[Bug 1970672] Re: makedumpfile falls back to cp with "__vtop4_x86_64: Can't get a valid pmd_pte."

[Ubuntu Foundations Team Bug Bot]

The attachment "lp1970672-focal.debdiff" seems to be a debdiff.  The
ubuntu-sponsors team has been subscribed to the bug report so that they
can review and hopefully sponsor the debdiff.  If the attachment isn't a
patch, please remove the "patch" flag from the attachment, remove the
"patch" tag, and if you are member of the ~ubuntu-sponsors, unsubscribe
the team.

[This is an automated message performed by a Launchpad user owned by
~brian-murray, for any issue please contact him.]

** Tags added: patch

-- 
You received this bug notification because you are a member of Ubuntu
Sponsors, which is subscribed to the bug report.
https://bugs.launchpad.net/bugs/1970672

Title:
  makedumpfile falls back to cp with "__vtop4_x86_64: Can't get a valid
  pmd_pte."

Status in makedumpfile package in Ubuntu:
  Confirmed
Status in makedumpfile source package in Focal:
  Confirmed

Bug description:
  [Impact] 
   * On Focal with an HWE (>=5.12) kernel, makedumpfile can sometimes fail with "__vtop4_x86_64: Can't get a valid pmd_pte."

   * makedumpfile falls back to cp for the dump, resulting in extremely
  large vmcores. This can impact both collection and analysis due to
  lack of space for the resulting vmcore.

   * This is fixed in upstream commit present in versions 1.7.0 and 1.7.1:
  https://github.com/makedumpfile/makedumpfile/commit/646456862df8926ba10dd7330abf3bf0f887e1b6

  commit 646456862df8926ba10dd7330abf3bf0f887e1b6
  Author: Kazuhito Hagio <k-hagio-ab at nec.com>
  Date:   Wed May 26 14:31:26 2021 +0900

      [PATCH] Increase SECTION_MAP_LAST_BIT to 5
      
      * Required for kernel 5.12
      
      Kernel commit 1f90a3477df3 ("mm: teach pfn_to_online_page() about
      ZONE_DEVICE section collisions") added a section flag
      (SECTION_TAINT_ZONE_DEVICE) and causes makedumpfile an error on
      some machines like this:
      
        __vtop4_x86_64: Can't get a valid pmd_pte.
        readmem: Can't convert a virtual address(ffffe2bdc2000000) to physical address.
        readmem: type_addr: 0, addr:ffffe2bdc2000000, size:32768
        __exclude_unnecessary_pages: Can't read the buffer of struct page.
        create_2nd_bitmap: Can't exclude unnecessary pages.
      
      Increase SECTION_MAP_LAST_BIT to 5 to fix this.  The bit had not
      been used until the change, so we can just increase the value.
      
      Signed-off-by: Kazuhito Hagio <k-hagio-ab at nec.com>

  [Test Plan]
   * Confirm that makedumpfile works as expected by triggering a kdump.

   * Confirm that the patched makedumpfile works as expected on a system
  known to experience the issue.

   * Confirm that the patched makedumpfile is able to work with a cp-
  generated known affected vmcore to compress it. The unpatched version
  fails.

  [Where problems could occur]

   * This change could adversely affect the collection/compression of
  vmcores during a kdump situation resulting in fallback to cp.

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/makedumpfile/+bug/1970672/+subscriptions