Re: [PATCH RFCv2 2/6] mm/memory_hotplug: make add_memory() take the device_hotplug_lock

From: Pasha Tatashin <Pavel.Tatashin@microsoft.com>
To: David Hildenbrand <david@redhat.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>
Cc: Michal Hocko <mhocko@suse.com>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	"linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Len Brown <lenb@kernel.org>,
	YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>,
	Nathan Fontenot <nfont@linux.vnet.ibm.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Oscar Salvador <osalvador@suse.de>,
	Juergen Gross <jgross@suse.com>,
	Mathieu Malaterre <malat@debian.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	John
Subject: Re: [PATCH RFCv2 2/6] mm/memory_hotplug: make add_memory() take the device_hotplug_lock
Date: Thu, 30 Aug 2018 19:36:43 +0000	[thread overview]
Message-ID: <b535ed4a-faa1-9281-e759-ef4b599298ae__43256.4387600452$1535657727$gmane$org@microsoft.com> (raw)
In-Reply-To: <20180821104418.12710-3-david@redhat.com>

On 8/21/18 6:44 AM, David Hildenbrand wrote:
> add_memory() currently does not take the device_hotplug_lock, however
> is aleady called under the lock from
> 	arch/powerpc/platforms/pseries/hotplug-memory.c
> 	drivers/acpi/acpi_memhotplug.c
> to synchronize against CPU hot-remove and similar.
> 
> In general, we should hold the device_hotplug_lock when adding memory
> to synchronize against online/offline request (e.g. from user space) -
> which already resulted in lock inversions due to device_lock() and
> mem_hotplug_lock - see 30467e0b3be ("mm, hotplug: fix concurrent memory
> hot-add deadlock"). add_memory()/add_memory_resource() will create memory
> block devices, so this really feels like the right thing to do.
> 
> Holding the device_hotplug_lock makes sure that a memory block device
> can really only be accessed (e.g. via .online/.state) from user space,
> once the memory has been fully added to the system.
> 
> The lock is not held yet in
> 	drivers/xen/balloon.c
> 	arch/powerpc/platforms/powernv/memtrace.c
> 	drivers/s390/char/sclp_cmd.c
> 	drivers/hv/hv_balloon.c
> So, let's either use the locked variants or take the lock.
> 
> Don't export add_memory_resource(), as it once was exported to be used
> by XEN, which is never built as a module. If somebody requires it, we
> also have to export a locked variant (as device_hotplug_lock is never
> exported).

Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>
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