linux-kernel - Re: [PATCH v3 07/11] mm: vmalloc: Offload free_vmap_area

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Date: Wed, 28 Feb 2024 11:39:32 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Matthew Wilcox <willy@...radead.org>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v3 07/11] mm: vmalloc: Offload free_vmap_area_lock lock

On Wed, Feb 28, 2024 at 05:48:53PM +0800, Baoquan He wrote:
> On 01/02/24 at 07:46pm, Uladzislau Rezki (Sony) wrote:
> .....snip...
> > +static void
> > +decay_va_pool_node(struct vmap_node *vn, bool full_decay)
> > +{
> > +	struct vmap_area *va, *nva;
> > +	struct list_head decay_list;
> > +	struct rb_root decay_root;
> > +	unsigned long n_decay;
> > +	int i;
> > +
> > +	decay_root = RB_ROOT;
> > +	INIT_LIST_HEAD(&decay_list);
> > +
> > +	for (i = 0; i < MAX_VA_SIZE_PAGES; i++) {
> > +		struct list_head tmp_list;
> > +
> > +		if (list_empty(&vn->pool[i].head))
> > +			continue;
> > +
> > +		INIT_LIST_HEAD(&tmp_list);
> > +
> > +		/* Detach the pool, so no-one can access it. */
> > +		spin_lock(&vn->pool_lock);
> > +		list_replace_init(&vn->pool[i].head, &tmp_list);
> > +		spin_unlock(&vn->pool_lock);
> > +
> > +		if (full_decay)
> > +			WRITE_ONCE(vn->pool[i].len, 0);
> > +
> > +		/* Decay a pool by ~25% out of left objects. */
> 
> This isn't true if the pool has less than 4 objects. If there are 3
> objects, n_decay = 0.
> 
This is expectable.

> > +		n_decay = vn->pool[i].len >> 2;
> > +
> > +		list_for_each_entry_safe(va, nva, &tmp_list, list) {
> > +			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;
> 
>                                Here, --n_decay will make n_decay 0xffffffffffffffff,
>                                then all left objects are reclaimed.
Right. Last three objects do not play a big game.

--
Uladzislau Rezki