linux-kernel - Re: [PATCH v2] s390: simplify memory notifier for protecting kdump crash kernel area

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Date:   Thu, 14 May 2020 14:41:30 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-s390@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Philipp Rudo <prudo@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v2] s390: simplify memory notifier for protecting kdump
 crash kernel area

On 24.04.20 10:39, David Hildenbrand wrote:
> Assume we have a crashkernel area of 256MB reserved:
> 
> root@vm0:~# cat /proc/iomem
> 00000000-6fffffff : System RAM
>   0f258000-0fcfffff : Kernel code
>   0fd00000-101d10e3 : Kernel data
>   105b3000-1068dfff : Kernel bss
> 70000000-7fffffff : Crash kernel
> 
> This exactly corresponds to memory block 7 (memory block size is 256MB).
> Trying to offline that memory block results in:
> 
> root@vm0:~# echo "offline" > /sys/devices/system/memory/memory7/state
> -bash: echo: write error: Device or resource busy
> 
> [  128.458762] page:000003d081c00000 refcount:1 mapcount:0 mapping:00000000d01cecd4 index:0x0
> [  128.458773] flags: 0x1ffff00000001000(reserved)
> [  128.458781] raw: 1ffff00000001000 000003d081c00008 000003d081c00008 0000000000000000
> [  128.458781] raw: 0000000000000000 0000000000000000 ffffffff00000001 0000000000000000
> [  128.458783] page dumped because: unmovable page
> 
> The craskernel area is marked reserved in the bootmem allocator. This
> results in the memmap getting initialized (refcount=1, PG_reserved), but
> the pages are never freed to the page allocator.
> 
> So these pages look like allocated pages that are unmovable (esp.
> PG_reserved), and therefore, memory offlining fails early, when trying to
> isolate the page range.
> 
> We only have to care about the exchange area, make that clear.
> 
> Cc: Heiko Carstens <heiko.carstens@...ibm.com>
> Cc: Vasily Gorbik <gor@...ux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@...ibm.com>
> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> Cc: Philipp Rudo <prudo@...ux.ibm.com>
> Cc: Gerald Schaefer <gerald.schaefer@...ibm.com>
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Thanks applied