linux-kernel - Re: "mm: use early_pfn_to_nid in page_ext

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Message-ID: <20170704065112.GA12068@dhcp22.suse.cz>
Date:   Tue, 4 Jul 2017 08:51:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Yang Shi <yang.shi@...aro.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: "mm: use early_pfn_to_nid in page_ext_init" broken on some
 configurations?

On Tue 04-07-17 14:11:41, Joonsoo Kim wrote:
> On Fri, Jun 30, 2017 at 05:44:16PM +0200, Michal Hocko wrote:
> > On Fri 30-06-17 17:42:24, Michal Hocko wrote:
> > [...]
> > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > index 16532fa0bb64..894697c1e6f5 100644
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -1055,6 +1055,7 @@ static inline struct zoneref *first_zones_zonelist(struct zonelist *zonelist,
> > >  	!defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP)
> > >  static inline unsigned long early_pfn_to_nid(unsigned long pfn)
> > >  {
> > > +	BUILD_BUG_ON(!IS_ENABLED(CONFIG_NUMA));
> > 
> > Err, this should read BUILD_BUG_ON(IS_ENABLED(CONFIG_NUMA)) of course
> 
> Agreed.
> 
> However, AFAIK, ARM can set CONFIG_NUMA but it doesn't have
> CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID and CONFIG_HAVE_MEMBLOCK_NODE_MAP.

$ git grep "config NUMA\|select NUMA" arch/arm
$

Did you mean arch64? If yes this one looks ok
$ git grep "HAVE_MEMBLOCK_NODE_MAP\|HAVE_ARCH_EARLY_PFN_TO_NID" arch/arm64/
arch/arm64/Kconfig:     select HAVE_MEMBLOCK_NODE_MAP if NUMA

> If page_ext uses early_pfn_to_nid(), it will cause build error in ARM.

Which would be intentional if it doesn't provide a proper implementation
of the function.
 
> Therefore, I suggest following change.
> CONFIG_DEFERRED_STRUCT_PAGE_INIT depends on proper early_pfn_to_nid().
> So, following code will always work as long as
> CONFIG_DEFERRED_STRUCT_PAGE_INIT works.

I haven't checked all other callers of early_pfn_to_nid yet but I have
run my original patch (with !IS_ENABLED...) just to see whether anybody
actually uses this function from an innvalid context and it hasn't blown
up. So I suspect that all current users simply use the function from the
proper context. So if nobody objects I would just post the patch for
inclusion. If the compilation breaks we can think of a proper
implementation.

> 
> Thanks.
> 
> ----------->8---------------
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 88ccc044..e3db259 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -384,6 +384,7 @@ void __init page_ext_init(void)
>  
>         for_each_node_state(nid, N_MEMORY) {
>                 unsigned long start_pfn, end_pfn;
> +               int page_nid;
>  
>                 start_pfn = node_start_pfn(nid);
>                 end_pfn = node_end_pfn(nid);
> @@ -405,8 +406,15 @@ void __init page_ext_init(void)
>                          *
>                          * Take into account DEFERRED_STRUCT_PAGE_INIT.
>                          */
> -                       if (early_pfn_to_nid(pfn) != nid)
> +#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> +                       page_nid = early_pfn_to_nid(pfn);
> +#else
> +                       page_nid = pfn_to_nid(pfn);
> +#endif

I cannot say I would be happy about this ifdefery. Especially when there
is no existing user which would need it. 

> +
> +                       if (page_nid != nid)
>                                 continue;
> +
>                         if (init_section_page_ext(pfn, nid))
>                                 goto oom;
>                 }

-- 
Michal Hocko
SUSE Labs