linux-kernel - Re: [PATCH 3/5] x86: pgtable: Drop pXd_none() checks from pXd_free_pYd

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Date:   Mon, 17 Sep 2018 12:33:22 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Kani, Toshi" <toshi.kani@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "cpandya@...eaurora.org" <cpandya@...eaurora.org>,
        "Hocko, Michal" <MHocko@...e.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/5] x86: pgtable: Drop pXd_none() checks from
 pXd_free_pYd_table()

On Fri, Sep 14, 2018 at 08:37:48PM +0000, Kani, Toshi wrote:
> On Wed, 2018-09-12 at 11:26 +0100, Will Deacon wrote:
> > Now that the core code checks this for us, we don't need to do it in the
> > backend.
> > 
> > Cc: Chintan Pandya <cpandya@...eaurora.org>
> > Cc: Toshi Kani <toshi.kani@....com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Michal Hocko <mhocko@...e.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Will Deacon <will.deacon@....com>
> > ---
> >  arch/x86/mm/pgtable.c | 6 ------
> >  1 file changed, 6 deletions(-)
> > 
> > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > index ae394552fb94..b4919c44a194 100644
> > --- a/arch/x86/mm/pgtable.c
> > +++ b/arch/x86/mm/pgtable.c
> > @@ -796,9 +796,6 @@ int pud_free_pmd_page(pud_t *pud, unsigned long addr)
> >  	pte_t *pte;
> >  	int i;
> >  
> > -	if (pud_none(*pud))
> > -		return 1;
> > -
> 
> Do we need to remove this safe guard?  I feel list this is same as
> kfree() accepting NULL.

I think two big differences with kfree() are (1) that this function has
exactly one caller in the tree and (2) it's implemented per-arch. Therefore
we're in a good position to give it some simple semantics and implement
those. Of course, if the x86 people would like to keep the redundant check,
that's up to them, but I think it makes the function more confusing and
tempts people into calling it for present entries.

Will