[PATCH v3 hmm 05/11] hmm: use mmu_notifier_get/put for 'struct hmm'

From: Jason Gunthorpe
Date: Tue Aug 06 2019 - 19:17:03 EST

From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

This is a significant simplification, it eliminates all the remaining
'hmm' stuff in mm_struct, eliminates krefing along the critical notifier
paths, and takes away all the ugly locking and abuse of page_table_lock.

mmu_notifier_get() provides the single struct hmm per struct mm which
eliminates mm->hmm.

It also directly guarantees that no mmu_notifier op callback is callable
while concurrent free is possible, this eliminates all the krefs inside
the mmu_notifier callbacks.

The remaining krefs in the range code were overly cautious, drivers are
already not permitted to free the mirror while a range exists.

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
---
drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c | 1 +
drivers/gpu/drm/nouveau/nouveau_drm.c | 3 +
include/linux/hmm.h | 12 +--
include/linux/mm_types.h | 6 --
kernel/fork.c | 1 -
mm/hmm.c | 121 ++++++------------------
6 files changed, 33 insertions(+), 111 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
index f2e8b4238efd49..d50774a5f98ef7 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
@@ -1464,6 +1464,7 @@ static void __exit amdgpu_exit(void)
amdgpu_unregister_atpx_handler();
amdgpu_sync_fini();
amdgpu_fence_slab_fini();
+ mmu_notifier_synchronize();
}

module_init(amdgpu_init);
diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c b/drivers/gpu/drm/nouveau/nouveau_drm.c
index 7c2fcaba42d6c3..a0e48a482452d7 100644
--- a/drivers/gpu/drm/nouveau/nouveau_drm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
@@ -28,6 +28,7 @@
#include <linux/pci.h>
#include <linux/pm_runtime.h>
#include <linux/vga_switcheroo.h>
+#include <linux/mmu_notifier.h>

#include <drm/drmP.h>
#include <drm/drm_crtc_helper.h>
@@ -1292,6 +1293,8 @@ nouveau_drm_exit(void)
#ifdef CONFIG_NOUVEAU_PLATFORM_DRIVER
platform_driver_unregister(&nouveau_platform_driver);
#endif
+ if (IS_ENABLED(CONFIG_DRM_NOUVEAU_SVM))
+ mmu_notifier_synchronize();
}

module_init(nouveau_drm_init);
diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 82265118d94abd..c3902449db6412 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -84,15 +84,12 @@
* @notifiers: count of active mmu notifiers
*/
struct hmm {
- struct mm_struct *mm;
- struct kref kref;
+ struct mmu_notifier mmu_notifier;
spinlock_t ranges_lock;
struct list_head ranges;
struct list_head mirrors;
- struct mmu_notifier mmu_notifier;
struct rw_semaphore mirrors_sem;
wait_queue_head_t wq;
- struct rcu_head rcu;
long notifiers;
};

@@ -436,13 +433,6 @@ long hmm_range_dma_unmap(struct hmm_range *range,
*/
#define HMM_RANGE_DEFAULT_TIMEOUT 1000

-/* Below are for HMM internal use only! Not to be used by device driver! */
-static inline void hmm_mm_init(struct mm_struct *mm)
-{
- mm->hmm = NULL;
-}
-#else /* IS_ENABLED(CONFIG_HMM_MIRROR) */
-static inline void hmm_mm_init(struct mm_struct *mm) {}
#endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */

#endif /* LINUX_HMM_H */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3a37a89eb7a7c3..525d25d93330f2 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -25,7 +25,6 @@

struct address_space;
struct mem_cgroup;
-struct hmm;

/*
* Each physical page in the system has a struct page associated with
@@ -502,11 +501,6 @@ struct mm_struct {
atomic_long_t hugetlb_usage;
#endif
struct work_struct async_put_work;
-
-#ifdef CONFIG_HMM_MIRROR
- /* HMM needs to track a few things per mm */
- struct hmm *hmm;
-#endif
} __randomize_layout;

/*
diff --git a/kernel/fork.c b/kernel/fork.c
index d8ae0f1b414802..bd4a0762f12f3e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1007,7 +1007,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mm_init_owner(mm, p);
RCU_INIT_POINTER(mm->exe_file, NULL);
mmu_notifier_mm_init(mm);
- hmm_mm_init(mm);
init_tlb_flush_pending(mm);
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
mm->pmd_huge_pte = NULL;
diff --git a/mm/hmm.c b/mm/hmm.c
index 9a908902e4cc38..00f94f94906afc 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -26,101 +26,37 @@
#include <linux/mmu_notifier.h>
#include <linux/memory_hotplug.h>

-static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
-
-/**
- * hmm_get_or_create - register HMM against an mm (HMM internal)
- *
- * @mm: mm struct to attach to
- * Return: an HMM object, either by referencing the existing
- * (per-process) object, or by creating a new one.
- *
- * This is not intended to be used directly by device drivers. If mm already
- * has an HMM struct then it get a reference on it and returns it. Otherwise
- * it allocates an HMM struct, initializes it, associate it with the mm and
- * returns it.
- */
-static struct hmm *hmm_get_or_create(struct mm_struct *mm)
+static struct mmu_notifier *hmm_alloc_notifier(struct mm_struct *mm)
{
struct hmm *hmm;

- lockdep_assert_held_write(&mm->mmap_sem);
-
- /* Abuse the page_table_lock to also protect mm->hmm. */
- spin_lock(&mm->page_table_lock);
- hmm = mm->hmm;
- if (mm->hmm && kref_get_unless_zero(&mm->hmm->kref))
- goto out_unlock;
- spin_unlock(&mm->page_table_lock);
-
- hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
+ hmm = kzalloc(sizeof(*hmm), GFP_KERNEL);
if (!hmm)
- return NULL;
+ return ERR_PTR(-ENOMEM);
+
init_waitqueue_head(&hmm->wq);
INIT_LIST_HEAD(&hmm->mirrors);
init_rwsem(&hmm->mirrors_sem);
- hmm->mmu_notifier.ops = NULL;
INIT_LIST_HEAD(&hmm->ranges);
spin_lock_init(&hmm->ranges_lock);
- kref_init(&hmm->kref);
hmm->notifiers = 0;
- hmm->mm = mm;
-
- hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
- if (__mmu_notifier_register(&hmm->mmu_notifier, mm)) {
- kfree(hmm);
- return NULL;
- }
-
- mmgrab(hmm->mm);
-
- /*
- * We hold the exclusive mmap_sem here so we know that mm->hmm is
- * still NULL or 0 kref, and is safe to update.
- */
- spin_lock(&mm->page_table_lock);
- mm->hmm = hmm;
-
-out_unlock:
- spin_unlock(&mm->page_table_lock);
- return hmm;
+ return &hmm->mmu_notifier;
}

-static void hmm_free_rcu(struct rcu_head *rcu)
+static void hmm_free_notifier(struct mmu_notifier *mn)
{
- struct hmm *hmm = container_of(rcu, struct hmm, rcu);
+ struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);

- mmdrop(hmm->mm);
+ WARN_ON(!list_empty(&hmm->ranges));
+ WARN_ON(!list_empty(&hmm->mirrors));
kfree(hmm);
}

-static void hmm_free(struct kref *kref)
-{
- struct hmm *hmm = container_of(kref, struct hmm, kref);
-
- spin_lock(&hmm->mm->page_table_lock);
- if (hmm->mm->hmm == hmm)
- hmm->mm->hmm = NULL;
- spin_unlock(&hmm->mm->page_table_lock);
-
- mmu_notifier_unregister_no_release(&hmm->mmu_notifier, hmm->mm);
- mmu_notifier_call_srcu(&hmm->rcu, hmm_free_rcu);
-}
-
-static inline void hmm_put(struct hmm *hmm)
-{
- kref_put(&hmm->kref, hmm_free);
-}
-
static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
{
struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
struct hmm_mirror *mirror;

- /* Bail out if hmm is in the process of being freed */
- if (!kref_get_unless_zero(&hmm->kref))
- return;
-
/*
* Since hmm_range_register() holds the mmget() lock hmm_release() is
* prevented as long as a range exists.
@@ -137,8 +73,6 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
mirror->ops->release(mirror);
}
up_read(&hmm->mirrors_sem);
-
- hmm_put(hmm);
}

static void notifiers_decrement(struct hmm *hmm)
@@ -169,9 +103,6 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn,
unsigned long flags;
int ret = 0;

- if (!kref_get_unless_zero(&hmm->kref))
- return 0;
-
spin_lock_irqsave(&hmm->ranges_lock, flags);
hmm->notifiers++;
list_for_each_entry(range, &hmm->ranges, list) {
@@ -206,7 +137,6 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn,
out:
if (ret)
notifiers_decrement(hmm);
- hmm_put(hmm);
return ret;
}

@@ -215,17 +145,15 @@ static void hmm_invalidate_range_end(struct mmu_notifier *mn,
{
struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);

- if (!kref_get_unless_zero(&hmm->kref))
- return;
-
notifiers_decrement(hmm);
- hmm_put(hmm);
}

static const struct mmu_notifier_ops hmm_mmu_notifier_ops = {
.release = hmm_release,
.invalidate_range_start = hmm_invalidate_range_start,
.invalidate_range_end = hmm_invalidate_range_end,
+ .alloc_notifier = hmm_alloc_notifier,
+ .free_notifier = hmm_free_notifier,
};

/*
@@ -237,18 +165,27 @@ static const struct mmu_notifier_ops hmm_mmu_notifier_ops = {
*
* To start mirroring a process address space, the device driver must register
* an HMM mirror struct.
+ *
+ * The caller cannot unregister the hmm_mirror while any ranges are
+ * registered.
+ *
+ * Callers using this function must put a call to mmu_notifier_synchronize()
+ * in their module exit functions.
*/
int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
{
+ struct mmu_notifier *mn;
+
lockdep_assert_held_write(&mm->mmap_sem);

/* Sanity check */
if (!mm || !mirror || !mirror->ops)
return -EINVAL;

- mirror->hmm = hmm_get_or_create(mm);
- if (!mirror->hmm)
- return -ENOMEM;
+ mn = mmu_notifier_get_locked(&hmm_mmu_notifier_ops, mm);
+ if (IS_ERR(mn))
+ return PTR_ERR(mn);
+ mirror->hmm = container_of(mn, struct hmm, mmu_notifier);

down_write(&mirror->hmm->mirrors_sem);
list_add(&mirror->list, &mirror->hmm->mirrors);
@@ -272,7 +209,7 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
down_write(&hmm->mirrors_sem);
list_del(&mirror->list);
up_write(&hmm->mirrors_sem);
- hmm_put(hmm);
+ mmu_notifier_put(&hmm->mmu_notifier);
}
EXPORT_SYMBOL(hmm_mirror_unregister);

@@ -880,14 +817,13 @@ int hmm_range_register(struct hmm_range *range,
range->end = end;

/* Prevent hmm_release() from running while the range is valid */
- if (!mmget_not_zero(hmm->mm))
+ if (!mmget_not_zero(hmm->mmu_notifier.mm))
return -EFAULT;

/* Initialize range to track CPU page table updates. */
spin_lock_irqsave(&hmm->ranges_lock, flags);

range->hmm = hmm;
- kref_get(&hmm->kref);
list_add(&range->list, &hmm->ranges);

/*
@@ -919,8 +855,7 @@ void hmm_range_unregister(struct hmm_range *range)
spin_unlock_irqrestore(&hmm->ranges_lock, flags);

/* Drop reference taken by hmm_range_register() */
- mmput(hmm->mm);
- hmm_put(hmm);
+ mmput(hmm->mmu_notifier.mm);

/*
* The range is now invalid and the ref on the hmm is dropped, so
@@ -970,14 +905,14 @@ long hmm_range_fault(struct hmm_range *range, unsigned int flags)
struct mm_walk mm_walk;
int ret;

- lockdep_assert_held(&hmm->mm->mmap_sem);
+ lockdep_assert_held(&hmm->mmu_notifier.mm->mmap_sem);

do {
/* If range is no longer valid force retry. */
if (!range->valid)
return -EBUSY;

- vma = find_vma(hmm->mm, start);
+ vma = find_vma(hmm->mmu_notifier.mm, start);
if (vma == NULL || (vma->vm_flags & device_vma))
return -EFAULT;

--
2.22.0