Re: [PATCH v8 5/8] mm: Device exclusive memory access

[Peter Xu]

On Tue, May 18, 2021 at 02:33:34PM -0300, Jason Gunthorpe wrote:
> On Tue, May 18, 2021 at 01:27:42PM -0400, Peter Xu wrote:
> 
> > I also have a pure and high level question regarding a process fork() when
> > there're device exclusive ptes: would the two processes then own the device
> > together?  Is this a real usecase?
> 
> If the pages are MAP_SHARED then yes. All VMAs should point at the
> same device_exclusive page and all VMA should migrate back to CPU
> pages together.

Makes sense.  If we keep the anonymous-only in this series (I think it's good
to separate these), maybe we can drop the !COW case, plus some proper installed
WARN_ON_ONCE()s.

> 
> > Indeed it'll be odd for a COW page since for COW page then it means after
> > parent/child writting to the page it'll clone into two, then it's a mistery on
> > which one will be the one that "exclusived owned" by the device..
> 
> For COW pages it is like every other fork case.. We can't reliably
> write-protect the device_exclusive page during fork so we must copy it
> at fork time.
> 
> Thus three reasonable choices:
>  - Copy to a new CPU page
>  - Migrate back to a CPU page and write protect it
>  - Copy to a new device exclusive page

IMHO the ownership question would really help us to answer this one..

If the device ownership should be kept in parent IMHO option (1) is the best
approach. To be explicit on page copy: we can do best-effort, even if the copy
is during a device atomic operation, perhaps?

If the ownership will be shared, seems option (3) will be easier as I don't see
a strong reason to do immediate restorinng of ptes; as long as we're careful on
the refcounting.

Thanks,

-- 
Peter Xu