mm/vmalloc.c: find the vmap of vmap_nodes in reverse order

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.

Link: https://lkml.kernel.org/r/20250418223653.243436-3-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index cd733387b1a072f4720223a80a7c6723d2fa912f..26d86ab5e59e1198ec8f4a458192eb0a9f65aa7c 100644 (file)
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2435,7 +2435,7 @@ 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;
 }
@@ -2461,7 +2461,7 @@ 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;
 }