This is to report a bug with Manjaro Linux 5.12.1-2 kernel that immediately affected 4 different, usually stable machines after update to kernel 5.12 from 5.11 or 5.10, and went away after reverting back to either 5.10 or 5.11.
Kernel affected : Linux 5.12.1-2-MANJARO
(Not sure if I tried Manjaro Linux 5.12.1-1-MANJARO)
Kernel not affected : All previous versions up to 5.11.18-1-MANJARO
Symptoms : Under heavy disk usage (such as performing a system backup onto external USB HD) the machine soon completely freezes and only a hard power cycle can get it out of it.
After reboot, systems on which BTRFS is built over bcache may show heavy filesystem corruption.
Happened on :
HP Laptop 1 (Intel Atom) : BTRFS over LUKS on SSD : System freeze, no BTRFS corruption after reboot.
Dell Laptop 1 (Intel Core2 duo) : BTRFS over LUKS on SSD : System freeze, no BTRFS corruption after reboot.
HP Laptop 2 : One BTRFS FS over LUKS on SSD, and one BTRFS over bcache over LUKS on HD+SSD : System freeze, SSD BTRFS was not corrupt but BTRFS over bcache was severely corrupt, beyond repair and had to be rebuilt and restored from backups.
Asus old desktop with AMD Athlon 64 X2 : BTRFS RAID-1 over bcache over LUKS on 2 HD + SSD : System freeze, heavy BTRFS corruption that could however be fixed by simply running a “btrfs scrub” after reverting back to a 5.10 Manjaro kernel.
To be thorough, I also have to report an Arch Linux Intel Celeron machine running 5.12.4-arch1-2 kernel, BTRFS over LUKS on SSD, that has been running for a while without showing any such symptom.
Hope these reports can be useful.