linux-kernel - Re: [PATCH] mmu notifiers #v2

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Date:	Tue, 15 Jan 2008 15:28:27 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Andrea Arcangeli <andrea@...ranet.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kvm-devel@...ts.sourceforge.net, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>, daniel.blueman@...drics.com,
	holt@....com, steiner@....com, Andrew Morton <akpm@...l.org>,
	Hugh Dickins <hugh@...itas.com>, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH] mmu notifiers #v2


On Mon, 2008-01-14 at 12:02 -0800, Christoph Lameter wrote:
> On Sun, 13 Jan 2008, Andrea Arcangeli wrote:
> 
> > About the locking perhaps I'm underestimating it, but by following the
> > TLB flushing analogy, by simply clearing the shadow ptes (with kvm
> > mmu_lock spinlock to avoid racing with other vcpu spte accesses of
> > course) and flushing the shadow-pte after clearing the main linux pte,
> > it should be enough to serialize against shadow-pte page faults that
> > would call into get_user_pages. Flushing the host TLB before or after
> > the shadow-ptes shouldn't matter.
> 
> Hmmm... In most of the callsites we hold a writelock on mmap_sem right?

Not in unmap_mapping_range() afaik.

> > Comments welcome... especially from SGI/IBM/Quadrics and all other
> > potential users of this functionality.
> 
> > There are also certain details I'm uncertain about, like passing 'mm'
> > to the lowlevel methods, my KVM usage of the invalidate_page()
> > notifier for example only uses 'mm' for a BUG_ON for example:
> 
> Passing mm is fine as long as mmap_sem is held.

Passing mm is always a good idea, regardless of the mmap_sem, it can be
useful for lots of other things :-)

> > diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> > --- a/include/asm-generic/pgtable.h
> > +++ b/include/asm-generic/pgtable.h
> > @@ -86,6 +86,7 @@ do {									\
> >  	pte_t __pte;							\
> >  	__pte = ptep_get_and_clear((__vma)->vm_mm, __address, __ptep);	\
> >  	flush_tlb_page(__vma, __address);				\
> > +	mmu_notifier(invalidate_page, (__vma)->vm_mm, __address);	\
> >  	__pte;								\
> >  })
> >  #endif
> 
> Hmmm... this is ptep_clear_flush? What about the other uses of 
> flush_tlb_page in asm-generic/pgtable.h and related uses in arch code?
> (would help if your patches would mention the function name in the diff 
> headers)

Note that last I looked, a lot of these were stale. Might be time to
resume my spring/summer cleaning of page table accessors...

> > +#define mmu_notifier(function, mm, args...)				\
> > +	do {								\
> > +		struct mmu_notifier *__mn;				\
> > +		struct hlist_node *__n;					\
> > +									\
> > +		hlist_for_each_entry(__mn, __n, &(mm)->mmu_notifier, hlist) \
> > +			if (__mn->ops->function)			\
> > +				__mn->ops->function(__mn, mm, args);	\
> > +	} while (0)
> 
> Does this have to be inline? ptep_clear_flush will become quite big
> 
> --
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