From: Adam Borowski <kilobyte@angband.pl>
Date: Tue, 28 Mar 2017 16:55:05 +0200
Subject: btrfs: warn about RAID5/6 being experimental at mount time
Bug-Debian: https://bugs.debian.org/863290
Origin: https://bugs.debian.org/863290#5

Too many people come complaining about losing their data -- and indeed,
there's no warning outside a wiki and the mailing list tribal knowledge.
Message severity chosen for consistency with XFS -- "alert" makes dmesg
produce nice red background which should get the point across.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
[bwh: Also add_taint() so this is flagged in bug reports]
[2023-01-10: still accurate according to btrfs-progs own manpage:
https://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git/commit/?id=922797e15590b836e377d6dc47b828356cafc2a9]
---
 fs/btrfs/disk-io.c | 8 ++++++++
 1 file changed, 8 insertions(+)

--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3832,6 +3832,15 @@ int __cold open_ctree(struct super_block *sb,
 		btrfs_set_and_info(fs_info, SSD, "enabling ssd optimizations");
 	}
 
+	if ((fs_info->avail_data_alloc_bits |
+	     fs_info->avail_metadata_alloc_bits |
+	     fs_info->avail_system_alloc_bits) &
+	    BTRFS_BLOCK_GROUP_RAID56_MASK) {
+		btrfs_alert(fs_info,
+		"btrfs RAID5/6 is EXPERIMENTAL and has known data-loss bugs");
+		add_taint(TAINT_AUX, LOCKDEP_STILL_OK);
+	}
+
 	/*
 	 * For devices supporting discard turn on discard=async automatically,
 	 * unless it's already set or disabled. This could be turned off by
