linux-kernel - Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area

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Message-ID: <ZP9GGW/q0nCFelLf@pc636>
Date:   Mon, 11 Sep 2023 18:53:45 +0200
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 v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree

On Mon, Sep 11, 2023 at 10:38:29AM +0800, Baoquan He wrote:
> On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
> > Store allocated objects in a separate nodes. A va->va_start
> > address is converted into a correct node where it should
> > be placed and resided. An addr_to_node() function is used
> > to do a proper address conversion to determine a node that
> > contains a VA.
> > 
> > Such approach balances VAs across nodes as a result an access
> > becomes scalable. Number of nodes in a system depends on number
> > of CPUs divided by two. The density factor in this case is 1/2.
> > 
> > Please note:
> > 
> > 1. As of now allocated VAs are bound to a node-0. It means the
> >    patch does not give any difference comparing with a current
> >    behavior;
> > 
> > 2. The global vmap_area_lock, vmap_area_root are removed as there
> >    is no need in it anymore. The vmap_area_list is still kept and
> >    is _empty_. It is exported for a kexec only;
> > 
> > 3. The vmallocinfo and vread() have to be reworked to be able to
> >    handle multiple nodes.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > ---
> >  mm/vmalloc.c | 209 +++++++++++++++++++++++++++++++++++++++------------
> >  1 file changed, 161 insertions(+), 48 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index b7deacca1483..ae0368c314ff 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -728,11 +728,9 @@ EXPORT_SYMBOL(vmalloc_to_pfn);
> >  #define DEBUG_AUGMENT_LOWEST_MATCH_CHECK 0
> >  
> >  
> > -static DEFINE_SPINLOCK(vmap_area_lock);
> >  static DEFINE_SPINLOCK(free_vmap_area_lock);
> >  /* Export for kexec only */
> >  LIST_HEAD(vmap_area_list);
> > -static struct rb_root vmap_area_root = RB_ROOT;
> >  static bool vmap_initialized __read_mostly;
> >  
> >  static struct rb_root purge_vmap_area_root = RB_ROOT;
> > @@ -772,6 +770,38 @@ static struct rb_root free_vmap_area_root = RB_ROOT;
> >   */
> >  static DEFINE_PER_CPU(struct vmap_area *, ne_fit_preload_node);
> >  
> > +/*
> > + * An effective vmap-node logic. Users make use of nodes instead
> > + * of a global heap. It allows to balance an access and mitigate
> > + * contention.
> > + */
> > +struct rb_list {
> > +	struct rb_root root;
> > +	struct list_head head;
> > +	spinlock_t lock;
> > +};
> > +
> > +struct vmap_node {
> > +	/* Bookkeeping data of this node. */
> > +	struct rb_list busy;
> > +};
> > +
> > +static struct vmap_node *nodes, snode;
> > +static __read_mostly unsigned int nr_nodes = 1;
> > +static __read_mostly unsigned int node_size = 1;
> 
> It could be better if calling these global variables a meaningful name,
> e.g vmap_nodes, static_vmap_nodes, nr_vmap_nodes. When I use vim+cscope
> to reference them, it gives me a super long list. Aside from that, a
> simple name often makes me mistake it as a local virable. A weak
> opinion.
> 
I am OK to add "vmap_" prefix:

vmap_nodes;
vmap_nr_nodes;
vmap_node_size;
..

If you are not OK with that, feel free to propose other variants.

Thank you!

--
Uladzislau Rezki