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Re: [Xen-devel] Re: kernel BUG at mm/swapfile.c:2527! [was 3.0.0 Xen pv guest - BUG: Unable to handle]

On 8/30/11 7:45 AM, Ian Campbell wrote:
On Mon, 2011-08-29 at 16:07 +0100, Konrad Rzeszutek Wilk wrote:
I just don't get how you are the only person seeing this - and you have
been seeing this from 2.6.32... The dom0 you have - is it printing at least
something when this happens (or before)? Or the Xen hypervisor:
maybe a message about L1 pages not found?

It'd be worth ensuring that the requires guest_loglvl and loglvl
parameters to allow this is in place on the hypervisor command line.

Nothing in Xen's output correlates at the time of the domUs crashing, however we don't have guest log levels turned up.

Are these reports against totally unpatched kernel.org domU kernels?

Yes - unpatched domUs.

And the dom0 is 2.6.18, right? - Did you update it (I know that the Red Hat guys
have been updating a couple of things on it).

2.6.18 from xenbits, all around changeset 931 vintage.

Any chance I can get access to your setup and try to work with somebody
to reproduce this?

Konrad, that's a fantastic offer and much appreciated. To make this happen I'll need to find a volunteer customer or two whose activity reproduces this problem and who can deal with some downtime -- then quarantine them off to an environment you can access. I'll send out the word...

------------[ cut here ]------------
kernel BUG at mm/swapfile.c:2527!

This is "BUG_ON(*map == 0);" which is subtly different from the error in
the original post from Peter which was a "unable to handle kernel paging
request" at EIP c01ab854, with a pagetable walk showing PTE==0.

I'd bet the dereference corresponds to the "*map" in that same place but
Peter can you convert that address to a line of code please?

root@build:/build/xen/domU/i386/3.0.0-linode35-debug# gdb vmlinux
GNU gdb (GDB) 7.1-ubuntu (...snip...)
Reading symbols from /build/xen/domU/i386/3.0.0-linode35-debug/vmlinux...done.
(gdb) list *0xc01ab854
0xc01ab854 is in swap_count_continued (mm/swapfile.c:2493).
2489 if (count == (SWAP_MAP_MAX | COUNT_CONTINUED)) { /* incrementing */
2490                    /*
2491                     * Think of how you add 1 to 999
2492                     */
2493                    while (*map == (SWAP_CONT_MAX | COUNT_CONTINUED)) {
2494                            kunmap_atomic(map, KM_USER0);
2495 page = list_entry(page->lru.next, struct page, lru);
2496                            BUG_ON(page == head);
2497                            map = kmap_atomic(page, KM_USER0) + offset;

map came from a kmap_atomic() not far before this point so it appears
that it is mapping the wrong page (so *map != 0) and/or mapping a
non-existent page (leading to the fault).

Warning, wild speculation follows...

Is it possible that we are in lazy paravirt mode at this point such that
the mapping hasn't really occurred yet, leaving either nothing or the
previous mapping? (would the current paravirt lazy state make a useful
general addition to the panic message?)

The definition of kmap_atomic is a bit confusing:
          * Make both: kmap_atomic(page, idx) and kmap_atomic(page) work.
         #define kmap_atomic(page, args...) __kmap_atomic(page)
but it appears that the KM_USER0 at the callsite is ignored and instead
we end up using the __kmap_atomic_idx stuff (fine). I wondered if it is
possible we are overflowing the number of slots but there is an explicit
BUG_ON for that case in kmap_atomic_idx_push. Oh, wait, that's iff
CONFIG_DEBUG_HIGHMEM, which appears to not be enabled. I think it would
be worth trying, it doesn't look to have too much overhead.

My next build will be sure to include CONFIG_DEBUG_HIGHMEM. Maybe that'll lead us to a discovery.

Another possibility which springs to mind is the pfn->mfn laundering
going wrong. Perhaps as a skanky debug hack remembering the last pte
val, address, mfn, pfn etc and dumping them on error would give a hint?
I wouldn't expect that to result in a non-present mapping though, rather
I would expect either the wrong thing or the guest to be killed by the

Would it be worth doing a __get_user(map) (or some other "safe" pointer
dereference) right after the mapping is established, catching a fault if
one occurs so we can dump some additional debug in that case? I'm not
entirely sure what to suggest dumping though.


invalid opcode: 0000 [#1] SMP
last sysfs file: /sys/devices/system/cpu/cpu3/topology/core_id
Modules linked in:

Pid: 17680, comm: postgres Tainted: G    B       2.6.39-linode33 #3
EIP: 0061:[<c01b4b26>] EFLAGS: 00210246 CPU: 0
EIP is at swap_count_continued+0x176/0x180
EAX: f57bac57 EBX: eba2c200 ECX: f57ba000 EDX: 00000000
ESI: ebfd7c20 EDI: 00000080 EBP: 00000c57 ESP: c670fe0c
  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0069
Process postgres (pid: 17680, ti=c670e000 task=e93415d0 task.ti=c670e000)
  e9e3a340 00013c57 ee15fc57 00000000 c01b60b1 c0731000 c06982d5 401b4b73
  ceebc988 e9e3a340 00013c57 00000000 c01b60f7 ceebc988 b7731000 c670ff04
  c01a7183 4646e045 80000005 e62ce348 28999063 c0103fc5 7f662000 00278ae0
Call Trace:
  [<c01b60b1>] ? swap_entry_free+0x121/0x140
  [<c06982d5>] ? _raw_spin_lock+0x5/0x10
  [<c01b60f7>] ? free_swap_and_cache+0x27/0xd0
  [<c01a7183>] ? zap_pte_range+0x1b3/0x480
  [<c0103fc5>] ? pte_pfn_to_mfn+0xb5/0xd0
  [<c01a7568>] ? unmap_page_range+0x118/0x1a0
  [<c0105b17>] ? xen_force_evtchn_callback+0x17/0x30
  [<c01a771b>] ? unmap_vmas+0x12b/0x1e0
  [<c01aba01>] ? exit_mmap+0x91/0x140
  [<c0134b2b>] ? mmput+0x2b/0xc0
  [<c01386ba>] ? exit_mm+0xfa/0x130
  [<c0698330>] ? _raw_spin_lock_irq+0x10/0x20
  [<c013a2b5>] ? do_exit+0x125/0x360
  [<c0105b17>] ? xen_force_evtchn_callback+0x17/0x30
  [<c013a52c>] ? do_group_exit+0x3c/0xa0
  [<c013a5a1>] ? sys_exit_group+0x11/0x20
  [<c0698631>] ? syscall_call+0x7/0xb
Code: ff 89 d8 e8 7d ec f6 ff 01 e8 8d 76 00 c6 00 00 ba 01 00 00 00
eb b2 89 f8 3c 80 0f 94 c0
e9 b9 fe ff ff 0f 0b eb fe 0f 0b eb fe<0f>  0b eb fe 0f 0b eb fe 66
90 53 31 db 83 ec 0c 85 c0 7
4 39 89
EIP: [<c01b4b26>] swap_count_continued+0x176/0x180 SS:ESP 0069:c670fe0c
---[ end trace c2dcb41c89b0a9f7 ]---

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