SRU Justification:
Impact: The mm zone page calculations are being miscalculated because of
a previous commit.
Fix: Add parentheses around >> operators in setup_per_zone_pages_min()
to correct the calculation. Elaboration here:
originally the calculation was:
zone->pages_low = zone->pages_min + tmp / 4;
zone->pages_high = zone->pages_min + tmp / 2;
commit 622f7b4c920a9362d506400c0da02e3e06442b27 changed it to:
zone->pages_low = zone->pages_min + tmp >> 2;
zone->pages_high = zone->pages_min + tmp >> 1;
unfortunately the + operator has higher precedence than >> hence the
bug.
it should be:
zone->pages_low = zone->pages_min + (tmp >> 2);
zone->pages_high = zone->pages_min + (tmp >> 1);
Testcase: One can see the zonelist from /proc/zoneinfo: The low and high
water marks should be different between the buggy and fixed versions.
Testing:
1. Install 6.0.6 LTS i386 Server in VirtualBox and
recording /proc/zoneinfo from /etc/rc.local to capture the zoneline low
and high settings at boot time.
2. Booted up 3 times to see if the figures differ per boot (they
don't).
3. Installed the fixed kernel and repeated the 3 boot test
4. Compare the buggy and fixed /proc/zoneinfo logs, e.g:
Buggy Fixed
low 15 38
high 31 46
Attached: zone info logs for the two cases
** Attachment added: "/proc/zoneinfo for broken buggy version"
http://launchpadlibrarian.net/17287900/zoneinfo_buggy.log
--
Precedence bug in patch
https://bugs.launchpad.net/bugs/86778
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