linux-kernel - Re: [PATCH v6 17/17] RFC: mm: add mmu_notifier argument to follow

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Date:   Tue, 24 Nov 2020 15:28:14 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v6 17/17] RFC: mm: add mmu_notifier argument to follow_pfn

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