Re: [PATCH 3/6] HMM: introduce heterogeneous memory management v2.

[Oded Gabbay]

On 01/06/2015 12:44 AM, j.glisse@xxxxxxxxx wrote:
> From: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> 
> 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: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Signed-off-by: Sherry Cheung <SCheung@xxxxxxxxxx>
> Signed-off-by: Subhash Gutti <sgutti@xxxxxxxxxx>
> Signed-off-by: Mark Hairgrove <mhairgrove@xxxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Jatin Kumar <jakumar@xxxxxxxxxx>
> ---
>  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:	JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> +L:	linux-mm@xxxxxxxxx
> +S:	Maintained
> +F:	mm/hmm.c
> +F:	include/linux/hmm.h
> +
>  HOST AP DRIVER
>  M:	Jouni Malinen <j@xxxxx>
>  L:	hostap@xxxxxxxxx (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: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> + */
> +/* 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: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> + */
> +/* 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);
Jerome,
Don't you need to handle a case where the process is terminated before it
calls hmm_mirror_unregister() ? or if it "forgets" to call that function in
the first place ?

This is the only place I saw "kfree(hmm)" and this function is only called
if there are no more references to hmm. I saw you unref only in
hmm_mirror_unregister() but as I said, that function may never be called.

	Oded

> +}
> +
> +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);
> 
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