Re: bad page flags booting 32bit dom0 on 64bit hypervisor using dom0_mem (kernel >=4.2)

[Juergen Gross <jgross@suse.com>]

On 02/05/16 12:47, Stefan Bader wrote:
> I recently tried to boot 32bit dom0 on 64bit Xen host which I configured to run
> with a limited, fix amount of memory for dom0. It seems that somewhere between
> kernel versions 3.19 and 4.2 (sorry that is still a wide range) the Linux kernel
> would report bad page flags for a range of pages (which seem to be around the
> end of the guest pfn range). For a 4.2 kernel that was easily missed as the boot
> finished ok and dom0 was accessible. However starting with 4.4 (tested 4.5 and a
> 4.6-rc) the serial console output freezes after some of those bad page flag
> messages and then (unfortunately without any further helpful output) the host
> reboots (I assume there is a panic that triggers a reset).
> 
> I suspect the problem is more a kernel side one. It is just possible to
> influence things by variation of dom0_mem=#,max:#. 512M seems ok, 1024M, 2048M,
> and 3072M cause bad page flags starting around kernel 4.2 and reboots around
> 4.4. Then 4096M and not clamping dom0 memory seem to be ok again (though not
> limiting dom0 memory seems to cause trouble on 32bit dom0 later when a domU
> tries to balloon memory, but I think that is a different problem).
> 
> I have not seen this on a 64bit dom0. Below is an example of those bad page
> errors. Somehow it looks to be a page marked as reserved. Initially I wondered
> whether this could be a problem of not clearing page flags when moving mappings
> to match the e820. But I never looked into i386 memory setup in that detail. So
> I am posting this, hoping that someone may have an idea from the detail about
> where to look next. PAE is enabled there. Usually its bpf init that gets hit but
> that likely is just because that is doing the first vmallocs.

Could you please post the kernel config, Xen and dom0 boot parameters?
I'm quite sure this is no common problem as there are standard tests
running for each kernel version including 32 bit dom0 with limited
memory size.


Juergen

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