Segmentation fault in GHC runtime system under low memory with USE_LARGE_ADDRESS_SPACE
I have here a ~600MB core dump on GHC 8.0.1 x86_64 linux triggered under low memory (1GB RAM total, no swap, trying to build aeson with optimizations), with a stacktrace blaming the GHC runtime system. Based on code review, it appears that when requesting memory from the OS fails, the error is ignored and GHC segfaults as soon as it writes to the newly "allocated" memory.
My diagnosis is that osCommitMemory calls the potentially failing
my_mmap but returns void (hence not propagating failures); it should instead probably use
my_mmap_or_barf or handle the failure anyhow:
However, while I'm very convinced that's a bug, I'm only mostly confident that's the culprit, since the segfault happens later; I've just looked for bugs on allocation failures in the code ran shortly before the crash.
The core dump is on a VM from the reporter of https://github.com/commercialhaskell/stack/issues/2575, https://github.com/jiakai0419, which graciously gave me access to the VM and bothered reporting in the first place—I'd like to thank her. I'd like to avoid copying the dump here unless strictly needed.
The bug is deterministic on that machine, but it requires a failure in mmap without triggering an OOM, so I'm not 100% sure how easy it is to reproduce but it should be easy on a VM with 1GB RAM and 1 core.
Here's the output of
$ free total used free shared buff/cache available Mem: 1015352 95864 71396 51244 848092 733132 Swap: 0 0 0
Instructions to produce a dump. First, as root:
echo /tmp/core > /proc/sys/kernel/core_pattern
that's needed because otherwise the core file is produced in a temporary folder that stack removes. Then (as non-root, root has privileged access to memory), install stack, and run:
ulimit -c unlimited git clone https://github.com/jiakai0419/snowflake.git stack build --verbose
Below is some raw data from the core dump. In short, an initializer for newly allocated data segfaults, and gdb tells that the address it's trying to write (bd->start) to can't be accessed, even though it's lower than mblock_high_watermark so it should have been committed (mmap'ed) by getFreshMBlocks (I don't get getReusableMBlocks in detail, but it also calls
osCommitMemory, so I guess there's the same invariant). The existence of mblock_high_watermark (beyond other reasons) should confirm this is using
USE_LARGE_ADDRESS_SPACE. If this diagnosis is correct, the bug exists since USE_LARGE_ADDRESS_SPACE was introduced in https://github.com/ghc/ghc/commit/0d1a8d09f452977aadef7897aa12a8d41c7a4af0, so it's a regression in 8.0.1.
(gdb) bt #0 initMBlock (mblock=0x225c00000) at rts/sm/BlockAlloc.c:676 #1 alloc_mega_group (mblocks=mblocks@entry=1) at rts/sm/BlockAlloc.c:328 #2 0x00007fe650438d98 in allocGroup (n=1) at rts/sm/BlockAlloc.c:379 #3 allocGroup (n=1) at rts/sm/BlockAlloc.c:336 #4 0x00007fe65043ed55 in allocBlock_sync () at rts/sm/GCUtils.c:38 #5 0x00007fe65043ef7d in alloc_todo_block (ws=ws@entry=0x13196b0, size=size@entry=3) at rts/sm/GCUtils.c:330 #6 0x00007fe65043f13a in todo_block_full (size=size@entry=3, ws=0x13196b0) at rts/sm/GCUtils.c:292 #7 0x00007fe65041baed in alloc_for_copy (gen_no=<optimized out>, size=3) at rts/sm/Evac.c:81 #8 copy_tag (tag=2, gen_no=<optimized out>, size=3, src=0x21b353868, info=<optimized out>, p=0x225b13f10) at rts/sm/Evac.c:99 #9 evacuate1 (p=p@entry=0x225b13f10) at rts/sm/Evac.c:596 #10 0x00007fe65041d65c in scavenge_block1 (bd=0x225b004c0) at rts/sm/Scav.c:570 #11 0x00007fe6504415dc in scavenge_find_work () at rts/sm/Scav.c:2040 #12 scavenge_loop1 () at rts/sm/Scav.c:2103 #13 0x00007fe65043cee2 in scavenge_until_all_done () at rts/sm/GC.c:968 #14 0x00007fe65043d75a in GarbageCollect (collect_gen=collect_gen@entry=1, do_heap_census=do_heap_census@entry=rtsFalse, gc_type=gc_type@entry=2, cap=cap@entry=0x7fe650669a80 <MainCapability>) at rts/sm/GC.c:403 #15 0x00007fe65042ff81 in scheduleDoGC (force_major=rtsFalse, task=0x13228a0, pcap=<optimized out>) at rts/Schedule.c:1652 #16 scheduleDoGC (pcap=<optimized out>, task=0x13228a0, force_major=rtsFalse) at rts/Schedule.c:1433 #17 0x00007fe650430c1a in schedule (initialCapability=initialCapability@entry=0x7fe650669a80 <MainCapability>, task=task@entry=0x13228a0) at rts/Schedule.c:551 #18 0x00007fe650431cf8 in scheduleWaitThread (tso=0x20000b388, ret=ret@entry=0x0, pcap=pcap@entry=0x7ffc4ee22c08) at rts/Schedule.c:2361 #19 0x00007fe65042c528 in rts_evalLazyIO (cap=cap@entry=0x7ffc4ee22c08, p=p@entry=0x7895c0, ret=ret@entry=0x0) at rts/RtsAPI.c:500 #20 0x00007fe65042e3a7 in hs_main (argc=96, argv=0x7ffc4ee22dc8, main_closure=0x7895c0, rts_config=...) at rts/RtsMain.c:64 #21 0x0000000000428414 in main () (gdb) info locals bd = 0x225c00100 block = 0x225c04000 <Address 0x225c04000 out of bounds> (gdb) print/x mblock_high_watermark $11 = 0x225d00000 (gdb) print bd->start Cannot access memory at address 0x225c00100 (gdb) print *bd Cannot access memory at address 0x225c00100