Since f0cc982e52, PostgreSQL 18 uses io_uring when available.
PostgreSQL creates an io_uring instance for each of the
`max_connections = 1000` possible backends. Linux 6.14 and newer
charge that usage (8-13MiB at our default PostgreSQL settings) against
RLIMIT_MEMLOCK. Unfortunately, the systemd default is only 8MiB; on
such kernels (e.g. Ubuntu 26.04), PostgreSQL 18 fails to start with:
FATAL: could not setup io_uring queue: Cannot allocate memory
Raise the limit to 256MiB. This accommodates io_uring usage even at
the maximum configurable io_max_concurrency (~100MiB), times two
clusters running concurrently during pg_upgradecluster (since the
kernel counts locked memory per-user, not per-process).
Installations on older kernels (e.g. Ubuntu 24.04's Linux 6.8) pin
the same memory without accounting it, so they start fine today but
would fail after a kernel upgrade; applying the higher limit
fixes them preemptively.
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