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SubjectRe: [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

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