OpenZFS bug causes mass segfaults and silent outages; fixed in versions 2.4.4 and 2.3.9

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A kernel-level arithmetic flaw in OpenZFS versions before 2.4.4 and 2.3.9 caused widespread, seemingly random process crashes on affected hosts. The bug in zfs_fillpage() triggered an unsigned integer underflow when a file was truncated while an mmap page fault was in progress, causing the kernel to zero-fill large swaths of physical memory belonging to unrelated processes. The resulting crash of systemd's PID 1, combined with its default CrashAction=freeze setting, left affected machines completely unresponsive for hours while still technically running. From the outside, the server appeared powered off — no SSH, no HTTP responses — though journal logs confirmed it had been silently failing the entire time. OpenZFS released fixes on 21 August 2026 in versions 2.4.4 and 2.3.9, and administrators can limit outage duration by setting CrashAction=reboot in a systemd drop-in file.

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