Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations

Jason Gunthorpe
2020-06-23T00:02:35Z

On Mon, Jun 22, 2020 at 04:15:40PM -0400, Jerome Glisse wrote:
> On Mon, Jun 22, 2020 at 08:46:17AM -0300, Jason Gunthorpe wrote:
> > On Fri, Jun 19, 2020 at 04:31:47PM -0400, Jerome Glisse wrote:
> > > Not doable as page refcount can change for things unrelated to GUP, with
> > > John changes we can identify GUP and we could potentialy copy GUPed page
> > > instead of COW but this can potentialy slow down fork() and i am not sure
> > > how acceptable this would be. Also this does not solve GUP against page
> > > that are already in fork tree ie page P0 is in process A which forks,
> > > we now have page P0 in process A and B. Now we have process A which forks
> > > again and we have page P0 in A, B, and C. Here B and C are two branches
> > > with root in A. B and/or C can keep forking and grow the fork tree.
> >
> > For a long time now RDMA has broken COW pages when creating user DMA
> > regions.
> >
> > The problem has been that fork re-COW's regions that had their COW
> > broken.
> >
> > So, if you break the COW upon mapping and prevent fork (and others)
> > from copying DMA pinned then you'd cover the cases.
>
> I am not sure we want to prevent COW for pinned GUP pages, this would
> change current semantic and potentialy break/slow down existing apps.

Isn't that basically exactly what 17839856fd588 does? It looks like it
uses the same approach RDMA does by sticking FOLL_WRITE even though it
is a read action.

After that change the reamining bug is that fork can re-establish a
COW./

> Anyway i think we focus too much on fork/COW, it is just an unfixable
> broken corner cases, mmu notifier allows you to avoid it. Forcing real
> copy on fork would likely be seen as regression by most people.

If you don't copy the there are data corruption bugs though. Real apps
probably don't hit a problem here as they are not forking while GUP's
are active (RDMA excluded, which does do this)

I think that implementing page pinning by blocking mmu notifiers for
the duration of the pin is a particularly good idea either, that
actually seems a lot worse than just having the pin in the first
place.

Particularly if it is only being done to avoid corner case bugs that
already afflict other GUP cases :(

> > What do you mean 'GUP fast is still succeptible to this' ?
>
> Not all GUP fast path are updated (intentionaly) __get_user_pages_fast()

Sure, that is is the 'raw' accessor

Jason