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

[Baoquan He <bhe@redhat.com>]

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