[PATCH 03/15] mm/hmm: HMM should have a callback before MM is destroyed v3

[jglisse@xxxxxxxxxx]

From: Ralph Campbell <rcampbell@xxxxxxxxxx>

The hmm_mirror_register() function registers a callback for when
the CPU pagetable is modified. Normally, the device driver will
call hmm_mirror_unregister() when the process using the device is
finished. However, if the process exits uncleanly, the struct_mm
can be destroyed with no warning to the device driver.

Changed since v1:
  - dropped VM_BUG_ON()
  - cc stable
Changed since v2:
  - drop stable
  - Split list removale and call to driver release callback. This
    allow the release callback to wait on any pending fault handler
    without deadlock.

Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
Cc: Evgeny Baskakov <ebaskakov@xxxxxxxxxx>
Cc: Mark Hairgrove <mhairgrove@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
---
 include/linux/hmm.h | 10 ++++++++++
 mm/hmm.c            | 29 ++++++++++++++++++++++++++++-
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 36dd21fe5caf..fa7b51f65905 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -218,6 +218,16 @@ enum hmm_update_type {
  * @update: callback to update range on a device
  */
 struct hmm_mirror_ops {
+	/* release() - release hmm_mirror
+	 *
+	 * @mirror: pointer to struct hmm_mirror
+	 *
+	 * This is called when the mm_struct is being released.
+	 * The callback should make sure no references to the mirror occur
+	 * after the callback returns.
+	 */
+	void (*release)(struct hmm_mirror *mirror);
+
 	/* sync_cpu_device_pagetables() - synchronize page tables
 	 *
 	 * @mirror: pointer to struct hmm_mirror
diff --git a/mm/hmm.c b/mm/hmm.c
index 320545b98ff5..34c16297f65e 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -160,6 +160,32 @@ static void hmm_invalidate_range(struct hmm *hmm,
 	up_read(&hmm->mirrors_sem);
 }
 
+static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
+{
+	struct hmm_mirror *mirror;
+	struct hmm *hmm = mm->hmm;
+
+	down_write(&hmm->mirrors_sem);
+	mirror = list_first_entry_or_null(&hmm->mirrors, struct hmm_mirror,
+					  list);
+	while (mirror) {
+		list_del_init(&mirror->list);
+		if (mirror->ops->release) {
+			/*
+			 * Drop mirrors_sem so callback can wait on any pending
+			 * work that might itself trigger mmu_notifier callback
+			 * and thus would deadlock with us.
+			 */
+			up_write(&hmm->mirrors_sem);
+			mirror->ops->release(mirror);
+			down_write(&hmm->mirrors_sem);
+		}
+		mirror = list_first_entry_or_null(&hmm->mirrors, struct hmm_mirror,
+						  list);
+	}
+	up_write(&hmm->mirrors_sem);
+}
+
 static void hmm_invalidate_range_start(struct mmu_notifier *mn,
 				       struct mm_struct *mm,
 				       unsigned long start,
@@ -185,6 +211,7 @@ static void hmm_invalidate_range_end(struct mmu_notifier *mn,
 }
 
 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,
 };
@@ -230,7 +257,7 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
 	struct hmm *hmm = mirror->hmm;
 
 	down_write(&hmm->mirrors_sem);
-	list_del(&mirror->list);
+	list_del_init(&mirror->list);
 	up_write(&hmm->mirrors_sem);
 }
 EXPORT_SYMBOL(hmm_mirror_unregister);
-- 
2.14.3