Re: Instability in current -git tree

[Pavel Tatashin <pasha.tatashin@oracle.com>]

On 07/13/2018 08:28 PM, Linus Torvalds wrote:
> On Fri, Jul 13, 2018 at 5:20 PM Pavel Tatashin
> <pasha.tatashin@oracle.com> wrote:
>>
>> I'd like to try to reproduce it as well, were you able to reproduce this problem in qemu? What were the qemu arguments if so?
> 
> No, this is actually on raw hardware. I've had a unstable machine for
> the last couple of weeks, and it just hung with no sign of where.
> 
> I finally reproduced it reliably by booting with less memory
> ("mem=6G") and then putting the machine under memory pressure and then
> I could get it on the console when the machine died. Before that it
> was just an occasional hung machine randomly every other day or
> whatever.
> 
> If it reproduces in emulation, that will certainly make it easier to
> see the messages.
> 
> But since I suspect it might be related to having that odd (read: real
> life) e820 table setup, it might not reproduce in emulation. At least
> when I boot up in lkvm-run, I don't see those ACPI tables and ACPI NVS
> sections, which seems to be related to this.
> 
> I'm attaching my kernel-config (this is the non-debug one - it does
> have CONFIG_DEBUG_VM, but none of the other debug options I ran with
> for the last few days in the hope of catching it earlier).
> 
>                 Linus
> 

I will try to reproduce it on bare metal. I believe, the problem was narrowed down to this commit:

124049decbb1 x86/e820: put !E820_TYPE_RAM regions into memblock.reserved

The commit intends to zero memmap (struct pages) for every hole in e820 ranges by marking them reserved in memblock. Later  zero_resv_unavail() walks through memmap ranges and zeroes struct pages for every page that is reserved, but does not have a physical backing known by kernel.

Pavel