[Vm-dev] Linux 4.4.7.2357 VM crash under memory pressure

[Bert Freudenberg]

Hilaire discovered that his newest DrGeo segfaults on the XO-1. It works fine elsewhere, including the XO-1.5, which has pretty much the same OS.

We narrowed down the problem to the XO-1 having only 256 MB of RAM and no swap space. I can reproduce the crash in a virtual Ubuntu 12 with 768 MB RAM (!) but no swap. Top reports:

Mem:    766204k total,   601588k used,   164616k free,    45624k buffers
Swap:        0k total,        0k used,        0k free,   277024k cached

but DrGeo still crashes. Etoys runs fine using the same Squeak VM on the same system (and on XO-1). DrGeo is based on Pharo 1.4, using a closure image. Etoys still is pre-closure. 

I ran the DrGeo under GDB. Here is the backtrace after the SEGFAULT, "gnu-interp.c:27272" maps to the "newOop = longAt(fwdBlock);" line in updatePointersInRootObjectsFromto():

=============
(gdb) bt
#0  0x0807acdb in updatePointersInRootObjectsFromto (memStart=-1314145112, 
    memEnd=-1312574048)
    at /home/olpc/Squeak-4.4.7.2357-src/bld/gnu-interp.c:27272
#1  0x08068586 in mapPointersInObjectsFromto (memStart=-1314145112, 
    memEnd=-1312574048)
    at /home/olpc/Squeak-4.4.7.2357-src/bld/gnu-interp.c:13495
#2  0x0805b53f in incCompBody ()
    at /home/olpc/Squeak-4.4.7.2357-src/bld/gnu-interp.c:5068
#3  0x0805bbe8 in incrementalGC ()
    at /home/olpc/Squeak-4.4.7.2357-src/bld/gnu-interp.c:5405
#4  0x0805c9c7 in instantiateContextsizeInBytes (classPointer=-1324064796, 
    sizeInBytes=92) at /home/olpc/Squeak-4.4.7.2357-src/bld/gnu-interp.c:6099
#5  0x080575b8 in allocateOrRecycleContext (needsLarge=0)
    at /home/olpc/Squeak-4.4.7.2357-src/bld/gnu-interp.c:2050
#6  0x08061898 in interpret ()
    at /home/olpc/Squeak-4.4.7.2357-src/bld/gnu-interp.c:9310
#7  0x0807f107 in main (argc=2, argv=0xbffff494, envp=0xbffff4a0)
    at /home/olpc/Squeak-4.4.7.2357-src/unix/vm/sqUnixMain.c:1444
(gdb) p/x memory
$3 = 0xb1094000
(gdb) p/x foo->endOfMemory
$4 = 0xb1c3b5a0
(gdb) p/x memStart
$5 = 0xb1abbca8
(gdb) p/x memEnd
$6 = 0xb1c3b5a0
(gdb) p/x fwdBlock
$7 = 0x63577a08
=============

fwdBlock appears to be clearly outside the object memory, that can't be right? Any idea how to fix it?


- Bert -