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Subject: Re: [PATCH v2] s390: simplify memory notifier for protecting kdump
 crash kernel area
To:     linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, linux-s390@vger.kernel.org,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Philipp Rudo <prudo@linux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@de.ibm.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Michal Hocko <mhocko@kernel.org>
References: <20200424083904.8587-1-david@redhat.com>
From:   David Hildenbrand <david@redhat.com>
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Message-ID: <1d79310d-8909-73e7-d2dd-90e8b9edf253@redhat.com>
Date:   Wed, 29 Apr 2020 16:55:38 +0200
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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@de.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Philipp Rudo <prudo@linux.ibm.com>
> Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> 
> Follow up of:
> - "[PATCH v1] s390: drop memory notifier for protecting kdump crash kernel
>    area"
> 
> v1 -> v2:
> - Keep the notifier, check for exchange area only
> 
> ---
>  arch/s390/kernel/setup.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
> index 0f0b140b5558..c0881f0a3175 100644
> --- a/arch/s390/kernel/setup.c
> +++ b/arch/s390/kernel/setup.c
> @@ -594,9 +594,10 @@ static void __init setup_memory_end(void)
>  #ifdef CONFIG_CRASH_DUMP
>  
>  /*
> - * When kdump is enabled, we have to ensure that no memory from
> - * the area [0 - crashkernel memory size] and
> - * [crashk_res.start - crashk_res.end] is set offline.
> + * When kdump is enabled, we have to ensure that no memory from the area
> + * [0 - crashkernel memory size] is set offline - it will be exchanged with
> + * the crashkernel memory region when kdump is triggered. The crashkernel
> + * memory region can never get offlined (pages are unmovable).
>   */
>  static int kdump_mem_notifier(struct notifier_block *nb,
>  			      unsigned long action, void *data)
> @@ -607,11 +608,7 @@ static int kdump_mem_notifier(struct notifier_block *nb,
>  		return NOTIFY_OK;
>  	if (arg->start_pfn < PFN_DOWN(resource_size(&crashk_res)))
>  		return NOTIFY_BAD;
> -	if (arg->start_pfn > PFN_DOWN(crashk_res.end))
> -		return NOTIFY_OK;
> -	if (arg->start_pfn + arg->nr_pages - 1 < PFN_DOWN(crashk_res.start))
> -		return NOTIFY_OK;
> -	return NOTIFY_BAD;
> +	return NOTIFY_OK;
>  }
>  
>  static struct notifier_block kdump_mem_nb = {
> 

Ping.

-- 
Thanks,

David / dhildenb