Regression for mmap writes through FUSE in 4.2+

[Jakob Unterwurzacher <jakobunt@gmail.com>]

I have noticed an annoying regression that was introduced in 4.2 and is
still there in 4.4. mmap writes to FUSE filesystems are throttled down
to basically zero.

Reproducer: https://github.com/rfjakob/mmapwrite , testing against encfs:

$ mmapwrite /tmp/encfs-mnt/foo
  1 .................................................. 107.01 MB/s
  2 .................................................. 101.98 MB/s
[...]
 68 .................................................. 106.79 MB/s
 69 .................................................. 105.09 MB/s
 70 ..................................................   2.02 MB/s
 71 ..................................................   1.77 MB/s
 72 ..................................................   0.42 MB/s
 73 .................................... (hangs)

I have tested kernels from 4.0 and this seems to have been introduced in
4.2:

4.0 ....... 140MB/s permanent
4.1 ....... 140MB/s permanent
4.2 ....... 100MB/s at the start, sudden slowdown to 1MB/s after ~5GB
4.3 ....... 100MB/s at the start, sudden slowdown to 1MB/s after ~1.5GB
4.4-rc4 ... 100MB/s at the start, slowly ramps down, 0.3MB/s after ~2GB
4.4 ....... 100MB/s at the start, sudden slowdown after ~3GB

Is there a way to disable the throttling? Or at least exempt FUSE until
there is a proper fix?

Thanks,
Jakob