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[Eric Desrochers <eric.desrochers@xxxxxxxxxxxxx>]

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

** Also affects: lxcfs (Ubuntu Eoan)
   Importance: Undecided
       Status: New

** Also affects: lxcfs (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Changed in: lxcfs (Ubuntu Bionic)
     Assignee: (unassigned) => Kellen Renshaw (krenshaw)

** Changed in: lxcfs (Ubuntu Bionic)
   Importance: Undecided => Medium

** Changed in: lxcfs (Ubuntu Bionic)
       Status: New => In Progress

** Tags added: sts-sponsor-slashd

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https://bugs.launchpad.net/bugs/1860813

Title:
  LXC container reports spike in swap occasionally

Status in lxcfs package in Ubuntu:
  Fix Released
Status in lxcfs source package in Xenial:
  New
Status in lxcfs source package in Bionic:
  In Progress
Status in lxcfs source package in Eoan:
  New

Bug description:
  [Impact]

   * lxcfs provides a container-specific view of /proc/meminfo.
  Occasionally, with near zero or zero swap usage *and* swap accounting
  turned on (kernel parameter swapaccount=1), the container will report
  100% swap utilization.

   * This issue has been encountered and could result in unecessary
  alerts or potential automated attempts at remediating a non-existent
  "full swap" issue.

   * This fix changed the logic used for SwapFree when swap accounting
  is enabled to better handle situations where memswusage is less than
  memusage, caused by the fuzziness of the usage_in_bytes counters used
  as the source. Specifically, it added a check for memusage being
  larger than memswusage and if so, sets 0 as the value of swapusage.
  Otherwise the calculation (memswusage - memusage) remains the same.

  [Test Case]

   * Requires a Bionic (18.04) or Eoan (19.10) host with swap space.

   * Enable swap accounting with the "swapaccount=1" kernel parameter on
  the kernel command line. Edit /etc/default/grub, add "swapaccount=1"
  to the GRUB_CMDLINE_LINUX_DEFAULT="other parameters" line, then run
  "update-grub" and reboot to make the change active.

   * Ensure lxd is installed, "sudo apt install lxd"

   * Create a lxd/lxc container with "lxc launch ubuntu:X
  container_name" with X being either b[ionic] or e[oan].

   * Open two shells to the container with "lxc shell container_name"

   * In one of the shells, run: watch -n 0.1 "grep Swap /proc/meminfo"

   * In the other, run: while true; do free; done

   * You should see SwapFree intermittently drop to zero in the first
  terminal.

   * The fix results in small non-zero swap "usage" intermittently
  instead of intermittent SwapFree = 0.

  
  [Regression Potential] 

   * Low, as swap accounting must be enabled to encounter the bug and
  the fix.

   * Potential for unanticipated edge cases in the values of memusage
  and memswusage to cause incorrect swap reporting within the container,
  with swap accounting turned on.

   * Any tooling that expected, compensated, or relied on the behavior
  may no longer work as expected.

  [Other Info]
   
   * Cherrypick of a one line fix to address this specific situation.

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