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

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Date:   Sat, 28 Mar 2020 08:51:44 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     John Hubbard <jhubbard@...dia.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, jgg@...pe.ca,
        david@...hat.com
Subject: Re: [Patch v2 2/2] mm/page_alloc.c: define node_order with all zero

On 03/28/20 at 12:26am, Wei Yang wrote:
> 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
> >> --- a/mm/page_alloc.c
> >> +++ 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...
> >
> 
> What a shame on me. You are right, I miss the static word. 
> 
> Well, then I am curious about why we want to define it as static. Each time we
> call this function, node_order would be set to 0 and find_next_best_node()
> would sort a proper value into it. I don't see the reason to reserve it in a
> global area and be used next time.
> 
> My suggestion is to remove the static and define it {0} instead of memset
> every time. Is my understanding correct here?

Removing static looks good, the node_order is calculated on the basis of
each node, it's useless for other node. It may be inherited from the old
code where it's a static global variable.