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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18374: Printing memory contexts on OOM condition might lead to segmentation fault
Date: Sat, 02 Mar 2024 13:49:58 -0500
Message-ID: <3140126.1709405398@sss.pgh.pa.us> (raw)
In-Reply-To: <b1a1eaf3-d5b7-da52-6bb7-c5b3fbe47f3e@gmail.com>
References: <18374-ebb8113ce4d02f0d@postgresql.org>
	<3120721.1709395887@sss.pgh.pa.us>
	<b1a1eaf3-d5b7-da52-6bb7-c5b3fbe47f3e@gmail.com>

Alexander Lakhin <exclusion@gmail.com> writes:
> 02.03.2024 19:11, Tom Lane wrote:
>> Hmph.  That's not an out-of-memory crash, that's a stack-too-deep
>> crash.

> (gdb) p $rsp
> $1 = (void *) 0x7ffcc83d4ff0
> (gdb) frame 13269
> #13269 0x000056289bc2685a in main (argc=8, argv=0x56289d3b4930) at main.c:198
> 198                     PostmasterMain(argc, argv);
> (gdb) p $rsp
> $2 = (void *) 0x7ffcc84834d0
> (gdb) p $rsp - 0x7ffcc83d4ff0
> $3 = (void *) 0xae4e0

> (Far less than ulimit -s == 8 MB.)

Yeah, I'm seeing something similar, also with ulimit -s = 8192 kbytes:

(gdb) i reg
...
rbp            0xb0a324            0xb0a324
rsp            0x7ffd07ce4fd0      0x7ffd07ce4fd0
...
(gdb) x/64 0x7ffd07ce4fd0
0x7ffd07ce4fd0: Cannot access memory at address 0x7ffd07ce4fd0

So it's definitely out-of-stack, yet

(gdb) p stack_base_ptr
$3 = 0x7ffd07dbf570 "\b"
(gdb) p 0x7ffd07dbf570 - 0x7ffd07ce4fd0
$4 = 894368

I'd have expected a diff in the vicinity of 8MB, but it isn't.

I think what must be happening is that the kernel is refusing
to expand our stack any more once we've hit the "ulimit -v" limit.
This is quite nasty, because it breaks all our assumptions about
having X amount of stack still available once check_stack_depth
triggers.

I tried inserting check_stack_depth() into MemoryContextStatsInternal,
and *that did not stop the crash*, confirming that we don't think
we're anywhere near the stack limit.  Ugh.

			regards, tom lane

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