mm/mmap/vma_merge: init cleanup, be explicit about the non-mergeable case

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.

Link: https://lkml.kernel.org/r/99259fbc6403e80e270e1cc4612abbc8620b121b.1679516210.git.lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vernon Yang <vernon2gm@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/mmap.c b/mm/mmap.c
index 343859b..6c32600 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -896,18 +896,18 @@ 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);
        /*
@@ -926,24 +926,14 @@ 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);
                }
@@ -951,13 +941,22 @@ 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)) {
@@ -970,8 +969,7 @@ 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 */
@@ -981,7 +979,7 @@ 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;
@@ -997,7 +995,6 @@ 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;
@@ -1006,7 +1003,7 @@ 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;