Re: [PATCH v7 08/17] mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail

[Vlastimil Babka]

On 12/26/24 18:07, Suren Baghdasaryan wrote:
> With upcoming replacement of vm_lock with vm_refcnt, we need to handle a
> possibility of vma_start_read_locked/vma_start_read_locked_nested failing
> due to refcount overflow. Prepare for such possibility by changing these
> APIs and adjusting their users.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

(I don't know if this can break some uffd users?)

> @@ -1483,10 +1484,16 @@ static int uffd_move_lock(struct mm_struct *mm,
>  	mmap_read_lock(mm);
>  	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
>  	if (!err) {
> -		vma_start_read_locked(*dst_vmap);
> -		if (*dst_vmap != *src_vmap)
> -			vma_start_read_locked_nested(*src_vmap,
> -						SINGLE_DEPTH_NESTING);
> +		if (vma_start_read_locked(*dst_vmap)) {
> +			if (*dst_vmap != *src_vmap) {
> +				if (!vma_start_read_locked_nested(*src_vmap,
> +							SINGLE_DEPTH_NESTING)) {
> +					vma_end_read(*dst_vmap);
> +					err = -EAGAIN;
> +				}
> +			}
> +		} else
> +			err = -EAGAIN;

Nit: { } per style

>  	}
>  	mmap_read_unlock(mm);
>  	return err;