[Bug 216178] ZFS ARC and L2ARC are unrealistically large, maybe after r307265

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=216178

            Bug ID: 216178
           Summary: ZFS ARC and L2ARC are unrealistically large, maybe
                    after r307265
           Product: Base System
           Version: 11.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: lev at FreeBSD.org

I have two ZFS pools: one with single device (zroot) and one raidz with 5
devices (zstor). zstor has L2ARC of 185GB.

% zpool list -v
NAME          SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH 
ALTROOT
zroot        39.9G  13.7G  26.2G         -    50%    34%  1.00x  ONLINE  -
  gpt/root   39.9G  13.7G  26.2G         -    50%    34%
zstor        13.6T  8.55T  5.08T         -    22%    62%  1.00x  ONLINE  -
  raidz1     13.6T  8.55T  5.08T         -    22%    62%
    ada1         -      -      -         -      -      -
    ada2         -      -      -         -      -      -
    ada3         -      -      -         -      -      -
    ada4         -      -      -         -      -      -
    ada5         -      -      -         -      -      -
cache            -      -      -         -      -      -
  gpt/l2arc   185G   185G   148M         -     0%    99%
%

Both pools has compression enabled (lz4), but compression ratio on zstor is
negligible (it contains mostly media files, like music, films and RAW digital
photos). zroot has compression rate about 1.4:

% zfs get compressratio zroot zstor
NAME   PROPERTY       VALUE  SOURCE
zroot  compressratio  1.43x  -
zstor  compressratio  1.00x  -
%

My system has 16GB of physical memory.

After upgrade to post-r307266 system (10-STABLE) different system tools starts
to show unrealistically large ARC. Upgrade to 11-STABLE (after r307265) doen't
help either. It looks like this:

(1) top output:
Mem: 195M Active, 4678M Inact, 9656M Wired, 1373M Free
ARC: 75G Total, 247M MFU, 6135M MRU, 1190K Anon, 77M Header, 556M Other
Swap: 8192M Total, 8192M Free

(2) zfs-stats -A output:
ARC Size:                               515.61% 74.92   GiB
        Target Size: (Adaptive)         100.00% 14.53   GiB
        Min Size (Hard Limit):          12.50%  1.82    GiB
        Max Size (High Water):          8:1     14.53   GiB


This starts after some uptime with disc activity (like buildworld & backup) and
could be reset to "normal" state with reboot only.

Sometimes, L2ARC starts to grow up to 400% of its size, too. L2ARC shows a lot
of checksum errors as it grow larger than physical size.

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