linux-kernel - Re: [mm PATCH v3 1/6] mm: Use mm_zero_struct_page from SPARC on all 64b architectures

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Date:   Wed, 17 Oct 2018 18:34:11 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        pavel.tatashin@...rosoft.com, dave.jiang@...el.com,
        linux-kernel@...r.kernel.org, willy@...radead.org,
        davem@...emloft.net, yi.z.zhang@...ux.intel.com,
        khalid.aziz@...cle.com, rppt@...ux.vnet.ibm.com, vbabka@...e.cz,
        sparclinux@...r.kernel.org, dan.j.williams@...el.com,
        ldufour@...ux.vnet.ibm.com, mgorman@...hsingularity.net,
        mingo@...nel.org, kirill.shutemov@...ux.intel.com
Subject: Re: [mm PATCH v3 1/6] mm: Use mm_zero_struct_page from SPARC on all
 64b architectures

On Wed 17-10-18 08:07:06, Alexander Duyck wrote:
> On 10/17/2018 1:47 AM, Michal Hocko wrote:
> > On Mon 15-10-18 13:26:56, Alexander Duyck wrote:
[...]
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index bb0de406f8e7..ec6e57a0c14e 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -102,8 +102,42 @@ static inline void set_max_mapnr(unsigned long limit) { }
> > >    * zeroing by defining this macro in <asm/pgtable.h>.
> > >    */
> > >   #ifndef mm_zero_struct_page
> > 
> > Do we still need this ifdef? I guess we can wait for an arch which
> > doesn't like this change and then add the override. I would rather go
> > simple if possible.
> 
> We probably don't, but as soon as I remove it somebody will probably
> complain somewhere. I guess I could drop it for now and see if anybody
> screams. Adding it back should be pretty straight forward since it would
> only be 2 lines.

Let's make it simpler please. If somebody really cares then this is
trivial to add later.
 
> > > +#if BITS_PER_LONG == 64
> > > +/* This function must be updated when the size of struct page grows above 80
> > > + * or reduces below 64. The idea that compiler optimizes out switch()
> > > + * statement, and only leaves move/store instructions
> > > + */
> > > +#define	mm_zero_struct_page(pp) __mm_zero_struct_page(pp)
> > > +static inline void __mm_zero_struct_page(struct page *page)
> > > +{
> > > +	unsigned long *_pp = (void *)page;
> > > +
> > > +	 /* Check that struct page is either 56, 64, 72, or 80 bytes */
> > > +	BUILD_BUG_ON(sizeof(struct page) & 7);
> > > +	BUILD_BUG_ON(sizeof(struct page) < 56);
> > > +	BUILD_BUG_ON(sizeof(struct page) > 80);
> > > +
> > > +	switch (sizeof(struct page)) {
> > > +	case 80:
> > > +		_pp[9] = 0;	/* fallthrough */
> > > +	case 72:
> > > +		_pp[8] = 0;	/* fallthrough */
> > > +	default:
> > > +		_pp[7] = 0;	/* fallthrough */
> > > +	case 56:
> > > +		_pp[6] = 0;
> > > +		_pp[5] = 0;
> > > +		_pp[4] = 0;
> > > +		_pp[3] = 0;
> > > +		_pp[2] = 0;
> > > +		_pp[1] = 0;
> > > +		_pp[0] = 0;
> > > +	}
> > 
> > This just hit my eyes. I have to confess I have never seen default: to
> > be not the last one in the switch. Can we have case 64 instead or does gcc
> > complain? I would be surprised with the set of BUILD_BUG_ONs.
> 
> I can probably just replace the "default:" with "case 64:". I think I have
> seen other switch statements in the kernel without a default so odds are it
> should be okay.

Please do, there shouldn't be any need to obfuscate the code more than
necessary.

-- 
Michal Hocko
SUSE Labs