0x169/0x1a1 [btrfs]
+? btrfs_recover_log_trees+0x195/0x29c [btrfs]
+? replay_one_dir_item+0xb5/0xb5 [btrfs]
+? btree_read_extent_buffer_pages+0x76/0xbc [btrfs]
+? open_ctree+0xff6/0x132c [btrfs]
+----
+
+If the errors are like above, then *zero-log* could be used to clear
+the log and the filesystem may be mounted normally again.
+
 EXIT STATUS
 -----------
 *btrfs rescue* returns a zero exit status if it succeeds. Non zero is

cmds-rescue.c

@@ -19,6 +19,9 @@
 #include "kerncompat.h"
 
 #include <getopt.h>
+#include "ctree.h"
+#include "transaction.h"
+#include "disk-io.h"
 #include "commands.h"
 #include "utils.h"
 
@@ -149,11 +152,61 @@ int cmd_super_recover(int argc, char **argv)
 	return ret;
 }
 
+const char * const cmd_rescue_zero_log_usage[] = {
+	"btrfs rescue zero-log <device>",
+	"Clear the tree log. Usable if it's corrupted and prevents mount.",
+	"",
+	NULL
+};
+
+int cmd_rescue_zero_log(int argc, char **argv)
+{
+	struct btrfs_root *root;
+	struct btrfs_trans_handle *trans;
+	struct btrfs_super_block *sb;
+	char *devname;
+	int ret;
+
+	if (check_argc_exact(argc, 2))
+		usage(cmd_rescue_zero_log_usage);
+
+	devname = argv[optind];
+	ret = check_mounted(devname);
+	if (ret < 0) {
+		fprintf(stderr, "Could not check mount status: %s\n", strerror(-ret));
+		goto out;
+	} else if (ret) {
+		fprintf(stderr, "%s is currently mounted. Aborting.\n", devname);
+		ret = -EBUSY;
+	}
+
+	root = open_ctree(devname, 0, OPEN_CTREE_WRITES | OPEN_CTREE_PARTIAL);
+	if (!root) {
+		fprintf(stderr, "Could not open ctree\n");
+		return 1;
+	}
+
+	sb = root->fs_info->super_copy;
+	printf("Clearing log on %s, previous log_root %llu, level %u\n",
+			devname,
+			(unsigned long long)btrfs_super_log_root(sb),
+			(unsigned)btrfs_super_log_root_level(sb));
+	trans = btrfs_start_transaction(root, 1);
+	btrfs_set_super_log_root(sb, 0);
+	btrfs_set_super_log_root_level(sb, 0);
+	btrfs_commit_transaction(trans, root);
+	close_ctree(root);
+
+out:
+	return !!ret;
+}
+
 const struct cmd_group rescue_cmd_group = {
 	rescue_cmd_group_usage, NULL, {
 		{ "chunk-recover", cmd_chunk_recover, cmd_chunk_recover_usage, NULL, 0},
 		{ "super-recover", cmd_super_recover, cmd_super_recover_usage, NULL, 0},
-		{ 0, 0, 0, 0, 0 }
+		{ "zero-log", cmd_rescue_zero_log, cmd_rescue_zero_log_usage, NULL, 0},
+		NULL_CMD_STRUCT
 	}
 };