Date:	Mon,  5 Jan 2015 17:44:46 -0500
From:	j.glisse@...il.com
To:	akpm@...ux-foundation.org
Cc:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	<joro@...tes.org>, Mel Gorman <mgorman@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Airlie <airlied@...hat.com>,
	Brendan Conoboy <blc@...hat.com>,
	Joe Donohue <jdonohue@...hat.com>,
	Duncan Poole <dpoole@...dia.com>,
	Sherry Cheung <SCheung@...dia.com>,
	Subhash Gutti <sgutti@...dia.com>,
	John Hubbard <jhubbard@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Lucien Dunning <ldunning@...dia.com>,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Arvind Gopalakrishnan <arvindg@...dia.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Liran Liss <liranl@...lanox.com>,
	Roland Dreier <roland@...estorage.com>,
	Ben Sander <ben.sander@....com>,
	Greg Stoner <Greg.Stoner@....com>,
	John Bridgman <John.Bridgman@....com>,
	Michael Mantor <Michael.Mantor@....com>,
	Paul Blinzer <Paul.Blinzer@....com>,
	Laurent Morichetti <Laurent.Morichetti@....com>,
	Alexander Deucher <Alexander.Deucher@....com>,
	Oded Gabbay <Oded.Gabbay@....com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Jatin Kumar <jakumar@...dia.com>
Subject: [PATCH 3/6] HMM: introduce heterogeneous memory management v2.

From: Jérôme Glisse <jglisse@...hat.com>

This patch only introduce core HMM functions for registering a new mirror and
stopping a mirror as well as registering and unregistering a device.

The lifecycle of HMM object is handled differently then one of mmu_notifier
because unlike mmu_notifier there can be concurrent call from both mm code to
HMM code and/or from device driver code to HMM code. Moreover lifetime of HMM
can be uncorrelated from the lifetime of the process that is being mirror.

Changed since v1:
  - Updated comment of hmm_device_register().

Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
Signed-off-by: Sherry Cheung <SCheung@...dia.com>
Signed-off-by: Subhash Gutti <sgutti@...dia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@...dia.com>
Signed-off-by: John Hubbard <jhubbard@...dia.com>
Signed-off-by: Jatin Kumar <jakumar@...dia.com>
---
 MAINTAINERS              |   7 +
 include/linux/hmm.h      | 129 ++++++++++++++++
 include/linux/mm.h       |  11 ++
 include/linux/mm_types.h |  14 ++
 kernel/fork.c            |   2 +
 mm/Kconfig               |  15 ++
 mm/Makefile              |   1 +
 mm/hmm.c                 | 373 +++++++++++++++++++++++++++++++++++++++++++++++
 8 files changed, 552 insertions(+)
 create mode 100644 include/linux/hmm.h
 create mode 100644 mm/hmm.c

diff --git a/MAINTAINERS b/MAINTAINERS
index c03bc6c..3ec87c4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4533,6 +4533,13 @@ F:	include/uapi/linux/if_hippi.h
 F:	net/802/hippi.c
 F:	drivers/net/hippi/
 
+HMM - Heterogeneous Memory Management
+M:	Jérôme Glisse <jglisse@...hat.com>
+L:	linux-mm@...ck.org
+S:	Maintained
+F:	mm/hmm.c
+F:	include/linux/hmm.h
+
 HOST AP DRIVER
 M:	Jouni Malinen <j@...fi>
 L:	hostap@...oo.com (subscribers-only)
diff --git a/include/linux/hmm.h b/include/linux/hmm.h
new file mode 100644
index 0000000..8eddc15
--- /dev/null
+++ b/include/linux/hmm.h
@@ -0,0 +1,129 @@
+/*
+ * Copyright 2013 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * Authors: Jérôme Glisse <jglisse@...hat.com>
+ */
+/* This is a heterogeneous memory management (hmm). In a nutshell this provide
+ * an API to mirror a process address on a device which has its own mmu using
+ * its own page table for the process. It supports everything except special
+ * vma.
+ *
+ * Mandatory hardware features :
+ *   - An mmu with pagetable.
+ *   - Read only flag per cpu page.
+ *   - Page fault ie hardware must stop and wait for kernel to service fault.
+ *
+ * Optional hardware features :
+ *   - Dirty bit per cpu page.
+ *   - Access bit per cpu page.
+ *
+ * The hmm code handle all the interfacing with the core kernel mm code and
+ * provide a simple API. It does support migrating system memory to device
+ * memory and handle migration back to system memory on cpu page fault.
+ *
+ * Migrated memory is considered as swaped from cpu and core mm code point of
+ * view.
+ */
+#ifndef _HMM_H
+#define _HMM_H
+
+#ifdef CONFIG_HMM
+
+#include <linux/list.h>
+#include <linux/spinlock.h>
+#include <linux/atomic.h>
+#include <linux/mm_types.h>
+#include <linux/mmu_notifier.h>
+#include <linux/workqueue.h>
+#include <linux/mman.h>
+
+
+struct hmm_device;
+struct hmm_mirror;
+struct hmm;
+
+
+/* hmm_device - Each device must register one and only one hmm_device.
+ *
+ * The hmm_device is the link btw HMM and each device driver.
+ */
+
+/* struct hmm_device_operations - HMM device operation callback
+ */
+struct hmm_device_ops {
+	/* release() - mirror must stop using the address space.
+	 *
+	 * @mirror: The mirror that link process address space with the device.
+	 *
+	 * This callback is call either on mm destruction or as result to a
+	 * call to hmm_mirror_release(). Device driver have to stop all hw
+	 * thread and all usage of the address space, it has to dirty all pages
+	 * that have been dirty by the device. But it must not clear any entry
+	 * from the mirror page table.
+	 */
+	void (*release)(struct hmm_mirror *mirror);
+};
+
+/* struct hmm_device - per device HMM structure
+ *
+ * @name: Device name (uniquely identify the device on the system).
+ * @ops: The hmm operations callback.
+ * @mirrors: List of all active mirrors for the device.
+ * @mutex: Mutex protecting mirrors list.
+ *
+ * Each device that want to mirror an address space must register one of this
+ * struct (only once per linux device).
+ */
+struct hmm_device {
+	const char			*name;
+	const struct hmm_device_ops	*ops;
+	struct list_head		mirrors;
+	struct mutex			mutex;
+};
+
+int hmm_device_register(struct hmm_device *device);
+int hmm_device_unregister(struct hmm_device *device);
+
+
+/* hmm_mirror - device specific mirroring functions.
+ *
+ * Each device that mirror a process has a uniq hmm_mirror struct associating
+ * the process address space with the device. Same process can be mirrored by
+ * several different devices at the same time.
+ */
+
+/* struct hmm_mirror - per device and per mm HMM structure
+ *
+ * @device: The hmm_device struct this hmm_mirror is associated to.
+ * @hmm: The hmm struct this hmm_mirror is associated to.
+ * @dlist: List of all hmm_mirror for same device.
+ * @mlist: List of all hmm_mirror for same process.
+ *
+ * Each device that want to mirror an address space must register one of this
+ * struct for each of the address space it wants to mirror. Same device can
+ * mirror several different address space. As well same address space can be
+ * mirror by different devices.
+ */
+struct hmm_mirror {
+	struct hmm_device	*device;
+	struct hmm		*hmm;
+	struct list_head	dlist;
+	struct hlist_node	mlist;
+};
+
+int hmm_mirror_register(struct hmm_mirror *mirror, struct hmm_device *device);
+void hmm_mirror_unregister(struct hmm_mirror *mirror);
+
+
+#endif /* CONFIG_HMM */
+#endif
diff --git a/include/linux/mm.h b/include/linux/mm.h
index f80d019..0e054f9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2208,5 +2208,16 @@ void __init setup_nr_node_ids(void);
 static inline void setup_nr_node_ids(void) {}
 #endif
 
+#ifdef CONFIG_HMM
+static inline void hmm_mm_init(struct mm_struct *mm)
+{
+	mm->hmm = NULL;
+}
+#else /* !CONFIG_HMM */
+static inline void hmm_mm_init(struct mm_struct *mm)
+{
+}
+#endif /* !CONFIG_HMM */
+
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6d34aa2..57a3e1c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -15,6 +15,10 @@
 #include <asm/page.h>
 #include <asm/mmu.h>
 
+#ifdef CONFIG_HMM
+struct hmm;
+#endif
+
 #ifndef AT_VECTOR_SIZE_ARCH
 #define AT_VECTOR_SIZE_ARCH 0
 #endif
@@ -426,6 +430,16 @@ struct mm_struct {
 #ifdef CONFIG_MMU_NOTIFIER
 	struct mmu_notifier_mm *mmu_notifier_mm;
 #endif
+#ifdef CONFIG_HMM
+	/*
+	 * hmm always register an mmu_notifier we rely on mmu notifier to keep
+	 * refcount on mm struct as well as forbiding registering hmm on a
+	 * dying mm
+	 *
+	 * This field is set with mmap_sem old in write mode.
+	 */
+	struct hmm *hmm;
+#endif
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
 	pgtable_t pmd_huge_pte; /* protected by page_table_lock */
 #endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 4dc2dda..0bb9dc4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -27,6 +27,7 @@
 #include <linux/binfmts.h>
 #include <linux/mman.h>
 #include <linux/mmu_notifier.h>
+#include <linux/hmm.h>
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/vmacache.h>
@@ -568,6 +569,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
 	mm_init_aio(mm);
 	mm_init_owner(mm, p);
 	mmu_notifier_mm_init(mm);
+	hmm_mm_init(mm);
 	clear_tlb_flush_pending(mm);
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
 	mm->pmd_huge_pte = NULL;
diff --git a/mm/Kconfig b/mm/Kconfig
index 1d1ae6b..b249db0 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -618,3 +618,18 @@ config MAX_STACK_SIZE_MB
 	  changed to a smaller value in which case that is used.
 
 	  A sane initial value is 80 MB.
+
+if STAGING
+config HMM
+	bool "Enable heterogeneous memory management (HMM)"
+	depends on MMU
+	select MMU_NOTIFIER
+	select GENERIC_PAGE_TABLE
+	default n
+	help
+	  Heterogeneous memory management provide infrastructure for a device
+	  to mirror a process address space into an hardware mmu or into any
+	  things supporting pagefault like event.
+
+	  If unsure, say N to disable hmm.
+endif # STAGING
diff --git a/mm/Makefile b/mm/Makefile
index 3548460..cb2f9ed 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -73,3 +73,4 @@ obj-$(CONFIG_GENERIC_EARLY_IOREMAP) += early_ioremap.o
 obj-$(CONFIG_CMA)	+= cma.o
 obj-$(CONFIG_MEMORY_BALLOON) += balloon_compaction.o
 obj-$(CONFIG_PAGE_EXTENSION) += page_ext.o
+obj-$(CONFIG_HMM) += hmm.o
diff --git a/mm/hmm.c b/mm/hmm.c
new file mode 100644
index 0000000..0b4e220
--- /dev/null
+++ b/mm/hmm.c
@@ -0,0 +1,373 @@
+/*
+ * Copyright 2013 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * Authors: Jérôme Glisse <jglisse@...hat.com>
+ */
+/* This is the core code for heterogeneous memory management (HMM). HMM intend
+ * to provide helper for mirroring a process address space on a device as well
+ * as allowing migration of data between system memory and device memory refer
+ * as remote memory from here on out.
+ *
+ * Refer to include/linux/hmm.h for further information on general design.
+ */
+#include <linux/export.h>
+#include <linux/bitmap.h>
+#include <linux/list.h>
+#include <linux/rculist.h>
+#include <linux/slab.h>
+#include <linux/mmu_notifier.h>
+#include <linux/mm.h>
+#include <linux/hugetlb.h>
+#include <linux/fs.h>
+#include <linux/file.h>
+#include <linux/ksm.h>
+#include <linux/rmap.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
+#include <linux/mmu_context.h>
+#include <linux/memcontrol.h>
+#include <linux/hmm.h>
+#include <linux/wait.h>
+#include <linux/mman.h>
+#include <linux/delay.h>
+#include <linux/workqueue.h>
+
+#include "internal.h"
+
+static struct srcu_struct srcu;
+
+
+/* struct hmm - per mm_struct HMM states.
+ *
+ * @mirrors: List of all mirror for this mm (one per device).
+ * @mm: The mm struct this hmm is associated with.
+ * @vm_end: Last valid address for this mm (exclusive).
+ * @lock: Serialize the mirror list modifications.
+ * @kref: Reference counter
+ * @mmu_notifier: The mmu_notifier of this mm.
+ *
+ * For each process address space (mm_struct) there is one and only one hmm
+ * struct. hmm functions will redispatch to each devices the change made to
+ * the process address space.
+ */
+struct hmm {
+	struct hlist_head	mirrors;
+	struct mm_struct	*mm;
+	unsigned long		vm_end;
+	spinlock_t		lock;
+	struct kref		kref;
+	struct mmu_notifier	mmu_notifier;
+};
+
+static struct mmu_notifier_ops hmm_notifier_ops;
+
+
+/* hmm - core HMM functions.
+ *
+ * Core HMM functions that deal with all the process mm activities.
+ */
+
+static int hmm_init(struct hmm *hmm)
+{
+	hmm->mm = current->mm;
+	hmm->vm_end = TASK_SIZE;
+	kref_init(&hmm->kref);
+	INIT_HLIST_HEAD(&hmm->mirrors);
+	spin_lock_init(&hmm->lock);
+
+	/* register notifier */
+	hmm->mmu_notifier.ops = &hmm_notifier_ops;
+	return __mmu_notifier_register(&hmm->mmu_notifier, current->mm);
+}
+
+static int hmm_add_mirror(struct hmm *hmm, struct hmm_mirror *mirror)
+{
+	struct hmm_mirror *tmp;
+
+	spin_lock(&hmm->lock);
+	hlist_for_each_entry_rcu(tmp, &hmm->mirrors, mlist)
+		if (tmp->device == mirror->device) {
+			/* Same device can mirror only once. */
+			spin_unlock(&hmm->lock);
+			return -EINVAL;
+		}
+	hlist_add_head(&mirror->mlist, &hmm->mirrors);
+	spin_unlock(&hmm->lock);
+
+	return 0;
+}
+
+static inline struct hmm *hmm_ref(struct hmm *hmm)
+{
+	if (!hmm || !kref_get_unless_zero(&hmm->kref))
+		return NULL;
+	return hmm;
+}
+
+static void hmm_destroy(struct kref *kref)
+{
+	struct hmm *hmm;
+
+	hmm = container_of(kref, struct hmm, kref);
+	BUG_ON(!hlist_empty(&hmm->mirrors));
+
+	down_write(&hmm->mm->mmap_sem);
+	/* A new hmm might have been register before reaching that point. */
+	if (hmm->mm->hmm == hmm)
+		hmm->mm->hmm = NULL;
+	up_write(&hmm->mm->mmap_sem);
+
+	mmu_notifier_unregister(&hmm->mmu_notifier, hmm->mm);
+
+	kfree(hmm);
+}
+
+static inline struct hmm *hmm_unref(struct hmm *hmm)
+{
+	if (hmm)
+		kref_put(&hmm->kref, hmm_destroy);
+	return NULL;
+}
+
+
+/* hmm_notifier - HMM callback for mmu_notifier tracking change to process mm.
+ *
+ * HMM use use mmu notifier to track change made to process address space.
+ */
+static void hmm_notifier_release(struct mmu_notifier *mn, struct mm_struct *mm)
+{
+	struct hmm *hmm;
+	int id;
+
+	/*
+	 * The hmm structure can not be free because the mmu_notifier srcu is
+	 * read locked thus any concurrent hmm_mirror_unregister that would
+	 * free hmm would have to wait on the mmu_notifier srcu.
+	 */
+	hmm = container_of(mn, struct hmm, mmu_notifier);
+
+	id = srcu_read_lock(&srcu);
+	spin_lock(&hmm->lock);
+	while (hmm->mirrors.first) {
+		struct hmm_mirror *mirror;
+
+		mirror = hlist_entry(hmm->mirrors.first,
+				     struct hmm_mirror,
+				     mlist);
+		hlist_del_init_rcu(&mirror->mlist);
+		spin_unlock(&hmm->lock);
+
+		mirror->device->ops->release(mirror);
+
+		mutex_lock(&mirror->device->mutex);
+		list_del_init(&mirror->dlist);
+		mutex_unlock(&mirror->device->mutex);
+
+		spin_lock(&hmm->lock);
+	}
+	spin_unlock(&hmm->lock);
+
+	srcu_read_unlock(&srcu, id);
+}
+
+static struct mmu_notifier_ops hmm_notifier_ops = {
+	.release		= hmm_notifier_release,
+};
+
+
+/* hmm_mirror - per device mirroring functions.
+ *
+ * Each device that mirror a process has a uniq hmm_mirror struct. A process
+ * can be mirror by several devices at the same time.
+ *
+ * Below are all the functions and their helpers use by device driver to mirror
+ * the process address space. Those functions either deals with updating the
+ * device page table (through hmm callback). Or provide helper functions use by
+ * the device driver to fault in range of memory in the device page table.
+ */
+
+/* hmm_mirror_register() - register mirror against current process for a device.
+ *
+ * @mirror: The mirror struct being registered.
+ * @device: The device struct to against which the mirror is registered.
+ * Returns: 0 on success or -ENOMEM, -EINVAL on error.
+ *
+ * Call when device driver want to start mirroring a process address space. The
+ * HMM shim will register mmu_notifier and start monitoring process address
+ * space changes. Hence callback to device driver might happen even before this
+ * function return.
+ *
+ * The task device driver want to mirror must be current !
+ *
+ * Only one mirror per mm and hmm_device can be created, it will return NULL if
+ * the hmm_device already has an hmm_mirror for the the mm.
+ */
+int hmm_mirror_register(struct hmm_mirror *mirror, struct hmm_device *device)
+{
+	struct mm_struct *mm = current->mm;
+	struct hmm *hmm = NULL;
+	int ret = 0;
+
+	/* Sanity checks. */
+	BUG_ON(!device);
+	BUG_ON(!mm);
+
+	/*
+	 * Initialize the mirror struct fields, the mlist init and del dance is
+	 * necessary to make the error path easier for driver and for hmm.
+	 */
+	INIT_HLIST_NODE(&mirror->mlist);
+	INIT_LIST_HEAD(&mirror->dlist);
+	mutex_lock(&device->mutex);
+	mirror->device = device;
+	list_add(&mirror->dlist, &device->mirrors);
+	mutex_unlock(&device->mutex);
+
+	down_write(&mm->mmap_sem);
+
+	hmm = mm->hmm ? hmm_ref(hmm) : NULL;
+	if (hmm == NULL) {
+		/* no hmm registered yet so register one */
+		hmm = kzalloc(sizeof(*mm->hmm), GFP_KERNEL);
+		if (hmm == NULL) {
+			up_write(&mm->mmap_sem);
+			ret = -ENOMEM;
+			goto error;
+		}
+
+		ret = hmm_init(hmm);
+		if (ret) {
+			up_write(&mm->mmap_sem);
+			kfree(hmm);
+			goto error;
+		}
+
+		mm->hmm = hmm;
+	}
+
+	mirror->hmm = hmm;
+	ret = hmm_add_mirror(hmm, mirror);
+	up_write(&mm->mmap_sem);
+	if (ret) {
+		mirror->hmm = NULL;
+		hmm_unref(hmm);
+		goto error;
+	}
+	return 0;
+
+error:
+	mutex_lock(&device->mutex);
+	list_del_init(&mirror->dlist);
+	mutex_unlock(&device->mutex);
+	return ret;
+}
+EXPORT_SYMBOL(hmm_mirror_register);
+
+static void hmm_mirror_release(struct hmm_mirror *mirror)
+{
+	spin_lock(&mirror->hmm->lock);
+	if (!hlist_unhashed(&mirror->mlist)) {
+		hlist_del_init_rcu(&mirror->mlist);
+		spin_unlock(&mirror->hmm->lock);
+		mirror->device->ops->release(mirror);
+
+		mutex_lock(&mirror->device->mutex);
+		list_del_init(&mirror->dlist);
+		mutex_unlock(&mirror->device->mutex);
+	} else
+		spin_unlock(&mirror->hmm->lock);
+}
+
+/* hmm_mirror_unregister() - unregister a mirror.
+ *
+ * @mirror: The mirror that link process address space with the device.
+ *
+ * Driver can call this function when it wants to stop mirroring a process.
+ * This will trigger a call to the ->stop() callback if it did not aleady
+ * happen.
+ */
+void hmm_mirror_unregister(struct hmm_mirror *mirror)
+{
+	int id;
+
+	id = srcu_read_lock(&srcu);
+	hmm_mirror_release(mirror);
+	srcu_read_unlock(&srcu, id);
+
+	/*
+	 * Wait for any running method to finish, of course including
+	 * ->release() if it was run by hmm_notifier_release instead of us.
+	 */
+	synchronize_srcu(&srcu);
+
+	mirror->hmm = hmm_unref(mirror->hmm);
+}
+EXPORT_SYMBOL(hmm_mirror_unregister);
+
+
+/* hmm_device - Each device driver must register one and only one hmm_device
+ *
+ * The hmm_device is the link btw HMM and each device driver.
+ */
+
+/* hmm_device_register() - register a device with HMM.
+ *
+ * @device: The hmm_device struct.
+ * Returns: 0 on success or -EINVAL otherwise.
+ *
+ *
+ * Call when device driver want to register itself with HMM. Device driver must
+ * only register once.
+ */
+int hmm_device_register(struct hmm_device *device)
+{
+	/* sanity check */
+	BUG_ON(!device);
+	BUG_ON(!device->name);
+	BUG_ON(!device->ops);
+	BUG_ON(!device->ops->release);
+
+	mutex_init(&device->mutex);
+	INIT_LIST_HEAD(&device->mirrors);
+
+	return 0;
+}
+EXPORT_SYMBOL(hmm_device_register);
+
+/* hmm_device_unregister() - unregister a device with HMM.
+ *
+ * @device: The hmm_device struct.
+ * Returns: 0 on success or -EBUSY otherwise.
+ *
+ * Call when device driver want to unregister itself with HMM. This will check
+ * that there is no any active mirror and returns -EBUSY if so.
+ */
+int hmm_device_unregister(struct hmm_device *device)
+{
+	mutex_lock(&device->mutex);
+	if (!list_empty(&device->mirrors)) {
+		mutex_unlock(&device->mutex);
+		return -EBUSY;
+	}
+	mutex_unlock(&device->mutex);
+	synchronize_srcu(&srcu);
+	return 0;
+}
+EXPORT_SYMBOL(hmm_device_unregister);
+
+
+static int __init hmm_subsys_init(void)
+{
+	return init_srcu_struct(&srcu);
+}
+subsys_initcall(hmm_subsys_init);
-- 
1.9.3

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