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From: Daniel Phillips <phillips@bonn-fries.net>
To: Hugh Dickins <hugh@veritas.com>
Subject: Re: [RFC] Page table sharing
Date: Tue, 19 Feb 2002 01:27:42 +0100
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On February 19, 2002 01:03 am, Hugh Dickins wrote:
> On Tue, 19 Feb 2002, Daniel Phillips wrote:
> > On February 18, 2002 08:04 pm, Hugh Dickins wrote:
> > > On Mon, 18 Feb 2002, Daniel Phillips wrote:
> > > > On February 18, 2002 09:09 am, Hugh Dickins wrote:
> > > > > Since copy_page_range would not copy shared page tables, I'm wrong to
> > > > > point there.  But __pte_alloc does copy shared page tables (to unshare
> > > > > them), and needs them to be stable while it does so: so locking against
> > > > > swap_out really is required.  It also needs locking against read faults,
> > > > > and they against each other: but there I imagine it's just a matter of
> > > > > dropping the write arg to __pte_alloc, going back to pte_alloc again.
> > 
> > I'm not sure what you mean here, you're not suggesting we should unshare the
> > page table on read fault are you?
> 
> I am.  But I can understand that you'd prefer not to do it that way.
> Hugh

No, that's not nearly studly enough ;-)

Since we have gone to all the trouble of sharing the page table, we should
swap in/out for all sharers at the same time.  That is, keep it shared, saving
memory and cpu.

Now I finally see what you were driving at: before, we could count on the
mm->page_table_lock for exclusion on read fault, now we can't, at least not
when ptb->count is great than one[1].  So let's come up with something nice as
a substitute, any suggestions?

[1] I think that's a big, broad hint.

-- 
Daniel
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