[PATCH 6/6] mm: move vma locking out of vma_prepare

[Suren Baghdasaryan <surenb@google.com>]

vma_prepare() is currently the central place where vmas are being locked
before vma_complete() applies changes to them. While this is convenient,
it also obscures vma locking and makes it hard to follow the locking rules.
Move vma locking out of vma_prepare() and take vma locks explicitly at the
locations where vmas are being modified.

Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/mmap.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 850a39dee075..e59d83cb1d7a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -476,16 +476,6 @@ static inline void init_vma_prep(struct vma_prepare *vp,
  */
 static inline void vma_prepare(struct vma_prepare *vp)
 {
-	vma_start_write(vp->vma);
-	if (vp->adj_next)
-		vma_start_write(vp->adj_next);
-	if (vp->insert)
-		vma_start_write(vp->insert);
-	if (vp->remove)
-		vma_start_write(vp->remove);
-	if (vp->remove2)
-		vma_start_write(vp->remove2);
-
 	if (vp->file) {
 		uprobe_munmap(vp->vma, vp->vma->vm_start, vp->vma->vm_end);
 
@@ -650,6 +640,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	bool remove_next = false;
 	struct vma_prepare vp;
 
+	vma_start_write(vma);
 	if (next && (vma != next) && (end == next->vm_end)) {
 		int ret;
 
@@ -657,6 +648,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		ret = dup_anon_vma(vma, next);
 		if (ret)
 			return ret;
+		vma_start_write(next);
 	}
 
 	init_multi_vma_prep(&vp, vma, NULL, remove_next ? next : NULL, NULL);
@@ -708,6 +700,8 @@ int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	if (vma_iter_prealloc(vmi))
 		return -ENOMEM;
 
+	vma_start_write(vma);
+
 	init_vma_prep(&vp, vma);
 	vma_prepare(&vp);
 	vma_adjust_trans_huge(vma, start, end, 0);
@@ -946,10 +940,12 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	/* Can we merge both the predecessor and the successor? */
 	if (merge_prev && merge_next &&
 	    is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {
+		vma_start_write(next);
 		remove = next;				/* case 1 */
 		vma_end = next->vm_end;
 		err = dup_anon_vma(prev, next);
 		if (curr) {				/* case 6 */
+			vma_start_write(curr);
 			remove = curr;
 			remove2 = next;
 			if (!next->anon_vma)
@@ -958,6 +954,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	} else if (merge_prev) {			/* case 2 */
 		if (curr) {
 			err = dup_anon_vma(prev, curr);
+			vma_start_write(curr);
 			if (end == curr->vm_end) {	/* case 7 */
 				remove = curr;
 			} else {			/* case 5 */
@@ -969,6 +966,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 		res = next;
 		if (prev && addr < prev->vm_end) {	/* case 4 */
 			vma_end = addr;
+			vma_start_write(next);
 			adjust = next;
 			adj_start = -(prev->vm_end - addr);
 			err = dup_anon_vma(next, prev);
@@ -983,6 +981,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			vma_pgoff = next->vm_pgoff - pglen;
 			if (curr) {			/* case 8 */
 				vma_pgoff = curr->vm_pgoff;
+				vma_start_write(curr);
 				remove = curr;
 				err = dup_anon_vma(next, curr);
 			}
@@ -996,6 +995,8 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	if (vma_iter_prealloc(vmi))
 		return NULL;
 
+	vma_start_write(vma);
+
 	init_multi_vma_prep(&vp, vma, adjust, remove, remove2);
 	VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
 		   vp.anon_vma != adjust->anon_vma);
@@ -2373,6 +2374,9 @@ int __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	if (new->vm_ops && new->vm_ops->open)
 		new->vm_ops->open(new);
 
+	vma_start_write(vma);
+	vma_start_write(new);
+
 	init_vma_prep(&vp, vma);
 	vp.insert = new;
 	vma_prepare(&vp);
@@ -3078,6 +3082,8 @@ static int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		if (vma_iter_prealloc(vmi))
 			goto unacct_fail;
 
+		vma_start_write(vma);
+
 		init_vma_prep(&vp, vma);
 		vma_prepare(&vp);
 		vma_adjust_trans_huge(vma, vma->vm_start, addr + len, 0);
-- 
2.41.0.487.g6d72f3e995-goog