[RFC PATCH 2/5] mm/mmap: Split do_vmi_align_munmap() into a gather and complete operation

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

Split the munmap function into a gathering of vmas and a cleanup of the
gathered vmas.  This is necessary for the later patches in the series.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
---
 mm/mmap.c | 143 ++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 101 insertions(+), 42 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 31d464e6a656..fad40d604c64 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2340,6 +2340,7 @@ static inline void remove_mt(struct mm_struct *mm, struct ma_state *mas)
 
 		if (vma->vm_flags & VM_ACCOUNT)
 			nr_accounted += nrpages;
+
 		vm_stat_account(mm, vma->vm_flags, -nrpages);
 		remove_vma(vma, false);
 	}
@@ -2545,33 +2546,45 @@ struct vm_area_struct *vma_merge_extend(struct vma_iterator *vmi,
 			 vma->vm_userfaultfd_ctx, anon_vma_name(vma));
 }
 
+
+static inline void abort_munmap_vmas(struct ma_state *mas_detach)
+{
+	struct vm_area_struct *vma;
+	int limit;
+
+	limit = mas_detach->index;
+	mas_set(mas_detach, 0);
+	/* Re-attach any detached VMAs */
+	mas_for_each(mas_detach, vma, limit)
+		vma_mark_detached(vma, false);
+
+	__mt_destroy(mas_detach->tree);
+}
+
 /*
- * do_vmi_align_munmap() - munmap the aligned region from @start to @end.
+ * vmi_gather_munmap_vmas() - Put all VMAs within a range into a maple tree
+ * for removal at a later date.  Handles splitting first and last if necessary
+ * and marking the vmas as isolated.
+ *
  * @vmi: The vma iterator
  * @vma: The starting vm_area_struct
  * @mm: The mm_struct
  * @start: The aligned start address to munmap.
  * @end: The aligned end address to munmap.
  * @uf: The userfaultfd list_head
- * @unlock: Set to true to drop the mmap_lock.  unlocking only happens on
- * success.
+ * @mas_detach: The maple state tracking the detached tree
  *
- * Return: 0 on success and drops the lock if so directed, error and leaves the
- * lock held otherwise.
+ * Return: 0 on success
  */
 static int
-do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
+vmi_gather_munmap_vmas(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		    struct mm_struct *mm, unsigned long start,
-		    unsigned long end, struct list_head *uf, bool unlock)
+		    unsigned long end, struct list_head *uf,
+		    struct ma_state *mas_detach, unsigned long *locked_vm)
 {
-	struct vm_area_struct *prev, *next = NULL;
-	struct maple_tree mt_detach;
-	int count = 0;
+	struct vm_area_struct *next = NULL;
 	int error = -ENOMEM;
-	unsigned long locked_vm = 0;
-	MA_STATE(mas_detach, &mt_detach, 0, 0);
-	mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
-	mt_on_stack(mt_detach);
+	int count = 0;
 
 	/*
 	 * If we need to split any vma, do it now to save pain later.
@@ -2610,15 +2623,14 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 				goto end_split_failed;
 		}
 		vma_start_write(next);
-		mas_set(&mas_detach, count);
-		error = mas_store_gfp(&mas_detach, next, GFP_KERNEL);
+		mas_set(mas_detach, count++);
+		if (next->vm_flags & VM_LOCKED)
+			*locked_vm += vma_pages(next);
+
+		error = mas_store_gfp(mas_detach, next, GFP_KERNEL);
 		if (error)
 			goto munmap_gather_failed;
 		vma_mark_detached(next, true);
-		if (next->vm_flags & VM_LOCKED)
-			locked_vm += vma_pages(next);
-
-		count++;
 		if (unlikely(uf)) {
 			/*
 			 * If userfaultfd_unmap_prep returns an error the vmas
@@ -2643,7 +2655,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 #if defined(CONFIG_DEBUG_VM_MAPLE_TREE)
 	/* Make sure no VMAs are about to be lost. */
 	{
-		MA_STATE(test, &mt_detach, 0, 0);
+		MA_STATE(test, mas_detach->tree, 0, 0);
 		struct vm_area_struct *vma_mas, *vma_test;
 		int test_count = 0;
 
@@ -2663,13 +2675,29 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	while (vma_iter_addr(vmi) > start)
 		vma_iter_prev_range(vmi);
 
-	error = vma_iter_clear_gfp(vmi, start, end, GFP_KERNEL);
-	if (error)
-		goto clear_tree_failed;
+	return 0;
 
-	/* Point of no return */
-	mm->locked_vm -= locked_vm;
+userfaultfd_error:
+munmap_gather_failed:
+end_split_failed:
+	abort_munmap_vmas(mas_detach);
+start_split_failed:
+map_count_exceeded:
+	return error;
+}
+
+static void
+vmi_complete_munmap_vmas(struct vma_iterator *vmi, struct vm_area_struct *vma,
+		struct mm_struct *mm, unsigned long start,
+		unsigned long end, bool unlock, struct ma_state *mas_detach,
+		unsigned long locked_vm)
+{
+	struct vm_area_struct *prev, *next;
+	int count;
+
+	count = mas_detach->index + 1;
 	mm->map_count -= count;
+	mm->locked_vm -= locked_vm;
 	if (unlock)
 		mmap_write_downgrade(mm);
 
@@ -2682,30 +2710,61 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	 * We can free page tables without write-locking mmap_lock because VMAs
 	 * were isolated before we downgraded mmap_lock.
 	 */
-	mas_set(&mas_detach, 1);
-	unmap_region(mm, &mas_detach, vma, prev, next, start, end, count,
+	mas_set(mas_detach, 1);
+	unmap_region(mm, mas_detach, vma, prev, next, start, end, count,
 		     !unlock);
 	/* Statistics and freeing VMAs */
-	mas_set(&mas_detach, 0);
-	remove_mt(mm, &mas_detach);
+	mas_set(mas_detach, 0);
+	remove_mt(mm, mas_detach);
 	validate_mm(mm);
 	if (unlock)
 		mmap_read_unlock(mm);
 
-	__mt_destroy(&mt_detach);
-	return 0;
+	__mt_destroy(mas_detach->tree);
+}
 
-clear_tree_failed:
-userfaultfd_error:
-munmap_gather_failed:
-end_split_failed:
-	mas_set(&mas_detach, 0);
-	mas_for_each(&mas_detach, next, end)
-		vma_mark_detached(next, false);
+/*
+ * do_vmi_align_munmap() - munmap the aligned region from @start to @end.
+ * @vmi: The vma iterator
+ * @vma: The starting vm_area_struct
+ * @mm: The mm_struct
+ * @start: The aligned start address to munmap.
+ * @end: The aligned end address to munmap.
+ * @uf: The userfaultfd list_head
+ * @unlock: Set to true to drop the mmap_lock.  unlocking only happens on
+ * success.
+ *
+ * Return: 0 on success and drops the lock if so directed, error and leaves the
+ * lock held otherwise.
+ */
+static int
+do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
+		    struct mm_struct *mm, unsigned long start,
+		    unsigned long end, struct list_head *uf, bool unlock)
+{
+	struct maple_tree mt_detach;
+	MA_STATE(mas_detach, &mt_detach, 0, 0);
+	mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
+	mt_on_stack(mt_detach);
+	int error;
+	unsigned long locked_vm = 0;
 
-	__mt_destroy(&mt_detach);
-start_split_failed:
-map_count_exceeded:
+	error = vmi_gather_munmap_vmas(vmi, vma, mm, start, end, uf,
+				       &mas_detach, &locked_vm);
+	if (error)
+		goto gather_failed;
+
+	error = vma_iter_clear_gfp(vmi, start, end, GFP_KERNEL);
+	if (error)
+		goto clear_area_failed;
+
+	vmi_complete_munmap_vmas(vmi, vma, mm, start, end, unlock, &mas_detach,
+				 locked_vm);
+	return 0;
+
+clear_area_failed:
+	abort_munmap_vmas(&mas_detach);
+gather_failed:
 	validate_mm(mm);
 	return error;
 }
-- 
2.43.0