[Xen-bugs] [Bug 733] New: Creating large Dom-U makes Dom-0's memory go negative

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http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=733

           Summary: Creating large Dom-U makes Dom-0's memory go negative
           Product: Xen
           Version: 3.0 (general)
          Platform: x86
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Unspecified
        AssignedTo: xen-bugs@xxxxxxxxxxxxxxxxxxx
        ReportedBy: anthony@xxxxxxxxxxxxxxxxx


If I start a domain with a large memory requirement that forces my Dom-0 to
reduce it's memory footprint, it is possible to make the Dom-0's memory usage
go negative (or it would be if it wasn't an unsigned value!).

This is true on all versions I've tested: Suse 10.1, 3.0.2-2 & 3.0-testing.

I give the Dom-0 all the available memory at startup. The contents of
/proc/meminfo before any domain is created is shown below.

meminfo - Before
----------------
MemTotal:      1199104 kB
MemFree:        624764 kB
Buffers:         94088 kB
Cached:         305792 kB
SwapCached:          0 kB
Active:         216364 kB
Inactive:       239556 kB
HighTotal:      354660 kB
HighFree:        26280 kB
LowTotal:       844444 kB
LowFree:        598484 kB
SwapTotal:           0 kB
SwapFree:            0 kB
Dirty:               0 kB
Writeback:           0 kB
Mapped:          96732 kB
Slab:            27280 kB
CommitLimit:    599552 kB
Committed_AS:   169904 kB
PageTables:       1824 kB
VmallocTotal:   114680 kB
VmallocUsed:     10860 kB
VmallocChunk:   103540 kB

I then attempt to create a domain with 'memory = 1000' in it's config file. The
contents of /proc/meminfo after the domain creation and destruction are shown
below (note LowTotal which is now 0xfffe769c in hex):

meminfo - After
---------------
MemTotal:       253952 kB
MemFree:         10856 kB
Buffers:         19580 kB
Cached:          55300 kB
SwapCached:          0 kB
Active:         110888 kB
Inactive:        20440 kB
HighTotal:      354660 kB
HighFree:         3544 kB
LowTotal:     4294866588 kB
LowFree:          7312 kB
SwapTotal:           0 kB
SwapFree:            0 kB
Dirty:              84 kB
Writeback:           0 kB
Mapped:          97308 kB
Slab:            20508 kB
CommitLimit:    126976 kB
Committed_AS:   178184 kB
PageTables:       1832 kB
VmallocTotal:   114680 kB
VmallocUsed:     10872 kB
VmallocChunk:   103540 kB

Note that this happens even if the domain doesn't actually run, for example if
the creation is aborted because one of the VBDs is already mounted in Dom-0.


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