Re: [PATCH] x86/pod: Do not fragment PoD memory allocations

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Jan 28, 2021, at 6:26 PM, Elliott Mitchell <ehem+xen@xxxxxxx> wrote:
> 
> On Thu, Jan 28, 2021 at 11:19:41AM +0100, Jan Beulich wrote:
>> On 27.01.2021 23:28, Elliott Mitchell wrote:
>>> On Wed, Jan 27, 2021 at 09:03:32PM +0000, Andrew Cooper wrote:
>>>> So.?? What *should* happen is that if QEMU/OVMF dirties more memory than
>>>> exists in the PoD cache, the domain gets terminated.
>>>> 
>>>> Irrespective, Xen/dom0 dying isn't an expected consequence of any normal
>>>> action like this.
>>>> 
>>>> Do you have a serial log of the crash??? If not, can you set up a crash
>>>> kernel environment to capture the logs, or alternatively reproduce the
>>>> issue on a different box which does have serial?
>>> 
>>> No, I don't.  I'm setup to debug ARM failures, not x86 ones.
>> 
>> Then alternatively can you at least give conditions that need to
>> be met to observe the problem, for someone to repro and then
>> debug? (The less complex the better, of course.)
> 
> Multiple prior messages have included statements of what I believed to be
> the minimal case to reproduce.  Presently I believe the minimal
> constraints are, maxmem >= host memory, memory < free Xen memory, type
> HVM.  A minimal kr45hme.cfg file:
> 
> type = "hvm"
> memory = 1024
> maxmem = 1073741824
> 
> I suspect maxmem > free Xen memory may be sufficient.  The instances I
> can be certain of have been maxmem = total host memory *7.

Can you include your Xen version and dom0 command-line?

For me, domain creation fails with an error like this:

root@immortal:/images# xl create c6-01.cfg maxmem=1073741824
Parsing config from c6-01.cfg
xc: error: panic: xc_dom_boot.c:120: xc_dom_boot_mem_init: can't allocate low 
memory for domain: Out of memory
libxl: error: libxl_dom.c:593:libxl__build_dom: xc_dom_boot_mem_init failed: 
Cannot allocate memory
libxl: error: libxl_create.c:1576:domcreate_rebuild_done: Domain 9:cannot 
(re-)build domain: -3
libxl: error: libxl_domain.c:1182:libxl__destroy_domid: Domain 9:Non-existant 
domain
libxl: error: libxl_domain.c:1136:domain_destroy_callback: Domain 9:Unable to 
destroy guest
libxl: error: libxl_domain.c:1063:domain_destroy_cb: Domain 9:Destruction of 
domain failed

This is on staging-4.14 from a month or two ago (i.e., what I happened to have 
on a random test  box), and `dom0_mem=1024M,max:1024M` in my command-line.  
That rune will give dom0 only 1GiB of RAM, but also prevent it from 
auto-ballooning down to free up memory for the guest.

 -George