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

[Baoquan He <bhe@redhat.com>]

On 04/15/25 at 05:25pm, Uladzislau Rezki wrote:
> 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.

Agree, on small system with few CPUs and big VA case, the advantage is
not apparent.

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

Thanks.