Re: [RFC] Page table sharing

[ebiederm@xmission.com (Eric W. Biederman)]

Daniel Phillips <phillips@bonn-fries.net> writes:

> 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.

Something like:
struct mm_share {
        spinlock_t page_table_lock;
        struct list_head mm_list;
};

struct mm {
	struct list_head mm_list;
        struct mm_share *mm_share;
        .....
};

So we have an overarching structure for all of the shared mm's.  

Eric