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

[Philipp Hahn <hahn@xxxxxxxxxxxxx>]

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.

Philipp
-- 
Philipp Hahn
Open Source Software Engineer

Univention GmbH
be open.
Mary-Somerville-Str. 1
D-28359 Bremen
Tel.: +49 421 22232-0
Fax : +49 421 22232-99
hahn@xxxxxxxxxxxxx

http://www.univention.de/
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