On Thu, Mar 09, 2023 at 12:12:50PM +0100, Vlastimil Babka wrote:
> In case 3 we we use 'next' for everything but vma_pgoff. So use 'next'
> for that as well, instead of 'mid', for consistency. Then in case 8 we
> have to use 'mid' explicitly, which should also make the intent more
> obvious.
>
> Adjust the diagram for cases 1-3 in the comment to match the code - we
> are using 'next' for case 3 so mark the range with XXXX instead of NNNN.
> For case 2 that's a no-op as the code doesn't touch 'next' or 'mid'. For
> case 1 it's now wrong but that will be fixed next.
>
> No functional change.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
> mm/mmap.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 0a8b052e3022..1af4c9bc2c87 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -857,11 +857,11 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
> * mmap, brk or case 4 below case 5 below
> * mremap move:
> * AAAA AAAA
> - * PPPP NNNN PPPPNNNNXXXX
> + * PPPP XXXX PPPPNNNNXXXX
> * might become might become
> * PPPPPPPPPPPP 1 or PPPPPPPPPPPP 6 or
> - * PPPPPPPPNNNN 2 or PPPPPPPPXXXX 7 or
> - * PPPPNNNNNNNN 3 PPPPXXXXXXXX 8
> + * PPPPPPPPXXXX 2 or PPPPPPPPXXXX 7 or
> + * PPPPXXXXXXXX 3 PPPPXXXXXXXX 8
> *
I'm glad you're making things more consistent and what you're addressing here is
a real clanger, but these diagrams while great to have do definitely feel
quite confusing even now. But that's something for a future patch!
> * It is important for case 8 that the vma NNNN overlapping the
> * region AAAA is never going to extended over XXXX. Instead XXXX must
> @@ -978,9 +978,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> vma = next; /* case 3 */
> vma_start = addr;
> vma_end = next->vm_end;
> - vma_pgoff = mid->vm_pgoff;
> + vma_pgoff = next->vm_pgoff;
> err = 0;
> if (mid != next) { /* case 8 */
> + vma_pgoff = mid->vm_pgoff;
> remove = mid;
> err = dup_anon_vma(next, mid);
> }
> --
> 2.39.2
>
This does fix a big incongruity in that previously everything but vm_pgoff was
relative to next, while in the non-8 case mid is equal to next anyway.
Good, clarifying improvement!
Reviewed-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
