On 3/22/23 08:13, Lorenzo Stoakes wrote:
> Previously, vma was an uninitialised variable which was only definitely
> assigned as a result of the logic covering all possible input cases - for
> it to have remained uninitialised, prev would have to be NULL, and next
> would _have_ to be mergeable.
>
> The value of res defaults to NULL, so we can neatly eliminate the
> assignment to res and vma in the if (prev) block and ensure that both res
> and vma are both explicitly assigned, by just setting both to prev.
>
> In addition we add an explanation as to under what circumstances both might
> change, and since we absolutely do rely on addr == curr->vm_start should
> curr exist, assert that this is the case.
Hm replied to v2 of this, sorry, so let me repeat that here:
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Nit suggestion below.
> ---
> mm/mmap.c | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index dbdbb92493b2..2a4f63716231 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -911,7 +911,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> {
> pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
> pgoff_t vma_pgoff;
> - struct vm_area_struct *curr, *next, *res = NULL;
> + struct vm_area_struct *curr, *next, *res;
> struct vm_area_struct *vma, *adjust, *remove, *remove2;
> int err = -1;
> bool merge_prev = false;
> @@ -939,14 +939,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> else
> next = NULL; /* case 5 */
>
> - /* verify some invariant that must be enforced by the caller */
> + /*
> + * 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;
Later in the function there's a line:
remove = remove2 = adjust = NULL;
Now it would make sense to move it up here?
> +
> + /* Verify some invariant that must be enforced by the caller. */
> VM_WARN_ON(prev && addr <= prev->vm_start);
> - VM_WARN_ON(curr && end > curr->vm_end);
> + VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> VM_WARN_ON(addr >= end);
>
> if (prev) {
> - res = prev;
> - vma = prev;
> vma_start = prev->vm_start;
> vma_pgoff = prev->vm_pgoff;
> /* Can we merge the predecessor? */
> @@ -957,6 +961,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> vma_prev(vmi);
> }
> }
> +
> /* Can we merge the successor? */
> if (next && mpol_equal(policy, vma_policy(next)) &&
> can_vma_merge_before(next, vm_flags,
> @@ -997,6 +1002,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> adj_start = -(prev->vm_end - addr);
> err = dup_anon_vma(next, prev);
> } else {
> + /*
> + * Note that cases 3 and 8 are the ONLY ones where prev
> + * is permitted to be (but is not necessarily) NULL.
> + */
> vma = next; /* case 3 */
> vma_start = addr;
> vma_end = next->vm_end;
