2019-07-08 22:44:04

by Pengfei Li

Subject: [PATCH] mm/vmalloc.c: Remove always-true conditional in vmap_init_free_space

When unsigned long variables are subtracted from one another,
the result is always non-negative.

The vmap_area_list is sorted by address.

So the following two conditions are always true.

1) if (busy->va_start - vmap_start > 0)
2) if (vmap_end - vmap_start > 0)

Just remove them.

Signed-off-by: Pengfei Li <[email protected]>
---
mm/vmalloc.c | 32 +++++++++++++-------------------
1 file changed, 13 insertions(+), 19 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 0f76cca32a1c..c7bdbdc18472 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1810,31 +1810,25 @@ static void vmap_init_free_space(void)
* |<--------------------------------->|
*/
list_for_each_entry(busy, &vmap_area_list, list) {
- if (busy->va_start - vmap_start > 0) {
- free = kmem_cache_zalloc(vmap_area_cachep, GFP_NOWAIT);
- if (!WARN_ON_ONCE(!free)) {
- free->va_start = vmap_start;
- free->va_end = busy->va_start;
-
- insert_vmap_area_augment(free, NULL,
- &free_vmap_area_root,
- &free_vmap_area_list);
- }
- }
-
- vmap_start = busy->va_end;
- }
-
- if (vmap_end - vmap_start > 0) {
free = kmem_cache_zalloc(vmap_area_cachep, GFP_NOWAIT);
if (!WARN_ON_ONCE(!free)) {
free->va_start = vmap_start;
- free->va_end = vmap_end;
+ free->va_end = busy->va_start;

insert_vmap_area_augment(free, NULL,
- &free_vmap_area_root,
- &free_vmap_area_list);
+ &free_vmap_area_root, &free_vmap_area_list);
}
+
+ vmap_start = busy->va_end;
+ }
+
+ free = kmem_cache_zalloc(vmap_area_cachep, GFP_NOWAIT);
+ if (!WARN_ON_ONCE(!free)) {
+ free->va_start = vmap_start;
+ free->va_end = vmap_end;
+
+ insert_vmap_area_augment(free, NULL,
+ &free_vmap_area_root, &free_vmap_area_list);
}
}

--
2.21.0

2019-07-08 22:44:53

by Matthew Wilcox

Subject: Re: [PATCH] mm/vmalloc.c: Remove always-true conditional in vmap_init_free_space

On Tue, Jul 09, 2019 at 01:06:31AM +0800, Pengfei Li wrote:
> When unsigned long variables are subtracted from one another,
> the result is always non-negative.
>
> The vmap_area_list is sorted by address.
>
> So the following two conditions are always true.
>
> 1) if (busy->va_start - vmap_start > 0)
> 2) if (vmap_end - vmap_start > 0)
>
> Just remove them.

That condition won't be true if busy->va_start == vmap_start.

2019-07-08 22:46:51

by Pengfei Li

Subject: Re: [PATCH] mm/vmalloc.c: Remove always-true conditional in vmap_init_free_space

On Tue, Jul 9, 2019 at 1:35 AM Matthew Wilcox <[email protected]> wrote:
>
> On Tue, Jul 09, 2019 at 01:06:31AM +0800, Pengfei Li wrote:
> > When unsigned long variables are subtracted from one another,
> > the result is always non-negative.
> >
> > The vmap_area_list is sorted by address.
> >
> > So the following two conditions are always true.
> >
> > 1) if (busy->va_start - vmap_start > 0)
> > 2) if (vmap_end - vmap_start > 0)
> >
> > Just remove them.
>
> That condition won't be true if busy->va_start == vmap_start.

Yes, you're right.
Sorry for my bad job.