From 34c4badfcbda4c8900b97667c8f7e62dd34e8fe5 Mon Sep 17 00:00:00 2001
From: Nikolay Borisov <nborisov@suse.com>
Date: Thu, 4 Oct 2018 14:41:17 +0300
Subject: [PATCH 3/6] btrfs: Add handling for disk split-brain scenario during
 fsid change
References: fate#325871
Patch-mainline: Submitted, awaiting review

Even though fsid change without rewrite is a very quick operations it's
still possible to experience a split brain scenario if power loss
occurs at the right time. This patch handle the case where power
failure occurs while the first transaction (the one setting
CHANGING_FSID_V2) flag is being persisted on disk. This can cause
the btrfs_fs_devices of this filesystem to be created by a device which:

 a) has the CHANGING_FSID_V2 flag set but its fsid value is intact

 b) or a device which doesn't have CHANGING_FSID_V2 flag set and its
 fsid value is intact

This situation is trivially handled by the current find_fsid code since
in both cases the devices are going to be treated like ordinary devices.
Since btrfs is always mounted using the superblock of the latest
device (the one with highest generation number), meaning it will have
the CHANGING_FSID_V2 flag set, ensure it's being cleared on mount. On
the first transaction commit following mount all disks will have it
cleared.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/disk-io.c |   15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2863,18 +2863,28 @@ int open_ctree(struct super_block *sb,
 	 * the whole block of INFO_SIZE
 	 */
 	memcpy(fs_info->super_copy, bh->b_data, sizeof(*fs_info->super_copy));
-	memcpy(fs_info->super_for_commit, fs_info->super_copy,
-	       sizeof(*fs_info->super_for_commit));
 	brelse(bh);
 
+	disk_super = fs_info->super_copy;
 	ASSERT(!memcmp(fs_info->fs_devices->fsid, fs_info->super_copy->fsid,
 		       BTRFS_FSID_SIZE));
+
 	if (btrfs_fs_incompat(fs_info, METADATA_UUID)) {
 		ASSERT(!memcmp(fs_info->fs_devices->metadata_uuid,
 				fs_info->super_copy->metadata_uuid,
 				BTRFS_FSID_SIZE));
 	}
 
+	features = btrfs_super_flags(disk_super);
+	if (features & BTRFS_SUPER_FLAG_CHANGING_FSID_V2) {
+		features &= ~BTRFS_SUPER_FLAG_CHANGING_FSID_V2;
+		btrfs_set_super_flags(disk_super, features);
+		btrfs_info(fs_info, "found metadata uuid in progress flag. Clearing");
+	}
+
+	memcpy(fs_info->super_for_commit, fs_info->super_copy,
+	       sizeof(*fs_info->super_for_commit));
+
 	ret = btrfs_check_super_valid(fs_info);
 	if (ret) {
 		btrfs_err(fs_info, "superblock contains fatal errors");
@@ -2882,7 +2892,6 @@ int open_ctree(struct super_block *sb,
 		goto fail_alloc;
 	}
 
-	disk_super = fs_info->super_copy;
 	if (!btrfs_super_root(disk_super))
 		goto fail_alloc;