Re: [PATCH] mm/mmu_notifier: fix mmu_notifier_range_init warning

[Jerome Glisse <jglisse@redhat.com>]

On Tue, Dec 11, 2018 at 01:12:54PM -0800, David Rientjes wrote:
> On Tue, 11 Dec 2018, Jerome Glisse wrote:
> 
> > On Tue, Dec 11, 2018 at 09:04:43PM +0100, Arnd Bergmann wrote:
> > > The macro version of mmu_notifier_range_init() for CONFIG_MMU_NOTIFIER=n
> > > does not evaluate all its arguments, leading to a warning in one case:
> > > 
> > > mm/migrate.c: In function 'migrate_vma_pages':
> > > mm/migrate.c:2711:20: error: unused variable 'mm' [-Werror=unused-variable]
> > >   struct mm_struct *mm = vma->vm_mm;
> > > 
> > > Pass down the 'mm' as into the inline function as well so gcc can
> > > see why the variable exists.
> > > 
> > > Fixes: 137d92bd73b1 ("mm/mmu_notifier: use structure for invalidate_range_start/end calls v2")
> > 
> > What about changing migrate.c (i actualy tried to do that everywhere in
> > the patchset but i missed that spot) So we avoid one useless variable on
> > such configuration:
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index f02bb4b22c1a..883fce631f47 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2701,7 +2701,6 @@ static void migrate_vma_pages(struct migrate_vma *migrate)
> >  	const unsigned long npages = migrate->npages;
> >  	const unsigned long start = migrate->start;
> >  	struct vm_area_struct *vma = migrate->vma;
> > -	struct mm_struct *mm = vma->vm_mm;
> >  	struct mmu_notifier_range range;
> >  	unsigned long addr, i;
> >  	bool notified = false;
> > @@ -2724,8 +2723,8 @@ static void migrate_vma_pages(struct migrate_vma *migrate)
> >  			if (!notified) {
> >  				notified = true;
> >  
> > -				mmu_notifier_range_init(&range, mm, addr,
> > -							migrate->end,
> > +				mmu_notifier_range_init(&range, vma->vm_mm,
> > +							addr, migrate->end,
> >  							MMU_NOTIFY_CLEAR);
> >  				mmu_notifier_invalidate_range_start(&range);
> >  			}
> 
> Wouldn't it be better to just declare mmu_notifier_range_init() as a 
> static inline function rather than a #define to avoid this warning?

The define trick is use to drop the event parameter so that we do not
need to define the event value for CONFIG_MMU_NOTIFIER=n case.

Cheers,
Jérôme