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[Po-Hsu Lin <po-hsu.lin@xxxxxxxxxxxxx>]

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

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

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

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

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https://bugs.launchpad.net/bugs/1833410

Title:
  idle-page oopses when accessing page frames that are out of range

Status in linux package in Ubuntu:
  In Progress
Status in linux source package in Xenial:
  New
Status in linux source package in Bionic:
  New
Status in linux source package in Cosmic:
  New
Status in linux source package in Disco:
  New

Bug description:
  == SRU [Xenial][Bionic][Cosmic][Disco] ==

  == Justification ==

  When accessing page frames that are greater than max_pfn using the
  idle-page sysfs interface an oops is triggered that kills the process
  that writes to the sysfs interface.

  == Fix ==

  Upstream fix currently in linux-next:

  https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-
  next.git/commit/mm/page_idle.c?id=d96d6145d9796d5f1eac242538d45559e9a23404

  This fixes the maximum pfn threshold allowed.

  == Test ==

  sudo stress-ng --idle-page 0

  this should trigger the oops in ~50% of the cases due to the way the
  threshold calculation in the kernel was handling the maximum threshold
  based on pfn alignments. 31 of 63 of the times it may be under the
  threshold so no oops occurs.  If it does not, increase or decrease the
  number of available pages in a system to trigger the sweet spot of the
  bug.

  == Regression Potential ==

  Minimal, this touches a sysfs kernel interface that is not used much.
  The fix narrows the scope of touching specific page frames, so the
  page frame scope is reduced by the fix.

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