[PATCH v4 4/4] mm/mmap/vma_merge: init cleanup, be explicit about the non-mergeable case

Lorenzo Stoakes
2023-03-22T20:35:59Z

Rather than setting err = -1 and only resetting if we hit merge cases,
explicitly check the non-mergeable case to make it abundantly clear that we
only proceed with the rest if something is mergeable, default err to 0 and
only update if an error might occur.

Move the merge_prev, merge_next cases closer to the logic determining curr,
next and reorder initial variables so they are more logically grouped.

This has no functional impact.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/mmap.c | 47 ++++++++++++++++++++++-------------------------
1 file changed, 22 insertions(+), 25 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 2a4f63716231..1064e6341a7a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -909,18 +909,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
struct anon_vma_name *anon_name)
{
- pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
- pgoff_t vma_pgoff;
struct vm_area_struct *curr, *next, *res;
struct vm_area_struct *vma, *adjust, *remove, *remove2;
- int err = -1;
+ struct vma_prepare vp;
+ pgoff_t vma_pgoff;
+ int err = 0;
bool merge_prev = false;
bool merge_next = false;
bool vma_expanded = false;
- struct vma_prepare vp;
+ unsigned long vma_start = addr;
unsigned long vma_end = end;
+ pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
long adj_start = 0;
- unsigned long vma_start = addr;

validate_mm(mm);
/*
@@ -939,24 +939,14 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
else
next = NULL; /* case 5 */

- /*
- * By default, we return prev. Cases 3, 4, 8 will instead return next
- * and cases 3, 8 will also update vma to point at next.
- */
- res = vma = prev;
-
- /* Verify some invariant that must be enforced by the caller. */
- VM_WARN_ON(prev && addr <= prev->vm_start);
- VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
- VM_WARN_ON(addr >= end);
-
if (prev) {
vma_start = prev->vm_start;
vma_pgoff = prev->vm_pgoff;
+
/* Can we merge the predecessor? */
- if (prev->vm_end == addr && mpol_equal(vma_policy(prev), policy)
+ if (addr == prev->vm_end && mpol_equal(vma_policy(prev), policy)
&& can_vma_merge_after(prev, vm_flags, anon_vma, file,
- pgoff, vm_userfaultfd_ctx, anon_name)) {
+ pgoff, vm_userfaultfd_ctx, anon_name)) {
merge_prev = true;
vma_prev(vmi);
}
@@ -964,13 +954,22 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,

/* Can we merge the successor? */
if (next && mpol_equal(policy, vma_policy(next)) &&
- can_vma_merge_before(next, vm_flags,
- anon_vma, file, pgoff+pglen,
+ can_vma_merge_before(next, vm_flags, anon_vma, file, pgoff+pglen,
vm_userfaultfd_ctx, anon_name)) {
merge_next = true;
}

+ if (!merge_prev && !merge_next)
+ return NULL; /* Not mergeable. */
+
+ res = vma = prev;
remove = remove2 = adjust = NULL;
+
+ /* Verify some invariant that must be enforced by the caller. */
+ VM_WARN_ON(prev && addr <= prev->vm_start);
+ VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
+ VM_WARN_ON(addr >= end);
+
/* 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)) {
@@ -983,8 +982,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
if (!next->anon_vma)
err = dup_anon_vma(prev, curr);
}
- } else if (merge_prev) {
- err = 0; /* case 2 */
+ } else if (merge_prev) { /* case 2 */
if (curr) {
err = dup_anon_vma(prev, curr);
if (end == curr->vm_end) { /* case 7 */
@@ -994,7 +992,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
adj_start = (end - curr->vm_start);
}
}
- } else if (merge_next) {
+ } else { /* merge_next */
res = next;
if (prev && addr < prev->vm_end) { /* case 4 */
vma_end = addr;
@@ -1010,7 +1008,6 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
vma_start = addr;
vma_end = next->vm_end;
vma_pgoff = next->vm_pgoff;
- err = 0;
if (curr) { /* case 8 */
vma_pgoff = curr->vm_pgoff;
remove = curr;
@@ -1019,7 +1016,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
}
}

- /* Cannot merge or error in anon_vma clone */
+ /* Error in anon_vma clone. */
if (err)
return NULL;

--
2.39.2