Re: [PATCH v2 05/40] iommu/sva: Track mm changes with an MMU notifier

Jonathan Cameron
2018-05-17T14:25:14Z

On Fri, 11 May 2018 20:06:06 +0100
Jean-Philippe Brucker <jean-philippe.brucker-5wv7dgnIgG8@public.gmane.org> wrote:

> When creating an io_mm structure, register an MMU notifier that informs
> us when the virtual address space changes and disappears.
>
> Add a new operation to the IOMMU driver, mm_invalidate, called when a
> range of addresses is unmapped to let the IOMMU driver send ATC
> invalidations. mm_invalidate cannot sleep.
>
> Adding the notifier complicates io_mm release. In one case device
> drivers free the io_mm explicitly by calling unbind (or detaching the
> device from its domain). In the other case the process could crash
> before unbind, in which case the release notifier has to do all the
> work.
>
> Allowing the device driver's mm_exit() handler to sleep adds another
> complication, but it will greatly simplify things for users. For example
> VFIO can take the IOMMU mutex and remove any trace of io_mm, instead of
> introducing complex synchronization to delicatly handle this race. But
> relaxing the user side does force unbind() to sleep and wait for all
> pending mm_exit() calls to finish.
>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker-5wv7dgnIgG8@public.gmane.org>
>
> ---
> v1->v2:
> * Unbind() waits for mm_exit to finish
> * mm_exit can sleep
> ---
> drivers/iommu/Kconfig | 1 +
> drivers/iommu/iommu-sva.c | 248 +++++++++++++++++++++++++++++++++++---
> include/linux/iommu.h | 10 ++
> 3 files changed, 244 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index cca8e06903c7..38434899e283 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -77,6 +77,7 @@ config IOMMU_DMA
> config IOMMU_SVA
> bool
> select IOMMU_API
> + select MMU_NOTIFIER
>
> config FSL_PAMU
> bool "Freescale IOMMU support"
> diff --git a/drivers/iommu/iommu-sva.c b/drivers/iommu/iommu-sva.c
> index 0700893c679d..e9afae2537a2 100644
> --- a/drivers/iommu/iommu-sva.c
> +++ b/drivers/iommu/iommu-sva.c
> @@ -7,6 +7,7 @@
>
> #include <linux/idr.h>
> #include <linux/iommu.h>
> +#include <linux/mmu_notifier.h>
> #include <linux/sched/mm.h>
> #include <linux/slab.h>
> #include <linux/spinlock.h>
> @@ -106,6 +107,9 @@ struct iommu_bond {
> struct list_head mm_head;
> struct list_head dev_head;
> struct list_head domain_head;
> + refcount_t refs;
> + struct wait_queue_head mm_exit_wq;
> + bool mm_exit_active;
>
> void *drvdata;
> };
> @@ -124,6 +128,8 @@ static DEFINE_IDR(iommu_pasid_idr);
> */
> static DEFINE_SPINLOCK(iommu_sva_lock);
>
> +static struct mmu_notifier_ops iommu_mmu_notifier;
> +
> static struct io_mm *
> io_mm_alloc(struct iommu_domain *domain, struct device *dev,
> struct mm_struct *mm, unsigned long flags)
> @@ -151,6 +157,7 @@ io_mm_alloc(struct iommu_domain *domain, struct device *dev,
>
> io_mm->flags = flags;
> io_mm->mm = mm;
> + io_mm->notifier.ops = &iommu_mmu_notifier;
> io_mm->release = domain->ops->mm_free;
> INIT_LIST_HEAD(&io_mm->devices);
>
> @@ -167,8 +174,29 @@ io_mm_alloc(struct iommu_domain *domain, struct device *dev,
> goto err_free_mm;
> }
>
> - /* TODO: keep track of mm. For the moment, abort. */
> - ret = -ENOSYS;
> + ret = mmu_notifier_register(&io_mm->notifier, mm);
> + if (ret)
> + goto err_free_pasid;
> +
> + /*
> + * Now that the MMU notifier is valid, we can allow users to grab this
> + * io_mm by setting a valid refcount. Before that it was accessible in
> + * the IDR but invalid.
> + *
> + * The following barrier ensures that users, who obtain the io_mm with
> + * kref_get_unless_zero, don't read uninitialized fields in the
> + * structure.
> + */
> + smp_wmb();
> + kref_init(&io_mm->kref);
> +
> + return io_mm;
> +
> +err_free_pasid:
> + /*
> + * Even if the io_mm is accessible from the IDR at this point, kref is
> + * 0 so no user could get a reference to it. Free it manually.
> + */
> spin_lock(&iommu_sva_lock);
> idr_remove(&iommu_pasid_idr, io_mm->pasid);
> spin_unlock(&iommu_sva_lock);
> @@ -180,9 +208,13 @@ io_mm_alloc(struct iommu_domain *domain, struct device *dev,
> return ERR_PTR(ret);
> }
>
> -static void io_mm_free(struct io_mm *io_mm)
> +static void io_mm_free(struct rcu_head *rcu)
> {
> - struct mm_struct *mm = io_mm->mm;
> + struct io_mm *io_mm;
> + struct mm_struct *mm;
> +
> + io_mm = container_of(rcu, struct io_mm, rcu);
> + mm = io_mm->mm;
>
> io_mm->release(io_mm);
> mmdrop(mm);
> @@ -197,7 +229,22 @@ static void io_mm_release(struct kref *kref)
>
> idr_remove(&iommu_pasid_idr, io_mm->pasid);
>
> - io_mm_free(io_mm);
> + /*
> + * If we're being released from mm exit, the notifier callback ->release
> + * has already been called. Otherwise we don't need ->release, the io_mm
> + * isn't attached to anything anymore. Hence no_release.
> + */
> + mmu_notifier_unregister_no_release(&io_mm->notifier, io_mm->mm);
> +
> + /*
> + * We can't free the structure here, because if mm exits during
> + * unbind(), then ->release might be attempting to grab the io_mm
> + * concurrently. And in the other case, if ->release is calling
> + * io_mm_release, then __mmu_notifier_release expects to still have a
> + * valid mn when returning. So free the structure when it's safe, after
> + * the RCU grace period elapsed.
> + */
> + mmu_notifier_call_srcu(&io_mm->rcu, io_mm_free);
> }
>
> /*
> @@ -206,8 +253,14 @@ static void io_mm_release(struct kref *kref)
> */
> static int io_mm_get_locked(struct io_mm *io_mm)
> {
> - if (io_mm)
> - return kref_get_unless_zero(&io_mm->kref);
> + if (io_mm && kref_get_unless_zero(&io_mm->kref)) {
> + /*
> + * kref_get_unless_zero doesn't provide ordering for reads. This
> + * barrier pairs with the one in io_mm_alloc.
> + */
> + smp_rmb();
> + return 1;
> + }
>
> return 0;
> }
> @@ -233,7 +286,8 @@ static int io_mm_attach(struct iommu_domain *domain, struct device *dev,
> struct iommu_bond *bond, *tmp;
> struct iommu_sva_param *param = dev->iommu_param->sva_param;
>
> - if (!domain->ops->mm_attach || !domain->ops->mm_detach)
> + if (!domain->ops->mm_attach || !domain->ops->mm_detach ||
> + !domain->ops->mm_invalidate)
> return -ENODEV;
>
> if (pasid > param->max_pasid || pasid < param->min_pasid)
> @@ -247,6 +301,8 @@ static int io_mm_attach(struct iommu_domain *domain, struct device *dev,
> bond->io_mm = io_mm;
> bond->dev = dev;
> bond->drvdata = drvdata;
> + refcount_set(&bond->refs, 1);
> + init_waitqueue_head(&bond->mm_exit_wq);
>
> spin_lock(&iommu_sva_lock);
> /*
> @@ -275,12 +331,37 @@ static int io_mm_attach(struct iommu_domain *domain, struct device *dev,
> return 0;
> }
>
> -static void io_mm_detach_locked(struct iommu_bond *bond)
> +static void io_mm_detach_locked(struct iommu_bond *bond, bool wait)
> {
> struct iommu_bond *tmp;
> bool detach_domain = true;
> struct iommu_domain *domain = bond->domain;
>
> + if (wait) {
> + bool do_detach = true;
> + /*
> + * If we're unbind() then we're deleting the bond no matter
> + * what. Tell the mm_exit thread that we're cleaning up, and
> + * wait until it finishes using the bond.
> + *
> + * refs is guaranteed to be one or more, otherwise it would
> + * already have been removed from the list. Check is someone is

Check if someone...

> + * already waiting, in which case we wait but do not free.
> + */
> + if (refcount_read(&bond->refs) > 1)
> + do_detach = false;
> +
> + refcount_inc(&bond->refs);
> + wait_event_lock_irq(bond->mm_exit_wq, !bond->mm_exit_active,
> + iommu_sva_lock);
> + if (!do_detach)
> + return;
> +
> + } else if (!refcount_dec_and_test(&bond->refs)) {
> + /* unbind() is waiting to free the bond */
> + return;
> + }
> +
> list_for_each_entry(tmp, &domain->mm_list, domain_head) {
> if (tmp->io_mm == bond->io_mm && tmp->dev != bond->dev) {
> detach_domain = false;
> @@ -298,6 +379,129 @@ static void io_mm_detach_locked(struct iommu_bond *bond)
> kfree(bond);
> }
>
> +static int iommu_signal_mm_exit(struct iommu_bond *bond)
> +{
> + struct device *dev = bond->dev;
> + struct io_mm *io_mm = bond->io_mm;
> + struct iommu_sva_param *param = dev->iommu_param->sva_param;
> +
> + /*
> + * We can't hold the device's param_lock. If we did and the device
> + * driver used a global lock around io_mm, we would risk getting the
> + * following deadlock:
> + *
> + * exit_mm() | Shutdown SVA
> + * mutex_lock(param->lock) | mutex_lock(glob lock)
> + * param->mm_exit() | sva_device_shutdown()
> + * mutex_lock(glob lock) | mutex_lock(param->lock)
> + *
> + * Fortunately unbind() waits for us to finish, and sva_device_shutdown
> + * requires that any bond is removed, so we can safely access mm_exit
> + * and drvdata without taking any lock.
> + */
> + if (!param || !param->mm_exit)
> + return 0;
> +
> + return param->mm_exit(dev, io_mm->pasid, bond->drvdata);
> +}
> +
> +/* Called when the mm exits. Can race with unbind(). */
> +static void iommu_notifier_release(struct mmu_notifier *mn, struct mm_struct *mm)
> +{
> + struct iommu_bond *bond, *next;
> + struct io_mm *io_mm = container_of(mn, struct io_mm, notifier);
> +
> + /*
> + * If the mm is exiting then devices are still bound to the io_mm.
> + * A few things need to be done before it is safe to release:
> + *
> + * - As the mmu notifier doesn't hold any reference to the io_mm when
> + * calling ->release(), try to take a reference.
> + * - Tell the device driver to stop using this PASID.
> + * - Clear the PASID table and invalidate TLBs.
> + * - Drop all references to this io_mm by freeing the bonds.
> + */
> + spin_lock(&iommu_sva_lock);
> + if (!io_mm_get_locked(io_mm)) {
> + /* Someone's already taking care of it. */
> + spin_unlock(&iommu_sva_lock);
> + return;
> + }
> +
> + list_for_each_entry_safe(bond, next, &io_mm->devices, mm_head) {
> + /*
> + * Release the lock to let the handler sleep. We need to be
> + * careful about concurrent modifications to the list and to the
> + * bond. Tell unbind() not to free the bond until we're done.
> + */
> + bond->mm_exit_active = true;
> + spin_unlock(&iommu_sva_lock);
> +
> + if (iommu_signal_mm_exit(bond))
> + dev_WARN(bond->dev, "possible leak of PASID %u",
> + io_mm->pasid);
> +
> + spin_lock(&iommu_sva_lock);
> + next = list_next_entry(bond, mm_head);
> +
> + /* If someone is waiting, let them delete the bond now */
> + bond->mm_exit_active = false;
> + wake_up_all(&bond->mm_exit_wq);
> +
> + /* Otherwise, do it ourselves */
> + io_mm_detach_locked(bond, false);
> + }
> + spin_unlock(&iommu_sva_lock);
> +
> + /*
> + * We're now reasonably certain that no more fault is being handled for
> + * this io_mm, since we just flushed them all out of the fault queue.
> + * Release the last reference to free the io_mm.
> + */
> + io_mm_put(io_mm);
> +}
> +
> +static void iommu_notifier_invalidate_range(struct mmu_notifier *mn,
> + struct mm_struct *mm,
> + unsigned long start,
> + unsigned long end)
> +{
> + struct iommu_bond *bond;
> + struct io_mm *io_mm = container_of(mn, struct io_mm, notifier);
> +
> + spin_lock(&iommu_sva_lock);
> + list_for_each_entry(bond, &io_mm->devices, mm_head) {
> + struct iommu_domain *domain = bond->domain;
> +
> + domain->ops->mm_invalidate(domain, bond->dev, io_mm, start,
> + end - start);
> + }
> + spin_unlock(&iommu_sva_lock);
> +}
> +
> +static int iommu_notifier_clear_flush_young(struct mmu_notifier *mn,
> + struct mm_struct *mm,
> + unsigned long start,
> + unsigned long end)
> +{
> + iommu_notifier_invalidate_range(mn, mm, start, end);
> + return 0;
> +}
> +
> +static void iommu_notifier_change_pte(struct mmu_notifier *mn,
> + struct mm_struct *mm,
> + unsigned long address, pte_t pte)
> +{
> + iommu_notifier_invalidate_range(mn, mm, address, address + PAGE_SIZE);
> +}
> +
> +static struct mmu_notifier_ops iommu_mmu_notifier = {
> + .release = iommu_notifier_release,
> + .clear_flush_young = iommu_notifier_clear_flush_young,
> + .change_pte = iommu_notifier_change_pte,
> + .invalidate_range = iommu_notifier_invalidate_range,
> +};
> +
> /**
> * iommu_sva_device_init() - Initialize Shared Virtual Addressing for a device
> * @dev: the device
> @@ -320,6 +524,12 @@ static void io_mm_detach_locked(struct iommu_bond *bond)
> * The handler gets an opaque pointer corresponding to the drvdata passed as
> * argument of bind().
> *
> + * The @mm_exit handler is allowed to sleep. Be careful about the locks taken in
> + * @mm_exit, because they might lead to deadlocks if they are also held when
> + * dropping references to the mm. Consider the following call chain:
> + * mutex_lock(A); mmput(mm) -> exit_mm() -> @mm_exit() -> mutex_lock(A)
> + * Using mmput_async() prevents this scenario.
> + *
> * The device should not be performing any DMA while this function is running,
> * otherwise the behavior is undefined.
> *
> @@ -484,15 +694,16 @@ int __iommu_sva_unbind_device(struct device *dev, int pasid)
> if (!param || WARN_ON(!domain))
> return -EINVAL;
>
> - spin_lock(&iommu_sva_lock);
> + /* spin_lock_irq matches the one in wait_event_lock_irq */
> + spin_lock_irq(&iommu_sva_lock);
> list_for_each_entry(bond, &param->mm_list, dev_head) {
> if (bond->io_mm->pasid == pasid) {
> - io_mm_detach_locked(bond);
> + io_mm_detach_locked(bond, true);
> ret = 0;
> break;
> }
> }
> - spin_unlock(&iommu_sva_lock);
> + spin_unlock_irq(&iommu_sva_lock);
>
> return ret;
> }
> @@ -503,18 +714,25 @@ EXPORT_SYMBOL_GPL(__iommu_sva_unbind_device);
> * @dev: the device
> *
> * When detaching @device from a domain, IOMMU drivers should use this helper.
> + * This function may sleep while waiting for bonds to be released.
> */
> void __iommu_sva_unbind_dev_all(struct device *dev)
> {
> struct iommu_sva_param *param;
> struct iommu_bond *bond, *next;
>
> + /*
> + * io_mm_detach_locked might wait, so we shouldn't call it with the dev
> + * param lock held. It's fine to read sva_param outside the lock because
> + * it can only be freed by iommu_sva_device_shutdown when there are no
> + * more bonds in the list.
> + */
> param = dev->iommu_param->sva_param;
> if (param) {
> - spin_lock(&iommu_sva_lock);
> + spin_lock_irq(&iommu_sva_lock);
> list_for_each_entry_safe(bond, next, &param->mm_list, dev_head)
> - io_mm_detach_locked(bond);
> - spin_unlock(&iommu_sva_lock);
> + io_mm_detach_locked(bond, true);
> + spin_unlock_irq(&iommu_sva_lock);
> }
> }
> EXPORT_SYMBOL_GPL(__iommu_sva_unbind_dev_all);
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 439c8fffd836..caa6f79785b9 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -24,6 +24,7 @@
> #include <linux/types.h>
> #include <linux/errno.h>
> #include <linux/err.h>
> +#include <linux/mmu_notifier.h>
> #include <linux/of.h>
> #include <uapi/linux/iommu.h>
>
> @@ -111,10 +112,15 @@ struct io_mm {
> unsigned long flags;
> struct list_head devices;
> struct kref kref;
> +#if defined(CONFIG_MMU_NOTIFIER)
> + struct mmu_notifier notifier;
> +#endif
> struct mm_struct *mm;
>
> /* Release callback for this mm */
> void (*release)(struct io_mm *io_mm);
> + /* For postponed release */
> + struct rcu_head rcu;
> };
>
> enum iommu_cap {
> @@ -249,6 +255,7 @@ struct iommu_sva_param {
> * @mm_attach: attach io_mm to a device. Install PASID entry if necessary
> * @mm_detach: detach io_mm from a device. Remove PASID entry and
> * flush associated TLB entries.
> + * @mm_invalidate: Invalidate a range of mappings for an mm
> * @map: map a physically contiguous memory region to an iommu domain
> * @unmap: unmap a physically contiguous memory region from an iommu domain
> * @map_sg: map a scatter-gather list of physically contiguous memory chunks
> @@ -298,6 +305,9 @@ struct iommu_ops {
> struct io_mm *io_mm, bool attach_domain);
> void (*mm_detach)(struct iommu_domain *domain, struct device *dev,
> struct io_mm *io_mm, bool detach_domain);
> + void (*mm_invalidate)(struct iommu_domain *domain, struct device *dev,
> + struct io_mm *io_mm, unsigned long vaddr,
> + size_t size);
> int (*map)(struct iommu_domain *domain, unsigned long iova,
> phys_addr_t paddr, size_t size, int prot);
> size_t (*unmap)(struct iommu_domain *domain, unsigned long iova,