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

[Christian Borntraeger]

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@xxxxxxxxxx>
> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> Cc: Philipp Rudo <prudo@xxxxxxxxxxxxx>
> Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks applied