[Bug 246886] Nginx + FUSE causes VM stall

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246886

            Bug ID: 246886
           Summary: Nginx + FUSE causes VM stall
           Product: Base System
           Version: 12.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: nishida at asusa.net

I'm developing a distributed file system using FUSE on FreeBSD 12.1-RELEASE and
STABLE. However, when Nginx is accessing the FUSE mounted filesystem, Nginx
stalls with 'grbmaw' state.
grbmaw seems to be used by vm_page_busy_sleep() and it seems to never awake
again for some reason.
My FUSE program is just waiting for the next command at fuse_session_loop_mt().

I have tested with 4 different kinds of hardware and get this problem with 3 of
them. The biggest difference between those 3 and the remaining 1 is the NIC;
the 3 are wired and the rest is wireless.
I guess this happens when Nginx is rushed to access or send the data.
Once it happens, I need to reboot the device but it does not always shut down.
It is easily reproducible.

I think the problem is fuse.ko related and would appreciate it if anybody could
fix it.

Thank you.

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