• Dave Hansen's avatar
    device-dax: "Hotplug" persistent memory for use like normal RAM · c221c0b0
    Dave Hansen authored
    This is intended for use with NVDIMMs that are physically persistent
    (physically like flash) so that they can be used as a cost-effective
    RAM replacement.  Intel Optane DC persistent memory is one
    implementation of this kind of NVDIMM.
    Currently, a persistent memory region is "owned" by a device driver,
    either the "Direct DAX" or "Filesystem DAX" drivers.  These drivers
    allow applications to explicitly use persistent memory, generally
    by being modified to use special, new libraries. (DIMM-based
    persistent memory hardware/software is described in great detail
    here: Documentation/nvdimm/nvdimm.txt).
    However, this limits persistent memory use to applications which
    *have* been modified.  To make it more broadly usable, this driver
    "hotplugs" memory into the kernel, to be managed and used just like
    normal RAM would be.
    To make this work, management software must remove the device from
    being controlled by the "Device DAX" infrastructure:
    	echo dax0.0 > /sys/bus/dax/drivers/device_dax/unbind
    and then tell the new driver that it can bind to the device:
    	echo dax0.0 > /sys/bus/dax/drivers/kmem/new_id
    After this, there will be a number of new memory sections visible
    in sysfs that can be onlined, or that may get onlined by existing
    udev-initiated memory hotplug rules.
    This rebinding procedure is currently a one-way trip.  Once memory
    is bound to "kmem", it's there permanently and can not be
    unbound and assigned back to device_dax.
    The kmem driver will never bind to a dax device unless the device
    is *explicitly* bound to the driver.  There are two reasons for
    this: One, since it is a one-way trip, it can not be undone if
    bound incorrectly.  Two, the kmem driver destroys data on the
    device.  Think of if you had good data on a pmem device.  It
    would be catastrophic if you compile-in "kmem", but leave out
    the "device_dax" driver.  kmem would take over the device and
    write volatile data all over your good data.
    This inherits any existing NUMA information for the newly-added
    memory from the persistent memory device that came from the
    firmware.  On Intel platforms, the firmware has guarantees that
    require each socket's persistent memory to be in a separate
    memory-only NUMA node.  That means that this patch is not expected
    to create NUMA nodes, but will simply hotplug memory into existing
    Because NUMA nodes are created, the existing NUMA APIs and tools
    are sufficient to create policies for applications or memory areas
    to have affinity for or an aversion to using this memory.
    There is currently some metadata at the beginning of pmem regions.
    The section-size memory hotplug restrictions, plus this small
    reserved area can cause the "loss" of a section or two of capacity.
    This should be fixable in follow-on patches.  But, as a first step,
    losing 256MB of memory (worst case) out of hundreds of gigabytes
    is a good tradeoff vs. the required code to fix this up precisely.
    This calculation is also the reason we export
    Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
    Reviewed-by: default avatarDan Williams <dan.j.williams@intel.com>
    Reviewed-by: default avatarKeith Busch <keith.busch@intel.com>
    Cc: Dave Jiang <dave.jiang@intel.com>
    Cc: Ross Zwisler <zwisler@kernel.org>
    Cc: Vishal Verma <vishal.l.verma@intel.com>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: linux-nvdimm@lists.01.org
    Cc: linux-kernel@vger.kernel.org
    Cc: linux-mm@kvack.org
    Cc: Huang Ying <ying.huang@intel.com>
    Cc: Fengguang Wu <fengguang.wu@intel.com>
    Cc: Borislav Petkov <bp@suse.de>
    Cc: Bjorn Helgaas <bhelgaas@google.com>
    Cc: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
    Cc: Takashi Iwai <tiwai@suse.de>
    Cc: Jerome Glisse <jglisse@redhat.com>
    Reviewed-by: default avatarVishal Verma <vishal.l.verma@intel.com>
    Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
Makefile 211 Bytes