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Date:   Thu, 30 Apr 2020 17:05:40 +0200
From:   Gerald Schaefer <gerald.schaefer@de.ibm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, 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>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v2] s390: simplify memory notifier for protecting kdump
 crash kernel area
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In-Reply-To: <1d79310d-8909-73e7-d2dd-90e8b9edf253@redhat.com>
References: <20200424083904.8587-1-david@redhat.com>
        <1d79310d-8909-73e7-d2dd-90e8b9edf253@redhat.com>
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On Wed, 29 Apr 2020 16:55:38 +0200
David Hildenbrand <david@redhat.com> wrote:

> 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.
> 

Looks good, thanks.

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>