Nouveau - Apr 2021 - [PATCH v7 5/8] mm: Device exclusive memory access

On Thursday, 1 April 2021 12:46:04 AM AEDT Jason Gunthorpe
wrote:
> On Thu, Apr 01, 2021 at 12:27:52AM +1100, Alistair Popple wrote:
> > On Thursday, 1 April 2021 12:18:54 AM AEDT Jason Gunthorpe wrote:
> > > On Wed, Mar 31, 2021 at 11:59:28PM +1100, Alistair Popple wrote:
> > > 
> > > > I guess that makes sense as the split could go either way at
the
> > > > moment but I should add a check to make sure this isn't
used with
> > > > pinned pages anyway.
> > > 
> > > Is it possible to have a pinned page under one of these things?
If I
> > > pin it before you migrate it then it remains pinned but hidden
under
> > > the swap entry?
> > 
> > At the moment yes. But I had planned (and this reminded me) to add a
check
to 
> > prevent marking pinned pages for exclusive access. 
> 
> How do you even do that without races with GUP fast?
Unless I've missed something I think I've convinced myself it should be
safe
to do the pin check after make_device_exclusive() has replaced all the PTEs 
with exclusive entries.

GUP fast sequence:
1. Read PTE
2. Pin page
3. Check PTE
4. if PTE changed -> unpin and fallback

If make_device_exclusive() runs after (1) it will either succeed or see the 
pin from (2) and fail (as desired). GUP should always see the PTE change and 
fallback which will revoke the exclusive access.

 - Alistair
> Jason
>

On Thu, Apr 01, 2021 at 11:45:57AM +1100, Alistair Popple
wrote:
> On Thursday, 1 April 2021 12:46:04 AM AEDT Jason Gunthorpe wrote:
> > On Thu, Apr 01, 2021 at 12:27:52AM +1100, Alistair Popple wrote:
> > > On Thursday, 1 April 2021 12:18:54 AM AEDT Jason Gunthorpe wrote:
> > > > On Wed, Mar 31, 2021 at 11:59:28PM +1100, Alistair Popple
wrote:
> > > > 
> > > > > I guess that makes sense as the split could go either
way at the
> > > > > moment but I should add a check to make sure this
isn't used with
> > > > > pinned pages anyway.
> > > > 
> > > > Is it possible to have a pinned page under one of these
things? If I
> > > > pin it before you migrate it then it remains pinned but
hidden under
> > > > the swap entry?
> > > 
> > > At the moment yes. But I had planned (and this reminded me) to
add a check
> to 
> > > prevent marking pinned pages for exclusive access. 
> > 
> > How do you even do that without races with GUP fast?
> 
> Unless I've missed something I think I've convinced myself it
should be safe
> to do the pin check after make_device_exclusive() has replaced all the PTEs
> with exclusive entries.
> 
> GUP fast sequence:
> 1. Read PTE
> 2. Pin page
> 3. Check PTE
> 4. if PTE changed -> unpin and fallback
> 
> If make_device_exclusive() runs after (1) it will either succeed or see the
> pin from (2) and fail (as desired). GUP should always see the PTE change
and
> fallback which will revoke the exclusive access.
AFAICT the user can trigger fork at that instant and fork will try to
copy the desposited migration entry before it has been checked

Jason