Re: [v2 2/2] device-dax: "Hotremove" persistent memory that is used like normal RAM

From: Pavel Tatashin <pasha.tatashin@soleen.com>
To: David Hildenbrand <david@redhat.com>
Cc: "Dan Williams" <dan.j.williams@intel.com>,
	"James Morris" <jmorris@namei.org>,
	"Sasha Levin" <sashal@kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Linux MM" <linux-mm@kvack.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Michal Hocko" <mhocko@suse.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Keith Busch" <keith.busch@intel.com>,
	"Vishal L Verma" <vishal.l.verma@intel.com>,
	"Dave Jiang" <dave.jiang@intel.com>,
	"Ross Zwisler" <zwisler@kernel.org>,
	"Tom Lendacky" <thomas.lendacky@amd.com>,
	"Huang, Ying" <ying.huang@intel.com>,
	"Fengguang Wu" <fengguang.wu@intel.com>,
	"Borislav Petkov" <bp@suse.de>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Yaowei Bai" <baiyaowei@cmss.chinamobile.com>,
	"Takashi Iwai" <tiwai@suse.de>,
	"Jérôme Glisse" <jglisse@redhat.com>
Subject: Re: [v2 2/2] device-dax: "Hotremove" persistent memory that is used like normal RAM
Date: Thu, 25 Apr 2019 08:30:40 -0400	[thread overview]
Message-ID: <CA+CK2bBt0vHr9D+BuvM=GmjCMESu5iBiUTdvid_TaoE6j2daQg@mail.gmail.com> (raw)
In-Reply-To: <180d6250-8a6a-0b5d-642a-ec6648cb45b1@redhat.com>

>
> Yes, also I think you can let go of the device_lock in
> check_memblocks_offline_cb, lock_device_hotplug() should take care of
> this (see Documentation/core-api/memory-hotplug.rst - "locking internals")
>
Hi David,

Thank you for your comments. I went through memory-hotplug.rst, and I
still think that device_lock() is needed here. In this particular case
it can be replaced with something like READ_ONCE(), but for simplicity
it is better to have device_lock()/device_unlock() as this is not a
performance critical code.

I do not see any lock ordering issues with this code, as we are
holding lock_device_hotplug() first that prevents userland from
adding/removing memory during this check.

https://soleen.com/source/xref/linux/arch/powerpc/platforms/powernv/memtrace.c?r=98fa15f3#248

Here we have a similar code:
lock_device_hotplug();
   online_mem_block();
    device_online()
     device_lock(dev);

Pasha