* Lorenzo Stoakes <lstoakes@xxxxxxxxx> [231009 16:53]:
> Only in mmap_region() and copy_vma() do we attempt to merge VMAs which
> occupy entirely new regions of virtual memory.
>
> We can abstract this logic and make the intent of this invocations of it
> completely explicit, rather than invoking vma_merge() with an inscrutable
> wall of parameters.
>
> This also paves the way for a simplification of the core vma_merge()
> implementation, as we seek to make it entirely an implementation detail.
>
> Note that on mmap_region(), VMA fields are initialised to zero, so we can
> simply reference these rather than explicitly specifying NULL.
I don't think that's accurate.. mmap_region() sets the start, end,
offset, flags. It also passes this vma into a driver, so I'm not sure
we can rely on them being anything after that? The whole reason
vma_merge() is attempted in this case is because the driver may have
changed vma->vm_flags on us. Your way may actually be better since the
driver may set something we assume is NULL today.
>
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> ---
> mm/mmap.c | 27 ++++++++++++++++++++-------
> 1 file changed, 20 insertions(+), 7 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 17c0dcfb1527..33aafd23823b 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2482,6 +2482,22 @@ struct vm_area_struct *vma_modify(struct vma_iterator *vmi,
> return NULL;
> }
>
> +/*
> + * Attempt to merge a newly mapped VMA with those adjacent to it. The caller
> + * must ensure that [start, end) does not overlap any existing VMA.
> + */
> +static struct vm_area_struct *vma_merge_new_vma(struct vma_iterator *vmi,
> + struct vm_area_struct *prev,
> + struct vm_area_struct *vma,
> + unsigned long start,
> + unsigned long end,
> + pgoff_t pgoff)
It's not a coding style, but if you used two tabs here, it may make this
more condensed.
> +{
> + return vma_merge(vmi, vma->vm_mm, prev, start, end, vma->vm_flags,
> + vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
> + vma->vm_userfaultfd_ctx, anon_vma_name(vma));
> +}
> +
> /*
> * do_vmi_align_munmap() - munmap the aligned region from @start to @end.
> * @vmi: The vma iterator
> @@ -2837,10 +2853,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> * vma again as we may succeed this time.
> */
> if (unlikely(vm_flags != vma->vm_flags && prev)) {
> - merge = vma_merge(&vmi, mm, prev, vma->vm_start,
> - vma->vm_end, vma->vm_flags, NULL,
> - vma->vm_file, vma->vm_pgoff, NULL,
> - NULL_VM_UFFD_CTX, NULL);
> + merge = vma_merge_new_vma(&vmi, prev, vma,
> + vma->vm_start, vma->vm_end,
> + pgoff);
└ vma->vm_pgoff
> if (merge) {
> /*
> * ->mmap() can change vma->vm_file and fput
> @@ -3382,9 +3397,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> if (new_vma && new_vma->vm_start < addr + len)
> return NULL; /* should never get here */
>
> - new_vma = vma_merge(&vmi, mm, prev, addr, addr + len, vma->vm_flags,
> - vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
> - vma->vm_userfaultfd_ctx, anon_vma_name(vma));
> + new_vma = vma_merge_new_vma(&vmi, prev, vma, addr, addr + len, pgoff);
> if (new_vma) {
> /*
> * Source vma may have been merged into new_vma
> --
> 2.42.0
>
