[PATCH v5 01/25] mm/mmu_notifiers: pass private data down to alloc_notifier()

[Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>]

The new allocation scheme introduced by commit 2c7933f53f6b
("mm/mmu_notifiers: add a get/put scheme for the registration") provides
a convenient way for users to attach notifier data to an mm. However, it
would be even better to create this notifier data atomically.

Since the alloc_notifier() callback only takes an mm argument at the
moment, some users have to perform the allocation in two times.
alloc_notifier() initially creates an incomplete structure, which is
then finalized using more context once mmu_notifier_get() returns. This
second step requires extra care to order memory accesses against live
invalidation.

The IOMMU SVA module, which attaches an mm to multiple devices,
exemplifies this situation. In essence it does:

	mmu_notifier_get()
	  alloc_notifier()
	     A = kzalloc()
	  /* MMU notifier is published */
	A->ctx = ctx;				// (1)
	device->A = A;
	list_add_rcu(device, A->devices);	// (2)

The invalidate notifier, which may start running before A is fully
initialized, does the following:

	io_mm_invalidate(A)
	  list_for_each_entry_rcu(device, A->devices)
	    device->invalidate(A->ctx)

The invalidate() thread must observe the initialization (1) before (2),
which is easily solved by fully initializing object A in
alloc_notifier(), before publishing the MMU notifier.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Dimitri Sivanich <sivanich@xxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
---
v4->v5: provide example in commit message, fix style.
---
 include/linux/mmu_notifier.h       | 11 +++++++----
 drivers/misc/sgi-gru/grutlbpurge.c |  5 +++--
 mm/mmu_notifier.c                  |  6 ++++--
 3 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index 736f6918335ed..0536fe85e7457 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -207,7 +207,8 @@ struct mmu_notifier_ops {
 	 * callbacks are currently running. It is called from a SRCU callback
 	 * and cannot sleep.
 	 */
-	struct mmu_notifier *(*alloc_notifier)(struct mm_struct *mm);
+	struct mmu_notifier *(*alloc_notifier)(struct mm_struct *mm,
+					       void *privdata);
 	void (*free_notifier)(struct mmu_notifier *subscription);
 };
 
@@ -271,14 +272,16 @@ static inline int mm_has_notifiers(struct mm_struct *mm)
 }
 
 struct mmu_notifier *mmu_notifier_get_locked(const struct mmu_notifier_ops *ops,
-					     struct mm_struct *mm);
+					     struct mm_struct *mm,
+					     void *privdata);
 static inline struct mmu_notifier *
-mmu_notifier_get(const struct mmu_notifier_ops *ops, struct mm_struct *mm)
+mmu_notifier_get(const struct mmu_notifier_ops *ops, struct mm_struct *mm,
+		 void *privdata)
 {
 	struct mmu_notifier *ret;
 
 	down_write(&mm->mmap_sem);
-	ret = mmu_notifier_get_locked(ops, mm);
+	ret = mmu_notifier_get_locked(ops, mm, privdata);
 	up_write(&mm->mmap_sem);
 	return ret;
 }
diff --git a/drivers/misc/sgi-gru/grutlbpurge.c b/drivers/misc/sgi-gru/grutlbpurge.c
index 10921cd2608df..336e1b1df072f 100644
--- a/drivers/misc/sgi-gru/grutlbpurge.c
+++ b/drivers/misc/sgi-gru/grutlbpurge.c
@@ -235,7 +235,8 @@ static void gru_invalidate_range_end(struct mmu_notifier *mn,
 		gms, range->start, range->end);
 }
 
-static struct mmu_notifier *gru_alloc_notifier(struct mm_struct *mm)
+static struct mmu_notifier *gru_alloc_notifier(struct mm_struct *mm,
+					       void *privdata)
 {
 	struct gru_mm_struct *gms;
 
@@ -266,7 +267,7 @@ struct gru_mm_struct *gru_register_mmu_notifier(void)
 {
 	struct mmu_notifier *mn;
 
-	mn = mmu_notifier_get_locked(&gru_mmuops, current->mm);
+	mn = mmu_notifier_get_locked(&gru_mmuops, current->mm, NULL);
 	if (IS_ERR(mn))
 		return ERR_CAST(mn);
 
diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 06852b896fa63..6b9bfb8ca94d2 100644
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -743,6 +743,7 @@ find_get_mmu_notifier(struct mm_struct *mm, const struct mmu_notifier_ops *ops)
  *                           the mm & ops
  * @ops: The operations struct being subscribe with
  * @mm : The mm to attach notifiers too
+ * @privdata: Initialization data passed down to ops->alloc_notifier()
  *
  * This function either allocates a new mmu_notifier via
  * ops->alloc_notifier(), or returns an already existing notifier on the
@@ -756,7 +757,8 @@ find_get_mmu_notifier(struct mm_struct *mm, const struct mmu_notifier_ops *ops)
  * and can be converted to an active mm pointer via mmget_not_zero().
  */
 struct mmu_notifier *mmu_notifier_get_locked(const struct mmu_notifier_ops *ops,
-					     struct mm_struct *mm)
+					     struct mm_struct *mm,
+					     void *privdata)
 {
 	struct mmu_notifier *subscription;
 	int ret;
@@ -769,7 +771,7 @@ struct mmu_notifier *mmu_notifier_get_locked(const struct mmu_notifier_ops *ops,
 			return subscription;
 	}
 
-	subscription = ops->alloc_notifier(mm);
+	subscription = ops->alloc_notifier(mm, privdata);
 	if (IS_ERR(subscription))
 		return subscription;
 	subscription->ops = ops;
-- 
2.26.0