linux-kernel - Re: [PATCH 1/2] x86/mm/pat: Change untrack

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Date:	Tue, 22 Dec 2015 10:02:05 -0700
From:	Toshi Kani <toshi.kani@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	mingo@...hat.com, hpa@...or.com, bp@...en8.de, stsp@...t.ru,
	x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 1/2] x86/mm/pat: Change untrack_pfn() to handle unmapped
 vma

On Sun, 2015-12-20 at 10:21 +0100, Thomas Gleixner wrote:
> Toshi,
> 
> On Wed, 9 Dec 2015, Toshi Kani wrote:
> > diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> > index 188e3e0..f3e391e 100644
> > --- a/arch/x86/mm/pat.c
> > +++ b/arch/x86/mm/pat.c
> > @@ -966,8 +966,14 @@ int track_pfn_insert(struct vm_area_struct *vma,
> > pgprot_t *prot,
> >  
> >  /*
> >   * untrack_pfn is called while unmapping a pfnmap for a region.
> > - * untrack can be called for a specific region indicated by pfn and
> > size or
> > - * can be for the entire vma (in which case pfn, size are zero).
> > + * untrack_pfn can be called for a specific region indicated by pfn
> > and
> > + * size or can be for the entire vma (in which case pfn, size are
> > zero).
> > + *
> > + * NOTE: mremap may move a virtual address of VM_PFNMAP, but keeps the
> > + * pfn and cache type.  In this case, untrack_pfn() is called with the
> > + * old vma after its translation has removed.  Hence, when
> > follow_phys()
> > + * fails, track_pfn() keeps the pfn tracked and clears VM_PAT from the
> > + * old vma.
> >   */
> >  void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
> >  		 unsigned long size)
> > @@ -981,14 +987,13 @@ void untrack_pfn(struct vm_area_struct *vma,
> > unsigned long pfn,
> >  	/* free the chunk starting from pfn or the whole chunk */
> >  	paddr = (resource_size_t)pfn << PAGE_SHIFT;
> >  	if (!paddr && !size) {
> > -		if (follow_phys(vma, vma->vm_start, 0, &prot, &paddr))
> > {
> > -			WARN_ON_ONCE(1);
> > -			return;
> > -		}
> > +		if (follow_phys(vma, vma->vm_start, 0, &prot, &paddr))
> > +			goto out;
> 
> Shouldn't we have an explicit call in the mremap code which clears the
> PAT flag on the mm instead of removing this sanity check?
>   
> Because that's what we end up with there. We just clear the PAT flag.
> 
> I rather prefer to do that explicitely, so the following call to
> untrack_pfn() from move_vma()->do_munmap() ... will see the PAT flag
> cleared. untrack_moved_pfn() or such.

Agreed.  I will add untrack_pfn_moved(), which clears the PAT flag.

Thanks!
-Toshi
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