[RFC PATCH 02/11] mm/hmm: Use hmm_mirror not mm as an argument for hmm_register_range

[Jason Gunthorpe <jgg@xxxxxxxx>]

From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

Ralph observes that hmm_register_range() can only be called by a driver
while a mirror is registered. Make this clear in the API by passing in the
mirror structure as a parameter.

This also simplifies understanding the lifetime model for struct hmm, as
the hmm pointer must be valid as part of a registered mirror so all we
need in hmm_register_range() is a simple kref_get.

Suggested-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
---
 include/linux/hmm.h |  7 ++++---
 mm/hmm.c            | 14 +++++---------
 2 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 8b91c90d3b88cb..87d29e085a69f7 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -503,7 +503,7 @@ static inline bool hmm_mirror_mm_is_alive(struct hmm_mirror *mirror)
  * Please see Documentation/vm/hmm.rst for how to use the range API.
  */
 int hmm_range_register(struct hmm_range *range,
-		       struct mm_struct *mm,
+		       struct hmm_mirror *mirror,
 		       unsigned long start,
 		       unsigned long end,
 		       unsigned page_shift);
@@ -539,7 +539,8 @@ static inline bool hmm_vma_range_done(struct hmm_range *range)
 }
 
 /* This is a temporary helper to avoid merge conflict between trees. */
-static inline int hmm_vma_fault(struct hmm_range *range, bool block)
+static inline int hmm_vma_fault(struct hmm_mirror *mirror,
+				struct hmm_range *range, bool block)
 {
 	long ret;
 
@@ -552,7 +553,7 @@ static inline int hmm_vma_fault(struct hmm_range *range, bool block)
 	range->default_flags = 0;
 	range->pfn_flags_mask = -1UL;
 
-	ret = hmm_range_register(range, range->vma->vm_mm,
+	ret = hmm_range_register(range, mirror,
 				 range->start, range->end,
 				 PAGE_SHIFT);
 	if (ret)
diff --git a/mm/hmm.c b/mm/hmm.c
index 824e7e160d8167..fa1b04fcfc2549 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -927,7 +927,7 @@ static void hmm_pfns_clear(struct hmm_range *range,
  * Track updates to the CPU page table see include/linux/hmm.h
  */
 int hmm_range_register(struct hmm_range *range,
-		       struct mm_struct *mm,
+		       struct hmm_mirror *mirror,
 		       unsigned long start,
 		       unsigned long end,
 		       unsigned page_shift)
@@ -935,7 +935,6 @@ int hmm_range_register(struct hmm_range *range,
 	unsigned long mask = ((1UL << page_shift) - 1UL);
 
 	range->valid = false;
-	range->hmm = NULL;
 
 	if ((start & mask) || (end & mask))
 		return -EINVAL;
@@ -946,15 +945,12 @@ int hmm_range_register(struct hmm_range *range,
 	range->start = start;
 	range->end = end;
 
-	range->hmm = hmm_get_or_create(mm);
-	if (!range->hmm)
-		return -EFAULT;
-
 	/* Check if hmm_mm_destroy() was call. */
-	if (range->hmm->mm == NULL || range->hmm->dead) {
-		hmm_put(range->hmm);
+	if (mirror->hmm->mm == NULL || mirror->hmm->dead)
 		return -EFAULT;
-	}
+
+	range->hmm = mirror->hmm;
+	kref_get(&range->hmm->kref);
 
 	/* Initialize range to track CPU page table update */
 	mutex_lock(&range->hmm->lock);
-- 
2.21.0