Re: [PATCH 4/4] mm: check the device private page owner in hmm_range_fault

[Christoph Hellwig <hch@lst.de>]

On Sat, Mar 21, 2020 at 09:38:04AM -0300, Jason Gunthorpe wrote:
> > I don't think there is any specific protection.  Let me see if we
> > can throw in a get_dev_pagemap here
> 
> The page tables are RCU protected right? could we do something like
> 
>  if (is_device_private_entry()) {
>        rcu_read_lock()
>        if (READ_ONCE(*ptep) != pte)
>            return -EBUSY;
>        hmm_is_device_private_entry()
>        rcu_read_unlock()
>  }
> 
> ?

Are they everywhere?  I'd really love to hear from people that really
know this ara..

> 
> Then pgmap needs a synchronize_rcu before the struct page's are
> destroyed (possibly gup_fast already requires this?)
> 
> I've got some other patches trying to close some of these styles of
> bugs, but 
> 
> > note that current mainline doesn't even use it for this path..
> 
> Don't follow?

If you look at mainline (or any other tree), we only do a
get_dev_pagemap for devmap ptes.  But device private pages are encoded
as non-present swap ptes.
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