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

Shivank Garg
2025-04-15T10:29:18Z

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@redhat.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;

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

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