linux-kernel - Re: [patch 1/4] mmu

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Date:	Fri, 25 Jan 2008 12:56:46 -0600
From:	Robin Holt <holt@....com>
To:	Christoph Lameter <clameter@....com>
Cc:	Robin Holt <holt@....com>, Andrea Arcangeli <andrea@...ranet.com>,
	Avi Kivity <avi@...ranet.com>, Izik Eidus <izike@...ranet.com>,
	Nick Piggin <npiggin@...e.de>, kvm-devel@...ts.sourceforge.net,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch 1/4] mmu_notifier: Core code

On Fri, Jan 25, 2008 at 10:47:04AM -0800, Christoph Lameter wrote:
> On Fri, 25 Jan 2008, Robin Holt wrote:
> 
> > I realize it is a minor nit, but since we put the continuation in column
> > 81 in the next define, can we do the same here and make this more
> > readable?
> 
> We need to fix the next define to not use column 81.
> Found a couple of more 80 column infractions. Will be fixed in next 
> release.
> 
> > > +void mmu_notifier_release(struct mm_struct *mm)
> > > +{
> > > +	struct mmu_notifier *mn;
> > > +	struct hlist_node *n;
> > > +
> > > +	if (unlikely(!hlist_empty(&mm->mmu_notifier.head))) {
> > > +		rcu_read_lock();
> > > +		hlist_for_each_entry_rcu(mn, n,
> > > +					  &mm->mmu_notifier.head, hlist) {
> > > +			if (mn->ops->release)
> > > +				mn->ops->release(mn, mm);
> > > +			hlist_del(&mn->hlist);
> > 
> > I think the hlist_del needs to be before the function callout so we can free
> > the structure without a use-after-free issue.
> 
> The list head is in the mm_struct. This will be freed later.
> 

I meant the structure pointed to by &mn.  I assume it is intended that
structure be kmalloc'd as part of a larger structure.  The driver is the
entity which created that structure and should be the one to free it.

> > > +void mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
> > > +{
> > > +	spin_lock(&mmu_notifier_list_lock);
> > 
> > Shouldn't this really be protected by the down_write(mmap_sem)?  Maybe:
> 
> Ok. We could switch this to mmap_sem protection for the mm_struct but the 
> rmap notifier is not associated with an mm_struct. So we would need to 
> keep it there. Since we already have a spinlock: Just use it for both to 
> avoid further complications.

But now you are putting a global lock in where it is inappropriate.

> 
> > > +	spin_lock(&mmu_notifier_list_lock);
> > > +	hlist_del(&mn->hlist);
> > 
> > hlist_del_rcu?  Ditto on the lock.
> 
> Peter already mentioned that and I have posted patches that address this 
> issue.
> 
> > > @@ -2043,6 +2044,7 @@ void exit_mmap(struct mm_struct *mm)
> > >  	vm_unacct_memory(nr_accounted);
> > >  	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, 0);
> > >  	tlb_finish_mmu(tlb, 0, end);
> > > +	mmu_notifier_release(mm);
> > 
> > Can we consider moving this notifier or introducing an additional notifier
> > in the release or a flag to this one indicating early/late.
> 
> There is only one call right now?
> 
> > The GRU that Jack is concerned with would benefit from the early in
> > that it could just invalidate the GRU context and immediately all GRU
> > TLB entries are invalid.  I believe Jack would like to also be able to
> > remove his entry from the mmu_notifier list in an effort to avoid the
> > page and range callouts.
> 
> The TLB entries are removed by earlier invalidate_range calls. I would 
> think that no TLBs are left at this point. Its simply a matter of 
> releasing any still allocated resources through this callback.

What I was asking for is a way to avoid those numerous callouts for
drivers that can do early cleanup.

>  
> > XPMEM, would also benefit from a call early.  We could make all the
> > segments as being torn down and start the recalls.  We already have
> > this code in and working (have since it was first written 6 years ago).
> > In this case, all segments are torn down with a single message to each
> > of the importing partitions.  In contrast, the teardown code which would
> > happen now would be one set of messages for each vma.
> 
> So we need an additional global teardown call? Then we'd need to switch 
> off the vma based invalidate_range()?

No, EXACTLY what I originally was asking for, either move this call site
up, introduce an additional mmu_notifier op, or place this one in two
locations with a flag indicating which call is being made.

Thanks,
Robin
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