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

[Alexander Duyck]

On 10/17/2018 1:47 AM, Michal Hocko wrote:
> On Mon 15-10-18 13:26:56, Alexander Duyck wrote:
> > This change makes it so that we use the same approach that was already in
> > use on Sparc on all the archtectures that support a 64b long.
> >
> > This is mostly motivated by the fact that 8 to 10 store/move instructions
> > are likely always going to be faster than having to call into a function
> > that is not specialized for handling page init.
> >
> > An added advantage to doing it this way is that the compiler can get away
> > with combining writes in the __init_single_page call. As a result the
> > memset call will be reduced to only about 4 write operations, or at least
> > that is what I am seeing with GCC 6.2 as the flags, LRU poitners, and
> > count/mapcount seem to be cancelling out at least 4 of the 8 assignments on
> > my system.
> >
> > One change I had to make to the function was to reduce the minimum page
> > size to 56 to support some powerpc64 configurations.
>
> This really begs for numbers. I do not mind the change itself with some
> minor comments below.
>
> [...]
> > 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.

> > +#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.