[PATCH 3/5] mm: eliminate adj_start parameter from commit_merge()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Introduce internal __VMG_FLAG_ADJUST_MIDDLE_START and
__VMG_FLAG_ADJUST_NEXT_START merge flags, enabling us to indicate to
commit_merge() that we are performing a merge which either spans only part
of vmg->middle, or part of vmg->next respectively.

In the former instance, we change the start of vmg->middle to match the
attributes of vmg->prev, without spanning all of vmg->middle.

This implies that vmg->prev->vm_end and vmg->middle->vm_start are both
increased to form the new merged VMA (vmg->prev) and the new subsequent VMA
(vmg->middle).

In the latter case, we change the end of vmg->middle to match the
attributes of vmg->next, without spanning all of vmg->next.

This implies that vmg->middle->vm_end and vmg->next->vm_start are both
decreased to form the new merged VMA (vmg->next) and the new prior VMA
(vmg->middle).

Since we now have a stable set of prev, middle, next VMAs threaded through
vmg and with these flags set know what is happening, we can perform
the calculation in commit_merge() instead.

This allows us to drop the confusing adj_start parameter and instead pass
semantic information to commit_merge().

In the latter case the -(middle->vm_end - start) calculation becomes
-(middle->vm-end - vmg->end), however this is correct as vmg->end is set to
the start parameter.

This is because in this case (rather confusingly), we manipulate
vmg->middle, but ultimately return vmg->next, whose range will be correctly
specified. At this point vmg->start, end is the new range for the prior VMA
rather than the merged one.

This patch has no change in functional behaviour.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/vma.c | 53 ++++++++++++++++++++++++++++++++---------------------
 mm/vma.h | 14 ++++++++++++--
 2 files changed, 44 insertions(+), 23 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index 955c5ebd5739..e78d65de734b 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -633,29 +633,45 @@ void validate_mm(struct mm_struct *mm)
  *
  * On success, returns the merged VMA. Otherwise returns NULL.
  */
-static struct vm_area_struct *commit_merge(struct vma_merge_struct *vmg,
-		long adj_start)
+static struct vm_area_struct *commit_merge(struct vma_merge_struct *vmg)
 {
-	struct vma_prepare vp;
 	struct vm_area_struct *remove = NULL;
 	struct vm_area_struct *remove2 = NULL;
+	unsigned long flags = vmg->merge_flags;
+	struct vma_prepare vp;
 	struct vm_area_struct *adjust = NULL;
+	long adj_start;
+	bool merge_target;
+
 	/*
-	 * In all cases but that of merge right, shrink next, we write
-	 * vmg->target to the maple tree and return this as the merged VMA.
+	 * If modifying an existing VMA and we don't remove vmg->middle, then we
+	 * shrink the adjacent VMA.
 	 */
-	bool merge_target = adj_start >= 0;
+	if (flags & __VMG_FLAG_ADJUST_MIDDLE_START) {
+		adjust = vmg->middle;
+		/* The POSITIVE value by which we offset vmg->middle->vm_start. */
+		adj_start = vmg->end - vmg->middle->vm_start;
+		merge_target = true;
+	} else if (flags & __VMG_FLAG_ADJUST_NEXT_START) {
+		adjust = vmg->next;
+		/* The NEGATIVE value by which we offset vmg->next->vm_start. */
+		adj_start = -(vmg->middle->vm_end - vmg->end);
+		/*
+		 * In all cases but this - merge right, shrink next - we write
+		 * vmg->target to the maple tree and return this as the merged VMA.
+		 */
+		merge_target = false;
+	} else {
+		adjust = NULL;
+		adj_start = 0;
+		merge_target = true;
+	}
 
-	if (vmg->merge_flags & __VMG_FLAG_REMOVE_MIDDLE)
+	if (flags & __VMG_FLAG_REMOVE_MIDDLE)
 		remove = vmg->middle;
 	if (vmg->merge_flags & __VMG_FLAG_REMOVE_NEXT)
 		remove2 = vmg->next;
 
-	if (adj_start > 0)
-		adjust = vmg->middle;
-	else if (adj_start < 0)
-		adjust = vmg->next;
-
 	init_multi_vma_prep(&vp, vmg->target, adjust, remove, remove2);
 
 	VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
@@ -739,7 +755,6 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
 	bool left_side = middle && start == middle->vm_start;
 	bool right_side = middle && end == middle->vm_end;
 	int err = 0;
-	long adj_start = 0;
 	bool merge_will_delete_middle, merge_will_delete_next;
 	bool merge_left, merge_right, merge_both;
 
@@ -860,11 +875,8 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
 		vmg->start = prev->vm_start;
 		vmg->pgoff = prev->vm_pgoff;
 
-		/*
-		 * We both expand prev and shrink middle.
-		 */
 		if (!merge_will_delete_middle)
-			adj_start = vmg->end - middle->vm_start;
+			vmg->merge_flags |= __VMG_FLAG_ADJUST_MIDDLE_START;
 
 		err = dup_anon_vma(prev, middle, &anon_dup);
 	} else { /* merge_right */
@@ -893,12 +905,11 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
 			 * IMPORTANT: This is the ONLY case where the final
 			 * merged VMA is NOT vmg->target, but rather vmg->next.
 			 */
+			vmg->merge_flags |= __VMG_FLAG_ADJUST_NEXT_START;
 			vmg->target = middle;
 			vmg->start = middle->vm_start;
 			vmg->end = start;
 			vmg->pgoff = middle->vm_pgoff;
-
-			adj_start = -(middle->vm_end - start);
 		}
 
 		err = dup_anon_vma(next, middle, &anon_dup);
@@ -912,7 +923,7 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
 	if (merge_will_delete_next)
 		vmg->merge_flags |= __VMG_FLAG_REMOVE_NEXT;
 
-	res = commit_merge(vmg, adj_start);
+	res = commit_merge(vmg);
 	if (!res) {
 		if (anon_dup)
 			unlink_anon_vmas(anon_dup);
@@ -1087,7 +1098,7 @@ int vma_expand(struct vma_merge_struct *vmg)
 	if (remove_next)
 		vmg->merge_flags |= __VMG_FLAG_REMOVE_NEXT;
 
-	if (!commit_merge(vmg, 0))
+	if (!commit_merge(vmg))
 		goto nomem;
 
 	return 0;
diff --git a/mm/vma.h b/mm/vma.h
index ffbfefb9a83d..ddf567359880 100644
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -67,16 +67,26 @@ enum vma_merge_flags {
 	 * at the gap.
 	 */
 	VMG_FLAG_JUST_EXPAND = 1 << 0,
+	/*
+	 * Internal flag indicating the merge increases vmg->middle->vm_start
+	 * (and thereby, vmg->prev->vm_end).
+	 */
+	__VMG_FLAG_ADJUST_MIDDLE_START = 1 << 1,
+	/*
+	 * Internal flag indicating the merge decreases vmg->next->vm_start
+	 * (and thereby, vmg->middle->vm_end).
+	 */
+	__VMG_FLAG_ADJUST_NEXT_START = 1 << 2,
 	/*
 	 * Internal flag used during the merge operation to indicate we will
 	 * remove vmg->middle.
 	 */
-	__VMG_FLAG_REMOVE_MIDDLE = 1 << 1,
+	__VMG_FLAG_REMOVE_MIDDLE = 1 << 3,
 	/*
 	 * Internal flag used during the merge operationr to indicate we will
 	 * remove vmg->next.
 	 */
-	__VMG_FLAG_REMOVE_NEXT = 1 << 2,
+	__VMG_FLAG_REMOVE_NEXT = 1 << 4,
 };
 
 /*
-- 
2.48.0