linux-kernel - Re: [PATCH 3/5] mm/vmalloc.c: optimize code in decay_va_pool

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Message-ID: <Z_-1ozajrbaVLq6m@pc636>
Date: Wed, 16 Apr 2025 15:50:27 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org, urezki@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] mm/vmalloc.c: optimize code in decay_va_pool_node()
 a little bit

On Tue, Apr 15, 2025 at 10:39:50AM +0800, Baoquan He wrote:
> When purge lazily freed vmap areas, VA stored in vn->pool[] will also be
> taken away into free vmap tree partially or completely accordingly, that
> is done in decay_va_pool_node(). When doing that, for each pool of node,
> the whole list is detached from the pool for handling. At this time,
> that pool is empty. It's not necessary to update the pool size each time
> when one VA is removed and addded into free vmap tree.
> 
> Here change code to update the pool size when attaching the pool back.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  mm/vmalloc.c | 23 +++++++++++------------
>  1 file changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 488d69b56765..bf735c890878 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2150,7 +2150,7 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
>  	LIST_HEAD(decay_list);
>  	struct rb_root decay_root = RB_ROOT;
>  	struct vmap_area *va, *nva;
> -	unsigned long n_decay;
> +	unsigned long n_decay, len;
>  	int i;
>  
>  	for (i = 0; i < MAX_VA_SIZE_PAGES; i++) {
> @@ -2164,22 +2164,20 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
>  		list_replace_init(&vn->pool[i].head, &tmp_list);
>  		spin_unlock(&vn->pool_lock);
>  
> -		if (full_decay)
> -			WRITE_ONCE(vn->pool[i].len, 0);
> +		len = n_decay = vn->pool[i].len;
> +		WRITE_ONCE(vn->pool[i].len, 0);
>  
>  		/* Decay a pool by ~25% out of left objects. */
> -		n_decay = vn->pool[i].len >> 2;
> +		if (!full_decay)
> +			n_decay >>= 2;
> +		len -= n_decay;
>  
>  		list_for_each_entry_safe(va, nva, &tmp_list, list) {
> +			if (!n_decay)
> +				break;
>  			list_del_init(&va->list);
>  			merge_or_add_vmap_area(va, &decay_root, &decay_list);
> -
> -			if (!full_decay) {
> -				WRITE_ONCE(vn->pool[i].len, vn->pool[i].len - 1);
> -
> -				if (!--n_decay)
> -					break;
> -			}
> +			n_decay--;
>  		}
>  
>  		/*
> @@ -2188,9 +2186,10 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
>  		 * can populate the pool therefore a simple list replace
>  		 * operation takes place here.
>  		 */
> -		if (!full_decay && !list_empty(&tmp_list)) {
> +		if (!list_empty(&tmp_list)) {
>  			spin_lock(&vn->pool_lock);
>  			list_replace_init(&tmp_list, &vn->pool[i].head);
> +			vn->pool[i].len = len;
>  			spin_unlock(&vn->pool_lock);
>  		}
>  	}
> -- 
> 2.41.0
> 
Which Linux version this patch is based on? I can not apply it.

Small nits:

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index c909b8fea6eb..0ae53c997219 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2150,7 +2150,7 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
 	LIST_HEAD(decay_list);
 	struct rb_root decay_root = RB_ROOT;
 	struct vmap_area *va, *nva;
-	unsigned long n_decay, len;
+	unsigned long n_decay, pool_len;
 	int i;
 
 	for (i = 0; i < MAX_VA_SIZE_PAGES; i++) {
@@ -2164,21 +2164,20 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
 		list_replace_init(&vn->pool[i].head, &tmp_list);
 		spin_unlock(&vn->pool_lock);
 
-		len = n_decay = vn->pool[i].len;
+		pool_len = n_decay = vn->pool[i].len;
 		WRITE_ONCE(vn->pool[i].len, 0);
 
 		/* Decay a pool by ~25% out of left objects. */
 		if (!full_decay)
 			n_decay >>= 2;
-		len -= n_decay;
+		pool_len -= n_decay;
 
 		list_for_each_entry_safe(va, nva, &tmp_list, list) {
-			if (!n_decay)
+			if (!n_decay--)
 				break;
 
 			list_del_init(&va->list);
 			merge_or_add_vmap_area(va, &decay_root, &decay_list);
-			n_decay--;
 		}
 
 		/*
@@ -2190,7 +2189,7 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
 		if (!list_empty(&tmp_list)) {
 			spin_lock(&vn->pool_lock);
 			list_replace_init(&tmp_list, &vn->pool[i].head);
-			vn->pool[i].len = len;
+			vn->pool[i].len = pool_len;
 			spin_unlock(&vn->pool_lock);
 		}
 	}
<snip>

on top of this?

a) decay in "if" statement, no need extra line;
b) rename len to something obvious - pool_len.

--
Uladzislau Rezki