Re: [PATCH v6 17/17] RFC: mm: add mmu_notifier argument to follow_pfn

From: Daniel Vetter <daniel@ffwll.ch>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-samsung-soc@vger.kernel.org, "Jan Kara" <jack@suse.cz>,
	"Kees Cook" <keescook@chromium.org>,
	kvm@vger.kernel.org, "Daniel Vetter" <daniel.vetter@ffwll.ch>,
	LKML <linux-kernel@vger.kernel.org>,
	"DRI Development" <dri-devel@lists.freedesktop.org>,
	"Christoph Hellwig" <hch@infradead.org>,
	linux-mm@kvack.org, "Jérôme Glisse" <jglisse@redhat.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Daniel Vetter" <daniel.vetter@intel.com>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-media@vger.kernel.org
Subject: Re: [PATCH v6 17/17] RFC: mm: add mmu_notifier argument to follow_pfn
Date: Tue, 24 Nov 2020 15:28:14 +0100	[thread overview]
Message-ID: <20201124142814.GM401619@phenom.ffwll.local> (raw)
In-Reply-To: <20201120183029.GQ244516@ziepe.ca>

On Fri, Nov 20, 2020 at 02:30:29PM -0400, Jason Gunthorpe wrote:
> On Thu, Nov 19, 2020 at 03:41:46PM +0100, Daniel Vetter wrote:
> > @@ -4805,21 +4824,15 @@ EXPORT_SYMBOL(follow_pte_pmd);
> >   * Return: zero and the pfn at @pfn on success, -ve otherwise.
> >   */
> >  int follow_pfn(struct vm_area_struct *vma, unsigned long address,
> > -	unsigned long *pfn)
> > +	unsigned long *pfn, struct mmu_notifier *subscription)
> >  {
> > -	int ret = -EINVAL;
> > -	spinlock_t *ptl;
> > -	pte_t *ptep;
> > +	if (WARN_ON(!subscription->mm))
> > +		return -EINVAL;
> >  
> > +	if (WARN_ON(subscription->mm != vma->vm_mm))
> > +		return -EINVAL;
> 
> These two things are redundant right? vma->vm_mm != NULL?

Yup, will remove.

> BTW, why do we even have this for nommu? If the only caller is kvm,
> can you even compile kvm on nommu??

Kinda makes sense, but I have no idea how to make sure with compile
testing this is really the case. And I didn't see any hard evidence in
Kconfig or Makefile that mmu notifiers requires CONFIG_MMU. So not sure
what to do here.

Should I just remove the nommu version of follow_pfn and see what happens?
We can't remove it earlier since it's still used by other subsystems.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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