Re: [PATCH 3/5] mm/vmalloc.c: optimize code in decay_va_pool_node() a little bit

[Shivank Garg]

On 4/15/2025 8:09 AM, 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@xxxxxxxxxx>
> ---
>  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;

Current logic uses WRITE_ONCE() to update vn->pool[i].len.
Could this lead to consistency issues?

>  			spin_unlock(&vn->pool_lock);
>  		}
>  	}