linux-kernel - Re: [PATCH] mm/slub: make add_full() condition more explicit

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Date:   Fri, 16 Oct 2020 18:58:30 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     wuyun.wu@...wei.com, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     liu.xiang6@....com.cn,
        "open list:SLAB ALLOCATOR" <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/slub: make add_full() condition more explicit

On 8/11/20 4:02 AM, wuyun.wu@...wei.com wrote:
> From: Abel Wu <wuyun.wu@...wei.com>
> 
> The commit below is incomplete, as it didn't handle the add_full() part.
> commit a4d3f8916c65 ("slub: remove useless kmem_cache_debug() before remove_full()")
> 
> This patch checks for SLAB_STORE_USER instead of kmem_cache_debug(),
> since that should be the only context in which we need the list_lock for
> add_full().
> 
> Signed-off-by: Abel Wu <wuyun.wu@...wei.com>
> ---
>   mm/slub.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f226d66408ee..df93a5a0e9a4 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2182,7 +2182,8 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>   		}
>   	} else {
>   		m = M_FULL;
> -		if (kmem_cache_debug(s) && !lock) {
> +#ifdef CONFIG_SLUB_DEBUG
> +		if ((s->flags & SLAB_STORE_USER) && !lock) {
>   			lock = 1;
>   			/*
>   			 * This also ensures that the scanning of full
> @@ -2191,6 +2192,7 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>   			 */
>   			spin_lock(&n->list_lock);
>   		}
> +#endif
>   	}
>   
>   	if (l != m) {
> 

Hm I missed this, otherwise I would have suggested the following

-----8<-----
 From 0b43c7e20c81241f4b74cdb366795fc0b94a25c9 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Fri, 16 Oct 2020 18:46:06 +0200
Subject: [PATCH] mm, slub: use kmem_cache_debug_flags() in deactivate_slab()

Commit 9cf7a1118365 ("mm/slub: make add_full() condition more explicit")
replaced an unnecessarily generic kmem_cache_debug(s) check with an explicit
check of SLAB_STORE_USER and #ifdef CONFIG_SLUB_DEBUG.

We can achieve the same specific check with the recently added
kmem_cache_debug_flags() which removes the #ifdef and restores the
no-branch-overhead benefit of static key check when slub debugging is not
enabled.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
  mm/slub.c | 4 +---
  1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 61d0d2968413..28d78238f31e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2245,8 +2245,7 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
  		}
  	} else {
  		m = M_FULL;
-#ifdef CONFIG_SLUB_DEBUG
-		if ((s->flags & SLAB_STORE_USER) && !lock) {
+		if (kmem_cache_debug_flags(s, SLAB_STORE_USER) && !lock) {
  			lock = 1;
  			/*
  			 * This also ensures that the scanning of full
@@ -2255,7 +2254,6 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
  			 */
  			spin_lock(&n->list_lock);
  		}
-#endif
  	}

  	if (l != m) {
-- 
2.28.0