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

Baoquan He
2025-04-15T23:41:46Z

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.