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Date:   Tue, 24 Nov 2020 15:28:14 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org,
        linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org, linux-media@vger.kernel.org,
        Daniel Vetter <daniel.vetter@intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Jan Kara <jack@suse.cz>
Subject: Re: [PATCH v6 17/17] RFC: mm: add mmu_notifier argument to follow_pfn
Message-ID: <20201124142814.GM401619@phenom.ffwll.local>
Mail-Followup-To: Jason Gunthorpe <jgg@ziepe.ca>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org,
        linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org, linux-media@vger.kernel.org,
        Daniel Vetter <daniel.vetter@intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Jan Kara <jack@suse.cz>
References: <20201119144146.1045202-1-daniel.vetter@ffwll.ch>
 <20201119144146.1045202-18-daniel.vetter@ffwll.ch>
 <20201120183029.GQ244516@ziepe.ca>
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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