linux-kernel - Re: linux-next: build failure after merge of the akpm-current tree

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Date:   Thu, 7 Sep 2017 00:46:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Stephen Rothwell <sfr@...b.auug.org.au>
Cc:     "Zi Yan" <zi.yan@...rutgers.edu>,
        "Linux-Next Mailing List" <linux-next@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: build failure after merge of the akpm-current tree

On Thu, 7 Sep 2017 15:23:55 +1000 Stephen Rothwell <sfr@...b.auug.org.au> wrote:

> Hi all,
> 
> On Wed, 2 Aug 2017 16:31:45 +1000 Stephen Rothwell <sfr@...b.auug.org.au> wrote:
> >
> > On Wed, 2 Aug 2017 15:45:54 +1000 Stephen Rothwell <sfr@...b.auug.org.au> wrote:
> > >
> > > On Tue, 01 Aug 2017 09:08:01 -0400 "Zi Yan" <zi.yan@...rutgers.edu> wrote:  
> > > >
> > > > I found two possible fixes.
> > > > 
> > > > 1. This uses C++ zero initializer, GCC is OK with it.
> > > > I tested with GCC 4.9.3 (has the initialization bug) and GCC 6.4.0.
> > > > 
> > > > --- a/include/linux/swapops.h~a
> > > > +++ a/include/linux/swapops.h
> > > > @@ -217,7 +217,7 @@ static inline swp_entry_t pmd_to_swp_ent
> > > > 
> > > >  static inline pmd_t swp_entry_to_pmd(swp_entry_t entry)
> > > >  {
> > > > -	return (pmd_t){ 0 };
> > > > +	return (pmd_t){};
> > > >  }    
> > > 
> > > I have done that for today ... please decide which is best (or find
> > > something better - maybe every platform really needs to have a __pmd()
> > > definition) and submit a real fix patch to Andrew.  
> > 
> > OK, that failed for my compiler (gcc 5.2.0) like this:
> > 
> > In file included from mm/vmscan.c:55:0:
> > include/linux/swapops.h: In function 'swp_entry_to_pmd':
> > include/linux/swapops.h:226:16: error: empty scalar initializer
> >   return (pmd_t){};
> >                 ^
> > include/linux/swapops.h:226:16: note: (near initialization for '(anonymous)')
> 
> This has reappeared today :-( (for the arm multi_v7_defconfig build at
> least)
> 
> > So I used the other idea (on top of Andrew's current tree):
> 
> The fix patch now looks like this:
> 
> From: Stephen Rothwell <sfr@...b.auug.org.au>
> Date: Wed, 2 Aug 2017 15:55:02 +1000
> Subject: [PATCH] mm-thp-enable-thp-migration-in-generic-path-fix-fix
> 
> Signed-off-by: Stephen Rothwell <sfr@...b.auug.org.au>
> ---
>  include/linux/swapops.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 45b092aa6419..61cffa148a79 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -223,7 +223,9 @@ static inline swp_entry_t pmd_to_swp_entry(pmd_t pmd)
>  
>  static inline pmd_t swp_entry_to_pmd(swp_entry_t entry)
>  {
> -	return (pmd_t){};
> +	pmd_t e;
> +	memset(&e, 0, sizeof(pmd_t));
> +	return e;
>  }

err, yeah.  I didn't want to add that one :(

At the very least we should add a good comment explaining why we had to
resort to this.