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

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Message-ID: <Pine.LNX.4.64.0801251041040.672@schroedinger.engr.sgi.com>
Date:	Fri, 25 Jan 2008 10:47:04 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Robin Holt <holt@....com>
cc:	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, 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.

> > +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.

> > +	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.
 
> 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()?

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