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[Kleber Sacilotto de Souza <kleber.souza@xxxxxxxxxxxxx>]

** Also affects: linux (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Changed in: linux (Ubuntu Xenial)
       Status: New => In Progress

** Changed in: linux (Ubuntu)
       Status: Incomplete => Fix Released

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https://bugs.launchpad.net/bugs/1750038

Title:
  user space process hung in 'D' state waiting for disk io to complete

Status in linux package in Ubuntu:
  Fix Released
Status in linux source package in Xenial:
  In Progress

Bug description:
  == SRU Justification ==

  [Impact]
  Occasionally an application gets stuck in "D" state on NFS reads/sync and close system calls. All the subsequent operations on the NFS mounts are stuck and reboot is required to rectify the situation.

  [Fix]
  Use GPF_NOIO in some allocations in writeback to avoid a deadlock. This is upstream in:
  ae97aa524ef4 ("NFS: Use GFP_NOIO for two allocations in writeback")

  [Testcase]
  See Test scenario in previous description.

  A test kernel with this patch was tested heavily (>100hrs of test
  suite) without issue.

  [Regression Potential]
  This changes memory allocation in NFS to use a different policy. This could potentially affect NFS. 

  However, the patch is already in Artful and Bionic without issue.

  The patch does not apply to Trusty.

  == Previous Description ==

  Using Ubuntu Xenial user reports processes hang in D state waiting for
  disk io.

  Ocassionally one of the applications gets into "D" state on NFS
  reads/sync and close system calls. based on the kernel backtraces
  seems to be stuck in kmalloc allocation during cleanup of dirty NFS
  pages.

  All the subsequent operations on the NFS mounts are stuck and reboot
  is required to rectify the situation.

  [Test scenario]

  1) Applications running in Docker environment
  2) Application have cgroup limits --cpu-shares --memory -shm-limit
  3) python and C++ based applications (torch and caffe)
  4) Applications read big lmdb files and write results to NFS shares
  5) use NFS v3 , hard and fscache is enabled
  6) now swap space is configured

  This prevents all other I/O activity on that mount to hang.

  we are running into this issue more frequently and identified few
  applications causing this problem.

  As updated in the description, the problem seems to be happening when
  exercising the stack

  try_to_free_mem_cgroup_pages+0xba/0x1a0

  we see this with docker containers with cgroup option --memory
  <USER_SPECIFIED_MEM>.

  whenever there is a deadlock, we see that the process that is hung has
  reached the maximum cgroup limit, multiple times and typically cleans
  up dirty data and caches to bring the usage under the limit.

  This reclaim path happens many times and finally we hit probably a
  race get into deadlock

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