Re: [PATCH 2/5] mm/vmalloc.c: find the vmap of vmap_nodes in reverse order

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Apr 15, 2025 at 10:39:49AM +0800, Baoquan He wrote:
> When finding VA in vn->busy, if VA spans several zones and the passed
> addr is not the same as va->va_start, we should scan the vn in reverse
> odrdr because the starting address of VA must be smaller than the passed
> addr if it really resides in the VA.
> 
> E.g on a system nr_vmap_nodes=100,
> 
>      <----va---->
>  -|-----|-----|-----|-----|-----|-----|-----|-----|-----|-
>     ...   n-1   n    n+1   n+2   ...   100     0     1
> 
> VA resides in node 'n' whereas it spans 'n', 'n+1' and 'n+2'. If passed
> addr is within 'n+2', we should try nodes backwards on 'n+1' and 'n',
> then succeed very soon.
> 
> Meanwhile we still need loop around because VA could spans node from 'n'
> to node 100, node 0, node 1.
> 
> Anyway, changing to find in reverse order can improve efficiency on
> many CPUs system.
> 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
>  mm/vmalloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index aca1905d3397..488d69b56765 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2436,7 +2436,7 @@ struct vmap_area *find_vmap_area(unsigned long addr)
>  
>  		if (va)
>  			return va;
> -	} while ((i = (i + 1) % nr_vmap_nodes) != j);
> +	} while ((i = (i + nr_vmap_nodes - 1) % nr_vmap_nodes) != j);
>  
>  	return NULL;
>  }
> @@ -2462,7 +2462,7 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
>  
>  		if (va)
>  			return va;
> -	} while ((i = (i + 1) % nr_vmap_nodes) != j);
> +	} while ((i = (i + nr_vmap_nodes - 1) % nr_vmap_nodes) != j);
>  
>  	return NULL;
>  }
> -- 
> 2.41.0
> 
It depends. Consider a below situation:

             addr
              |
        VA    V
  <------------>
<---|---|---|---|---|---|---|--->
  0   1   2   3   0   1   2   3

basically it matters how big VA and how many nodes it spans. But i
agree that an assumption to reverse back is more convinced in most
cases.

Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

--
Uladzislau Rezki