linux-kernel - Re: [Patch v2 2/2] mm/page_alloc.c: define node

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Message-ID: <20200327231820.GA20941@ziepe.ca>
Date:   Fri, 27 Mar 2020 20:18:20 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Wei Yang <richard.weiyang@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, david@...hat.com
Subject: Re: [Patch v2 2/2] mm/page_alloc.c: define node_order with all zero

On Fri, Mar 27, 2020 at 03:37:57PM -0700, John Hubbard wrote:
> On 3/27/20 3:01 PM, Wei Yang wrote:
> > Since we always clear node_order before getting it, we can leverage
> > compiler to do this instead of at run time.
> > 
> > Signed-off-by: Wei Yang <richard.weiyang@...il.com>
> >   mm/page_alloc.c | 3 +--
> >   1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index dfcf2682ed40..49dd1f25c000 100644
> > +++ b/mm/page_alloc.c
> > @@ -5585,7 +5585,7 @@ static void build_thisnode_zonelists(pg_data_t *pgdat)
> >   static void build_zonelists(pg_data_t *pgdat)
> >   {
> > -	static int node_order[MAX_NUMNODES];
> > +	static int node_order[MAX_NUMNODES] = {0};
> 
> 
> Looks wrong: now the single instance of node_order is initialized just once by
> the compiler. And that means that only the first caller of this function
> gets a zeroed node_order array...

It is also redundant, all static data is 0 initialized in Linux and
should not be explicitly initialized so it can remain in .bss

> > @@ -5595,7 +5595,6 @@ static void build_zonelists(pg_data_t *pgdat)
> >   	load = nr_online_nodes;
> >   	prev_node = local_node;
> > -	memset(node_order, 0, sizeof(node_order));
> 
> ...and all subsequent callers are left with whatever debris is remaining in
> node_order. So this is not good.

Indeed

Jason