Re: [PATCH RFC 1/2] s390/zcore: traverse resources instead of memblocks

[Heiko Carstens]

On Fri, Apr 17, 2020 at 05:01:50PM +0200, David Hildenbrand wrote:
> The zcore memmap basically contains the first level of all system RAM from
> /proc/iomem. We want to disable CONFIG_ARCH_KEEP_MEMBLOCK (e.g., to not
> create memblocks for hotplugged/standby memory and save space), switch to
> traversing system ram resources instead. During early boot, we create
> resources for all early memblocks (including the crash kernel area). When
> adding standby memory, we currently create both, memblocks and resources.
> 
> Note: As we don't have memory hotplug after boot (standby memory is added
> via sclp during boot), we don't have to worry about races.
> 
> I am only able to test under KVM (where I hacked up zcore to still
> create the memmap file)
> 
> root@vm0:~# cat /proc/iomem
> 00000000-2fffffff : System RAM
>   10424000-10ec6fff : Kernel code
>   10ec7000-1139a0e3 : Kernel data
>   1177a000-11850fff : Kernel bss
> 30000000-3fffffff : Crash kernel
> 
> Result without this patch:
> root@vm0:~# cat /sys/kernel/debug/zcore/memmap
> 0000000000000000 0000000040000000
> 
> Result with this patch:
> root@vm0:~# cat /sys/kernel/debug/zcore/memmap
> 0000000000000000 0000000030000000 0000000030000000 0000000010000000
> 
> The difference is due to memblocks getting merged, resources (currently)
> not. So we might have some more entries, but they describe the same
> memory map.
> 
> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Cc: Philipp Rudo <prudo@xxxxxxxxxxxxx>
> Cc: Kirill Smelkov <kirr@xxxxxxxxxx>
> Cc: Michael Holzheu <holzheu@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  drivers/s390/char/zcore.c | 61 ++++++++++++++++++++++++++++++---------
>  1 file changed, 48 insertions(+), 13 deletions(-)

I'm having a hard time to find any code that ever made use of this
file. After all this was only meant for our zfcp dumper, but as far as
I can tell there was never code out there that read the memmap file.

So the pragmatic approach would be to just change its contents, or a
more progressive variant would be to remove the file completely.
But maybe I'm entirely wrong...

I'm leaving this up to Philipp and Alexander :)