linux-kernel - Re: [PATCH 1/3] mm: introduce mm_[p4d|pud|pmd]

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Message-Id: <20181031103536.0cab673d@mschwideX1>
Date:   Wed, 31 Oct 2018 10:35:36 +0100
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Li Wang <liwang@...hat.com>, Guenter Roeck <linux@...ck-us.net>,
        Janosch Frank <frankja@...ux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/3] mm: introduce mm_[p4d|pud|pmd]_folded

On Wed, 31 Oct 2018 12:02:55 +0300
"Kirill A. Shutemov" <kirill@...temov.name> wrote:

> On Mon, Oct 15, 2018 at 06:42:37PM +0200, Martin Schwidefsky wrote:
> > Add three architecture overrideable function to test if the
> > p4d, pud, or pmd layer of a page table is folded or not.
> > 
> > Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> > ---
> >  include/linux/mm.h | 40 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 0416a7204be3..d1029972541c 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h  
> 
> Shouldn't it be somewhere in asm-generic/pgtable*?

If you prefer the definitions in asm-generic that is fine with me.
I'll give it a try to see if it still compiles.

> > @@ -105,6 +105,46 @@ extern int mmap_rnd_compat_bits __read_mostly;
> >  #define mm_zero_struct_page(pp)  ((void)memset((pp), 0, sizeof(struct page)))
> >  #endif
> >  
> > +/*
> > + * On some architectures it depends on the mm if the p4d/pud or pmd
> > + * layer of the page table hierarchy is folded or not.
> > + */
> > +#ifndef mm_p4d_folded
> > +#define mm_p4d_folded(mm) mm_p4d_folded(mm)  
> 
> Do we need to define it in generic header?

That is true, it should work without the #define in the generic header.

> > +static inline bool mm_p4d_folded(struct mm_struct *mm)
> > +{
> > +#ifdef __PAGETABLE_P4D_FOLDED
> > +	return 1;
> > +#else
> > +	return 0;
> > +#endif  
> 
> Maybe
> 	return __is_defined(__PAGETABLE_P4D_FOLDED);
> 
> ?
 
I have tried that, doesn't work. The reason is that the
__PAGETABLE_xxx_FOLDED defines to not have a value.

#define __PAGETABLE_P4D_FOLDED
#define __PAGETABLE_PMD_FOLDED
#define __PAGETABLE_PUD_FOLDED

While the definition of CONFIG_xxx symbols looks like this

#define CONFIG_xxx 1

The __is_defined needs the value for the __take_second_arg trick.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.