Re: [PATCH v2 6/9] mm: vmalloc: Offload free_vmap_area_lock lock

From: Baoquan He <bhe@redhat.com>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	"Liam R . Howlett" <Liam.Howlett@oracle.com>,
	Dave Chinner <david@fromorbit.com>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Subject: Re: [PATCH v2 6/9] mm: vmalloc: Offload free_vmap_area_lock lock
Date: Thu, 7 Sep 2023 08:06:09 +0800	[thread overview]
Message-ID: <ZPkT8RJJgY0HGwmC@MiWiFi-R3L-srv> (raw)
In-Reply-To: <ZPjQCSkKesWmWdB8@pc636>

On 09/06/23 at 09:16pm, Uladzislau Rezki wrote:
> > >  static void free_vmap_area(struct vmap_area *va)
> > >  {
> > >  	struct vmap_node *vn = addr_to_node(va->va_start);
> > > +	int vn_id = decode_vn_id(va->flags);
> > >  
> > >  	/*
> > >  	 * Remove from the busy tree/list.
> > > @@ -1594,12 +1629,19 @@ static void free_vmap_area(struct vmap_area *va)
> > >  	unlink_va(va, &vn->busy.root);
> > >  	spin_unlock(&vn->busy.lock);
> > >  
> > > -	/*
> > > -	 * Insert/Merge it back to the free tree/list.
> > > -	 */
> > > -	spin_lock(&free_vmap_area_lock);
> > > -	merge_or_add_vmap_area_augment(va, &free_vmap_area_root, &free_vmap_area_list);
> > > -	spin_unlock(&free_vmap_area_lock);
> > > +	if (vn_id >= 0) {
> > 
> > In alloc_vmap_area(), the vn_id is encoded into va->flags. When
> > allocation failed, the vn_id = 0. Here should we change to check 'if
> > (vn_id > 0)' becasue the vn_id == 0 means no available vn_id encoded
> > into. And I do not get how we treat the case vn_id truly is 0.
> > 
> > 	va->flags = (addr != vend) ? encode_vn_id(vn_id) : 0;
> >
> Yes, vn_id always >= 0, so it is positive since it is an index.
> We encode a vn_id as vn_id + 1. For example if it is zero we write 1.
> 
> If not node allocation path or an error zero is written. Decoding
> is done as: zero - 1 = -1, so it is negative value, i.e. decode_vn_id()
> function returns -1.

Ah, I see it now, thanks. It would be helpful to add some explanation
above decode_vn_id() lest people misunderstand this like me?