On Wednesday, 19 May 2021 9:45:05 AM AEST Peter Xu wrote: > External email: Use caution opening links or attachments > > On Tue, May 18, 2021 at 08:03:27PM -0300, Jason Gunthorpe wrote: > > Logically during fork all these device exclusive pages should be > > reverted back to their CPU pages, write protected and the CPU page PTE > > copied to the fork. > > > > We should not copy the device exclusive page PTE to the fork. I think > > I pointed to this on an earlier rev.. > > Agreed. Though please see the question I posted in the other thread: now I > am not very sure whether we'll be able to mark a page as device exclusive > if that page has mapcount>1. > > > We can optimize this into the various variants above, but logically > > device exclusive stop existing during fork. > > Makes sense, I think that's indeed what this patch did at least for the COW > case, so I think Alistair did address that comment. It's just that I think > we need to drop the other !COW case (imho that should correspond to the > changes in copy_nonpresent_pte()) in this patch to guarantee it. Right. The main change from v7 -> v8 was to remove device exclusive entries on fork instead of copying them. The change in copy_nonpresent_pte() is for the !COW case. I think what you are getting at is given exclusive entries are (currently) only supported for PageAnon pages is_cow_mapping() will always be true and therefore the change to copy_nonpresent_pte() can be dropped. That logic seems reasonable so I will change the exclusive case in copy_nonpresent_pte() to a VM_WARN_ON. > I also hope we don't make copy_pte_range() even more complicated just to do > the lock_page() right, so we could fail the fork() if the lock is hard to > take. Failing fork() because we couldn't take a lock doesn't seem like the right approach though, especially as there is already existing code that retries. I get this adds complexity though, so would be happy to take a look at cleaning copy_pte_range() up in future. > -- > Peter Xu
