Re: [Xen-devel] [xen-4.1.6.1] SIGSEGV libxc/xc_save_domain.c: p2m_size >> configured_ram_size

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Fri, Jun 10, 2016 at 4:22 PM, Philipp Hahn <hahn@xxxxxxxxxxxxx> wrote:
> Hi,
>
> while trying to live migrate some VMs from an xen-4.1.6.1 host "xc_save"
> crashes with a segmentation fault in tools/libxc/xc_domain_save.c:1141
>>         /*
>>          * Quick belt and braces sanity check.
>>          */
>>         for ( i = 0; i < dinfo->p2m_size; i++ )
>>         {
>>             mfn = pfn_to_mfn(i);
>>             if( (mfn != INVALID_P2M_ENTRY) && (mfn_to_pfn(mfn) != i) )
>                                                  ^^^^^^^^^^^^^^^
> due to a de-reference through
>> #define pfn_to_mfn(_pfn)                                            \
>>   ((xen_pfn_t) ((dinfo->guest_width==8)                               \
>>                 ? (((uint64_t *)ctx->live_p2m)[(_pfn)])                  \
>>                 : ((((uint32_t *)ctx->live_p2m)[(_pfn)]) == 0xffffffffU  \
>>                    ? (-1UL) : (((uint32_t *)ctx->live_p2m)[(_pfn)]))))
>                                                             ^^^^^^^^
> The VM is a 32bit Linux-PV-domain having maxmem=1997[MB]
>> (gdb) print _ctx
>> $1 = {hvirt_start = 4118806528, pt_levels = 3, max_mfn = 25690112, live_p2m 
>> = 0x7f421cc2e000, live_m2p = 0x7f421d02e000, m2p_mfn0 = 8649728, dinfo = 
>> {guest_width = 4, p2m_size = 1048576}}
>
> Note that p2m_size = 0x100000 = 4GiB_RAM/4KiB_page_size »
> maxmem_of_domU, so so loop doesn't end around the allocated 2GB, but
> tries to go up to the 32-maximum of 4GB and fails.
>
> I've added more debugging to verify that:
>> xc: detail: p2m_size=0x100000
> ...
>> xc: detail: i=0x7cfff mfn=183ecf2 live_m2p=7cfff
>> segfault
>
> I can reproduce that easily doing
>  /usr/lib/xen/bin/xc_save 28 $domid 0 0 1 28>/dev/null
>
> Doing a non-live-migration (28 $domid 0 0 0) also fails, so it doesn't
> depend on being live.
>
> Increasing the configured memory size of the domU to 2001 doesn't change
> the problem:
>> xc: detail: p2m_size=0x100000
>> xc: detail: i=0x7cfff mfn=183ecf2 live_m2p=7cfff
>> xc: detail: i=0x7d000 mfn=183ecf1 live_m2p=7d000
> ...
>> xc: detail: i=0x7d0fd mfn=10aebce live_m2p=7d0fd
>> xc: detail: i=0x7d0fe mfn=10aebcd live_m2p=7d0fe
>> xc: detail: i=0x7d0ff mfn=10aebcc live_m2p=7d0ff
>> Speicherzugriffsfehler (Speicherabzug geschrieben)
>
> Rebooting the domU also doesn't fix the problem.
>
> There has been an 8 year old report about live migration mailing:
> <http://lists.xenproject.org/archives/html/xen-devel/2008-10/msg00107.html>
>
> The host is Linux-3.10.71 amd64, while the guest is Linux-4.1.16 i386
> with PAE. The guest kernel reports
>> [    0.000000] e820: last_pfn = 0x7d900 max_arch_pfn = 0x1000000
>
>> # cat iomem
>> 00000000-00000fff : reserved
>> 00001000-0009ffff : System RAM
>> 000a0000-000fffff : reserved
>>   000f0000-000fffff : System ROM
>> 00100000-7d8fffff : System RAM
>>   01000000-014e51d9 : Kernel code
>>   014e51da-016efd7f : Kernel data
>>   017a9000-0181cfff : Kernel bss
>> fee00000-fee00fff : Local APIC
>
>
> To me that looks like a bad mixture of
> 1. xc_domain_maximum_gpfn() returning not the right maximum,
> 2. xc_map_foreign_pages() mapping only the really allocated pages,
>
> Any idea?
> If you need more info, I can provide that on request.
>
> I know that the old migration implementation has been removed with
> xen-4.6, but I still would like to know how to fix that since we will
> not have 4.6 in the next few month and I need working live migration.
>
> Thank you in advance and have a nice weekend.

Phillip,

Given that 4.1 is long out of support, we won't be making a proper fix
in-tree (since it will never be released).  So what kind of resolution
would be the most help to you?  A patch you can apply locally to allow
the save/restore to work?

 -George

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