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Subject: Re: Instability in current -git tree
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From: Pavel Tatashin <pasha.tatashin@oracle.com>
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Date: Fri, 13 Jul 2018 20:46:31 -0400
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, Thomas Gleixner <tglx@linutronix.de>, Matthew Wilcox <willy@infradead.org>, Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, David Miller <davem@davemloft.net>, Al Viro <viro@zeniv.linux.org.uk>, Dave Airlie <airlied@gmail.com>, Tejun Heo <tj@kernel.org>, Ted Ts'o <tytso@google.com>, Mike Snitzer <snitzer@redhat.com>, linux-mm <linux-mm@kvack.org>, Daniel Vacek <neelx@redhat.com>, Mel Gorman <mgorman@techsingularity.net>

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