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

[Baoquan He]

On 04/15/25 at 03:59pm, Shivank Garg wrote:
> 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?

Seems no necessary to use WRITE_ONCE(). I can change back to use
WRITE_ONCE() just in case. Currently, it's only updated in
node_alloc(), decay_va_pool_node(), purge_vmap_node(). And the latter
two are inside vmap_purge_lock area.

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