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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Sun, Jan 31, 2021 at 10:13:49AM -0800, Elliott Mitchell wrote:
> On Thu, Jan 28, 2021 at 10:42:27PM +0000, George Dunlap wrote:
> > 
> > > On Jan 28, 2021, at 6:26 PM, Elliott Mitchell <ehem+xen@m5p.com> wrote:
> > > 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?
> 
> > 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.
> > 
> 
> As this is a server, not a development target, Debian's build of 4.11 is
> in use.  Your domain 0 memory allocation is extremely generous compared
> to mine.  One thing which is on the command-line though is
> "watchdog=true".
> 
> I've got 3 candidates which presently concern me:ble:
> 
> 1> There is a limited range of maxmem values where this occurs.  Perhaps
> 1TB is too high on your machine for the problem to reproduce.  As
> previously stated my sample configuration has maxmem being roughly 7
> times actual machine memory.
> 
> 2> Between issuing the `xl create` command and the machine rebooting a
> few moments of slow response have been observed.  Perhaps the memory
> allocator loop is hogging processor cores long enough for the watchdog to
> trigger?
> 
> 3> Perhaps one of the patches on Debian broke things?  This seems
> unlikely since nearly all of Debian's patches are either strictly for
> packaging or else picks from Xen's main branch, but this is certainly
> possible.

If you have a reliable way to reproduce this, and such error happens
on one of your server boxes, is it impossible for you to connect to
the serial console and get the output of the crash?

I would assume this being a server it must have some kind of serial
console support, even if Serial over LAN.

That way we could remove all the speculation about what has gone
wrong.

Thanks, Roger.