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

Jason Gunthorpe
2020-06-22T11:46:23Z

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.

> 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' ?

Jason