linux-kernel - Re: [PATCH 2/5] mm/vmalloc.c: find the vmap of vmap

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Message-ID: <a19f409d-3507-459c-ab1f-b28dc0cb021d@amd.com>
Date: Wed, 16 Apr 2025 00:39:55 +0530
From: Shivank Garg <shivankg@....com>
To: Baoquan He <bhe@...hat.com>, linux-mm@...ck.org
Cc: akpm@...ux-foundation.org, urezki@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] mm/vmalloc.c: find the vmap of vmap_nodes in reverse
 order

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

Thanks,
Shivank