linux-kernel - Re: [PATCH] mm/mmu_notifier: fix mmu_notifier_range

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Message-ID: <alpine.DEB.2.21.1812111310320.87855@chino.kir.corp.google.com>
Date:   Tue, 11 Dec 2018 13:12:54 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Jerome Glisse <jglisse@...hat.com>
cc:     Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mmu_notifier: fix mmu_notifier_range_init warning

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?