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

Jerome Glisse
2020-06-22T20:15:58Z

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.

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.

> > Semantic was change with 17839856fd588f4ab6b789f482ed3ffd7c403e1f to some
> > what "fix" that but GUP fast is still succeptible to this.
>
> Ah, so everyone breaks the COW now, not just RDMA..
>
> What do you mean 'GUP fast is still succeptible to this' ?

Not all GUP fast path are updated (intentionaly) __get_user_pages_fast()
for instance still keeps COW intact. People using GUP should really knows
what they are doing.

Cheers,
Jérôme