Subject: Re: visible memory seems wrong in kexec crash dump kernel

From: Scott Wood <[email-masked]>
Date: Mon, 29 Jul 2013 18:10:36 -0500

On 07/13/2013 01:30:50 AM, Chris Friesen wrote:
> On 07/12/2013 04:59 PM, Chris Friesen wrote:
>> On 07/12/2013 03:08 PM, Chris Friesen wrote:
>>=20
>>> I turned on the instrumentation in early_init_dt_scan_memory() and =20
>>> got
>>> the following when jumping to the capture kernel:
>>>=20
>>> memory scan node memory, reg size 16, data: 0 0 2 0,
>>> - 0 , 200000000
>>>=20
>>> That 0x200000000 matches the fact that I'm seeing 8GB of memory
>>> available in the recovery kernel.
>>>=20
>>> If I boot the original kernel with "crashkernel=3D224M@32M", should I
>>> expect that only 224MB is marked as "linux,usable-memory" in the
>>> recovery kernel?
>>=20
>> I started looking at the kexec side of things, and I noticed =20
>> something a
>> bit odd. In most places dealing with the device tree in kexec it =20
>> accepts
>> either "memory" or "memory@" for the memory node name. In
>> add_usable_mem_property() in arch/ppc64/fs2dt.c it seems to only =20
>> accept
>> "memory@".
>>=20
>> Is this expected behaviour? It seems to be the same in current git
>> versions of kexec-tools.
>>=20
>> On my system I see "/proc/device-tree/memory".
>>=20
>> If I modify add_usable_mem_property() to also accept "/memory" then =20
>> my
>> recovery kernel boots up with
>>=20
>> physicalMemorySize =3D 0x10000000
>>=20
>> which is 256MB (which is still a bit odd since I specified 224MB for =20
>> the
>> crashkernel).
>>=20
>> However, it then hits the BUG() call at the end of mark_bootmem() in
>> mm/bootmem.c.
>=20
> One final thing and I'll stop replying to myself. :)
>=20
> It looks like the problem is that some board-specific freescale code =20
> was calling lmb_reserve() with a base address in the 4GB range.  It =20
> seems odd that lmb_reserve() didn't throw some kind of error when the =20
> recovery kernel was supposed to be limited to 224MB.
>=20
> Rather than try and fix the bug, I turned off the (unneeded) config =20
> options related to the above lmb_reserve() calls and was able to =20
> successfully access the information I needed via /dev/oldmem.
>=20
> The upshot is that there seems to be a number of things that could be =20
> improved:
>=20
> 1) kexec should accept "/memory" and not just "/memory@"
> 2) lmb_reserve() should really respect the crashkernel memory limit
> 3) the freescale stuff really shouldn't assume it can map things =20
> wherever it feels like

What "board-specific freescale code" are you referring to?

-Scott=