On Tue, May 18, 2021 at 04:45:09PM -0300, Jason Gunthorpe wrote: > On Tue, May 18, 2021 at 02:01:36PM -0400, Peter Xu wrote: > > > > 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.. > > I'm confused about what device ownership you are talking about My question was more about the user scenario rather than anything related to the kernel code, nor does it related to page struct at all. Let me try to be a little bit more verbose... Firstly, I think one simple solution to handle fork() of device exclusive ptes is to do just like device private ptes: if COW we convert writable ptes into readable ptes. Then when CPU access happens (in either parent/child) page restore triggers which will convert those readable ptes into read-only present ptes (with the original page backing it). Then do_wp_page() will take care of page copy. However... if you see that also means parent/child have the equal opportunity to reuse that original page: who access first will do COW because refcount>1 for that page (note! it's possible that mapcount==1 here, as we drop mapcount when converting to device exclusive ptes; however with the most recent do_wp_page change from Linus where we'll also check page_count(), we'll still do COW just like when this page was GUPed by someone else). While that matters because the device is writting to that original page only, not the COWed one. Then here comes the ownership question: If we still want to have the parent process behave like before it fork()ed, IMHO we must make sure that original page (that exclusively owned by the device once) still belongs to the parent process not the child. That's why I think if that's the case we'd do early cow in fork(), because it guarantees that. I can't say I fully understand the whole picture, so sorry if I missed something important there. Thanks, -- Peter Xu
