Description of problem:
https://github.com/gluster/glusterfs/issues/435 adds support for writeback cache with fuse.

However, it doesn't seem to be working as I would expect. In write tests, I don't see an dirty data in page cache increasing.

Version-Release number of selected component (if applicable):
glusterfs-*5.2-1.el7.x86_64
kernel-3.10.0-957.el7.x86_64 (RHEL 7.6)

How reproducible:
Consistently

Steps to Reproduce:
1. create single brick distribute volume with default settings.

2. mount gluster volume with the kernel-writeback-cache option. output of ps showing that this has been done:
/usr/sbin/glusterfs --kernel-writeback-cache=yes --process-name fuse --volfile-server=172.16.70.128 --volfile-id=/perfvol /mnt/glustervol

3. run an fio write test without fsync options:
fio --name=initialwrite --ioengine=sync --rw=write \
--direct=0 --create_on_open=1 --bs=128k --directory=/mnt/glustervol/ \
--filename_format=f.\$jobnum.\$filenum --filesize=16g \
--size=16g --numjobs=4


Actual results:
Not seeing any dirty data accumulating in the page cache:
<excerpt>
# sar -r 5
Linux 3.10.0-957.el7.x86_64 (c09-h08-r630.rdu.openstack.engineering.redhat.com)         01/10/2019      _x86_64_        (56 CPU)

11:50:23 AM kbmemfree kbmemused  %memused kbbuffers  kbcached  kbcommit   %commit  kbactive   kbinact   kbdirty
11:50:28 AM 126637320   4972364      3.78         0   2984068    363436      0.26   1884000   1156308        32
11:50:33 AM 126637320   4972364      3.78         0   2984068    363436      0.26   1884000   1156308        32
11:50:38 AM 126637320   4972364      3.78         0   2984068    363436      0.26   1884000   1156308        32
11:50:43 AM 125801556   5808128      4.41         0   3808880    937648      0.67   1896732   1980992         0
11:50:48 AM 120168932  11440752      8.69         0   9428756    937648      0.67   1896912   7599108         4
11:50:53 AM 114769368  16840316     12.80         0  14815316    937648      0.67   1896912  12986512         4
11:50:58 AM 109458768  22150916     16.83         0  20116092    937648      0.67   1897396  18287780         4
11:51:03 AM 104207304  27402380     20.82         0  25364236    937648      0.67   1897424  23535716         0
11:51:08 AM  98995764  32613920     24.78         0  30564848    937648      0.67   1897408  28735148         0
11:51:13 AM  93582944  38026740     28.89         0  35965720    937648      0.67   1897408  34136384         0
11:51:18 AM  88071656  43538028     33.08         0  41463728    937648      0.67   1897408  39634616         0
11:51:23 AM  82411904  49197780     37.38         0  47106212    937648      0.67   1897408  45275676         0
11:51:28 AM  76742608  54867076     41.69         0  52761136    937648      0.67   1897408  50932124         0
11:51:33 AM  71736380  59873304     45.49         0  57754148    937648      0.67   1897408  55924636         0
11:51:38 AM  66740952  64868732     49.29         0  62738164    937648      0.67   1897408  60908384         0
11:51:43 AM  61620148  69989536     53.18         0  67843088    937648      0.67   1897408  66014100         0
11:51:48 AM  59375388  72234296     54.89         0  70091108    363792      0.26   1893552  68261796         0
11:51:53 AM  59375388  72234296     54.89         0  70091108    363792      0.26   1893552  68261796         0
</excerpt>


Expected results:
Evidence of dirty data building up in the page cache.


Additional info:

For comparison, the same test was run on an XFS file system on the server (the FS that would serve as the brick for the gluster volume). (The h/w spec of server is not same as of client; it has more RAM e.g.). In this case, we can see buildup of dirty data:
<excerpt>
# sar -r 5
Linux 3.10.0-957.el7.x86_64 (c06-h05-6048r.rdu.openstack.engineering.redhat.com)        01/10/2019     _x86_64_        (56 CPU)

11:46:33 AM kbmemfree kbmemused  %memused kbbuffers  kbcached  kbcommit   %commit  kbactive   kbinact   kbdirty
11:46:38 AM 261061052   2794896      1.06      1124    244356    326432      0.12    132284    162168        40
11:46:43 AM 261061052   2794896      1.06      1124    244356    326432      0.12    132284    162168        40
11:46:48 AM 261061052   2794896      1.06      1124    244356    326432      0.12    132284    162168        40
11:46:53 AM 261061052   2794896      1.06      1124    244356    326432      0.12    132284    162168        40
11:46:58 AM 261061052   2794896      1.06      1124    244356    326432      0.12    132284    162168        40
11:47:03 AM 261061052   2794896      1.06      1124    244356    326432      0.12    132284    162168        40
11:47:08 AM 245576160  18279788      6.93      1124  15005000    896264      0.33    134232  14922152   4537836
11:47:13 AM 237023884  26832064     10.17      1124  23303832    896264      0.33    134236  23220484   4845900
11:47:18 AM 228223240  35632708     13.50      1124  31822796    896264      0.33    134236  31741232   4901984
11:47:23 AM 219775288  44080660     16.71      1124  40001140    896264      0.33    134236  39917604   4654116
11:47:28 AM 211272552  52583396     19.93      1124  48319980    896264      0.33    134236  48235832   4702104
11:47:33 AM 202607168  61248780     23.21      1124  56654988    896264      0.33    134236  56571356   4592700
11:47:38 AM 193999760  69856188     26.48      1124  65109548    896264      0.33    134236  65025612   4904092
11:47:43 AM 192078956  71776992     27.20      1124  67352876    326676      0.12    133228  67268776   4629040
11:47:48 AM 192078736  71777212     27.20      1124  67353220    326676      0.12    132644  67270272         0
11:47:53 AM 192078736  71777212     27.20      1124  67353220    326676      0.12    132644  67270272         0
11:47:58 AM 192078736  71777212     27.20      1124  67353220    326676      0.12    132644  67270272         0
11:48:03 AM 192078736  71777212     27.20      1124  67353220    326676      0.12    132644  67270272         0
</excerpt>