[Bug 276402] tmpfs: memory reserve does not account for ARC

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=276402

            Bug ID: 276402
           Summary: tmpfs: memory reserve does not account for ARC
           Product: Base System
           Version: 15.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: throwaway_vthgwq4@protonmail.com

When there is tmpfs entry in fstab without size specified, e.g.

none /var/run/user tmpfs rw

And ZFS ARC fills available memory fully, writes to tmpfs will fail with no
space left on device, ARC won't be reclaimed. This results in hangs when
switching browser tabs or spawning Wayland clients in e.g. x11-wm/sway which
uses XDG_RUNTIME_DIR on tmpfs.

Disabling memory reserve with

sysctl vfs.tmpfs.memory_percent=100

restores old behavior (ARC is reclaimed for tmpfs) and gets rid of hangs in
x11-wm/sway.

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