[Bug 230903] remount root readonly slow on Alix

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230903

            Bug ID: 230903
           Summary: remount root readonly slow on Alix
           Product: Base System
           Version: 11.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: sthalik at tehran.lain.pl

Hey,

On this Alix machine, mounting root readonly after mounting it read-write is
slow, as in:

% mount -w /
% time mount -r / # 15-20 seconds

A workaround is to mount the partition async in which case the remount is
almost instant. My typical workflow is to mount read-write, break something,
then remount read-only for the sake of fsck(8).

The original Alix series use a Compact Flash -> ATA controller. I've observed
this behavior with multiple CF cards. Typical WCHAN is `biowr', `wdrain' or
both. This behavior persists since at least FreeBSD 10 series.

Note, the CF storage isn't that slow, see:
  cat /usr/lib/* > /dev/null  0.10s user 1.21s system 46% cpu 2.821 total
  cat /usr/bin/* > /dev/null  0.12s user 3.09s system 67% cpu 4.756 total

There are some nasty things in KERNCONF, only to reduce logspam and save time
otherwise spent probing for nonexistent controllers. The Alix series have none
of these controllers:

-->--
include GENERIC
[trivia]

nooptions   SMP
nooptions   APIC_IO
nodevice    acpi

nodevice    xenpci # fix build
nooptions   XENHVM # idem
nodevice    hyperv # idem
--<--

I can attach various config files (sysctl.conf, rc.conf, dmesg.boot, fstab,
KERNCONF) if necessary but they look sane other than the aforementioned
KERNCONF.

cheers,
sh

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