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

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Date:   Wed, 10 Jun 2020 13:45:23 +0200
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Philipp Rudo <prudo@...ux.ibm.com>,
        Kirill Smelkov <kirr@...edi.com>,
        Alexander Egorenkov <egorenar@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC 1/2] s390/zcore: traverse resources instead of
 memblocks

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@...ibm.com>
> Cc: Vasily Gorbik <gor@...ux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@...ibm.com>
> Cc: Philipp Rudo <prudo@...ux.ibm.com>
> Cc: Kirill Smelkov <kirr@...edi.com>
> Cc: Michael Holzheu <holzheu@...ux.vnet.ibm.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  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 :)