[Bug 1845011] [NEW] mcontend stressor SIGBUS's on older armhf platforms

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You have been subscribed to a public bug by Mathew Hodson (mathew-hodson):

== SRU Justification BIONIC, DISCO ==

When running the mcontend stressor on slow armhf devices that have slow
memory backed I/O one can get SIGBUS on a file backed mmap because the
flush has been slow to allocate the page and we're mmapping and
accessing a as yet allocated page on the backing store.

== Test Proceedure ==

Run:


stress-ng --mcontend 1 -t 10 -v

without the fix, we get SIGBUS failures because we touch a page that has
not yet had the backing mmap page allocated and flushed to the file
system.

== Fix ==

commit c3678dadee23f9b8783038f54463cff5741a9cbe
Author: Colin Ian King <colin.king at canonical.com>
Date:   Mon Sep 23 15:54:13 2019 +0100

    stress-mcontend: ensure mmap'd file is sync'd with zero data (LP: #1845011)
    
    Write zero's to backing store of at least one page size and sync
    it so that we can be 100% sure that mmap'ing the page into memory
    is backed with data so we don't get BUS errors on some platforms
    such as armhf Raspi2.
    
    Also fix broken sanity check on 2nd mmap out of memory failure.
    
    Signed-off-by: Colin Ian King <colin.king at canonical.com>


== Regression Potential ==

Minimal. This replaces the more esoteric fallocate() with a direct set
of file writes and a sync to ensure data is written back before we mmap
onto it.  The fixed behaviour is slower as we introduce a sync, so we
have the possibility of the sync blocking for a long time if the system
has a lot of dirty data, but this is necessary as fsync() is less
portable and sync guarantees we really have data to mmap onto to avoid
the SIGBUS.

** Affects: stress-ng (Ubuntu)
     Importance: Medium
     Assignee: Colin Ian King (colin-king)
         Status: Fix Released

** Affects: stress-ng (Ubuntu Bionic)
     Importance: Medium
     Assignee: Colin Ian King (colin-king)
         Status: In Progress

** Affects: stress-ng (Ubuntu Disco)
     Importance: Medium
     Assignee: Colin Ian King (colin-king)
         Status: Fix Released

** Affects: stress-ng (Ubuntu Eoan)
     Importance: Medium
     Assignee: Colin Ian King (colin-king)
         Status: Fix Released


** Tags: verification-done verification-done-disco
-- 
mcontend stressor SIGBUS's on older armhf platforms
https://bugs.launchpad.net/bugs/1845011
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