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

[Launchpad Bug Tracker]

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[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.

** Affects: makedumpfile (Ubuntu)
     Importance: Medium
     Assignee: Kellen Renshaw (krenshaw)
         Status: Confirmed

** Affects: makedumpfile (Ubuntu Focal)
     Importance: Medium
     Assignee: Heather Lemon (hypothetical-lemon)
         Status: Confirmed


** Tags: patch sts
-- 
makedumpfile falls back to cp with "__vtop4_x86_64: Can't get a valid pmd_pte."
https://bugs.launchpad.net/bugs/1970672
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