Commit 4765600f authored Apr 24, 2020 by David Hildenbrand Committed by Vasily Gorbik May 20, 2020

s390: simplify memory notifier for protecting kdump crash kernel area



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.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@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>
Link: https://lore.kernel.org/r/20200424083904.8587-1-david@redhat.com


Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

parent e5794cf1

arch/s390/kernel/setup.c

+5 −8

Original line number	Diff line number	Diff line
		@@ -597,9 +597,10 @@ 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)
		@@ -610,11 +611,7 @@ 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;
		}

		static struct notifier_block kdump_mem_nb = {

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