[PATCH] slub: fix off by one in number of slab tests

[PATCH] slub: fix off by one in number of slab tests

[Joonsoo Kim]

min_partial means minimum number of slab cached in node partial
list. So, if nr_partial is less than it, we keep newly empty slab
on node partial list rather than freeing it. But if nr_partial is
equal or greater than it, it means that we have enough partial slabs
so should free newly empty slab. Current implementation missed
the equal case so if we set min_partial is 0, then, at least one slab
could be cached. This is critical problem to kmemcg destroying logic
because it doesn't works properly if some slabs is cached. This patch
fixes this problem.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

diff --git a/mm/slub.c b/mm/slub.c
index c567927..67da14d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1851,7 +1851,7 @@ redo:
 
 	new.frozen = 0;
 
-	if (!new.inuse && n->nr_partial > s->min_partial)
+	if (!new.inuse && n->nr_partial >= s->min_partial)
 		m = M_FREE;
 	else if (new.freelist) {
 		m = M_PARTIAL;
@@ -1962,7 +1962,7 @@ static void unfreeze_partials(struct kmem_cache *s,
 				new.freelist, new.counters,
 				"unfreezing slab"));
 
-		if (unlikely(!new.inuse && n->nr_partial > s->min_partial)) {
+		if (unlikely(!new.inuse && n->nr_partial >= s->min_partial)) {
 			page->next = discard_page;
 			discard_page = page;
 		} else {
@@ -2595,7 +2595,7 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
                 return;
         }
 
-	if (unlikely(!new.inuse && n->nr_partial > s->min_partial))
+	if (unlikely(!new.inuse && n->nr_partial >= s->min_partial))
 		goto slab_empty;
 
 	/*
-- 
1.7.9.5

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Re: [PATCH] slub: fix off by one in number of slab tests

[Vladimir Davydov]

On Tue, Jun 24, 2014 at 04:44:01PM +0900, Joonsoo Kim wrote:
> min_partial means minimum number of slab cached in node partial
> list. So, if nr_partial is less than it, we keep newly empty slab
> on node partial list rather than freeing it. But if nr_partial is
> equal or greater than it, it means that we have enough partial slabs
> so should free newly empty slab. Current implementation missed
> the equal case so if we set min_partial is 0, then, at least one slab
> could be cached. This is critical problem to kmemcg destroying logic
> because it doesn't works properly if some slabs is cached. This patch
> fixes this problem.

Oops, my fault :-(

Thank you for catching this!

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: Vladimir Davydov <vdavydov@parallels.com>

> 
> diff --git a/mm/slub.c b/mm/slub.c
> index c567927..67da14d 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1851,7 +1851,7 @@ redo:
>  
>  	new.frozen = 0;
>  
> -	if (!new.inuse && n->nr_partial > s->min_partial)
> +	if (!new.inuse && n->nr_partial >= s->min_partial)
>  		m = M_FREE;
>  	else if (new.freelist) {
>  		m = M_PARTIAL;
> @@ -1962,7 +1962,7 @@ static void unfreeze_partials(struct kmem_cache *s,
>  				new.freelist, new.counters,
>  				"unfreezing slab"));
>  
> -		if (unlikely(!new.inuse && n->nr_partial > s->min_partial)) {
> +		if (unlikely(!new.inuse && n->nr_partial >= s->min_partial)) {
>  			page->next = discard_page;
>  			discard_page = page;
>  		} else {
> @@ -2595,7 +2595,7 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>                  return;
>          }
>  
> -	if (unlikely(!new.inuse && n->nr_partial > s->min_partial))
> +	if (unlikely(!new.inuse && n->nr_partial >= s->min_partial))
>  		goto slab_empty;
>  
>  	/*
> -- 
> 1.7.9.5
> 

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Re: [PATCH] slub: fix off by one in number of slab tests

[David Rientjes]

On Tue, 24 Jun 2014, Joonsoo Kim wrote:

> min_partial means minimum number of slab cached in node partial
> list. So, if nr_partial is less than it, we keep newly empty slab
> on node partial list rather than freeing it. But if nr_partial is
> equal or greater than it, it means that we have enough partial slabs
> so should free newly empty slab. Current implementation missed
> the equal case so if we set min_partial is 0, then, at least one slab
> could be cached. This is critical problem to kmemcg destroying logic
> because it doesn't works properly if some slabs is cached. This patch
> fixes this problem.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: David Rientjes <rientjes@google.com>

Needed for 3.16 to fix commit 91cb69620284 ("slub: make dead memcg caches 
discard free slabs immediately").

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