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

Shivank Garg
2025-04-15T19:10:06Z

On 4/15/2025 8:09 AM, 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;
> }

Reviewed-by: Shivank Garg <shivankg@amd.com>
Tested-by: Shivank Garg <shivankg@amd.com>

Thanks,
Shivank