[Bug 1900438] Re: Bcache bypasse writeback on caching device with fragmentation

I tried to run the apport-collect on one of the server we can see the
issue on :

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Traceback (most recent call last):
File "/usr/bin/apport-cli", line 370, in <module>
if not app.run_argv():
File "/usr/lib/python2.7/dist-packages/apport/ui.py", line 666, in run_argv
return self.run_update_report()
File "/usr/lib/python2.7/dist-packages/apport/ui.py", line 564, in run_update_report
self.report.add_proc_environ()
File "/usr/lib/python2.7/dist-packages/apport/report.py", line 577, in add_proc_environ
proc_pid_fd = os.open('/proc/%s' % pid, os.O_RDONLY | os.O_PATH | os.O_DIRECTORY)
AttributeError: 'module' object has no attribute 'O_PATH'
Error in sys.excepthook:
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/apport_python_hook.py", line 109, in apport_excepthook
pr.add_proc_info(extraenv=['PYTHONPATH', 'PYTHONHOME'])
File "/usr/lib/python2.7/dist-packages/apport/report.py", line 507, in add_proc_info
proc_pid_fd = os.open('/proc/%s' % pid, os.O_RDONLY | os.O_PATH | os.O_DIRECTORY)
AttributeError: 'module' object has no attribute 'O_PATH'


But since the bug has already been marked as "Confirmed".

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https://bugs.launchpad.net/bugs/1900438

Title:
Bcache bypasse writeback on caching device with fragmentation

Status in linux package in Ubuntu:
Confirmed

Bug description:
Hello,

An upstream bug has been opened on the matter for quite some time now [0].
I can reproduce easily on our production compute node instance, which are trusty host with xenial hwe kernels (4.15.0-101-generic).
However due to heavy backport and such, doing real tracing is a bit hard there.

I was able to reproduce the behavior on a hwe-bionic kernel as well.
Since most of our critical deployments use bcache, I think this is a kinda nasty bug to have.

Reproducing the issue is relatively easy with the script provided in the bug [1].
The script used to capture the stats is this one [2].

[0]: https://bugzilla.kernel.org/show_bug.cgi?id=206767
[1]: https://pastebin.ubuntu.com/p/YnnvvSRhXK/
[2]: https://pastebin.ubuntu.com/p/XfVpzg32sN/

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