On Fri, Jan 20, 2023 at 05:55:48PM +0000, David Howells wrote: > (3) Make the bio struct carry a pair of flags to indicate the cleanup > mode. BIO_NO_PAGE_REF is replaced with BIO_PAGE_REFFED (equivalent to > FOLL_GET) and BIO_PAGE_PINNED (equivalent to BIO_PAGE_PINNED) is > added. I think there's a simpler solution than all of this. As I understand the fundamental problem here, the question is when to copy a page on fork. We have the optimisation of COW, but O_DIRECT/RDMA/... breaks it. So all this page pinning is to indicate to the fork code "You can't do COW to this page". Why do we want to track that information on a per-page basis? Wouldn't it be easier to have a VM_NOCOW flag in vma->vm_flags? Set it the first time somebody does an O_DIRECT read or RDMA pin. That's it. Pages in that VMA will now never be COWed, regardless of their refcount/mapcount. And the whole "did we pin or get this page" problem goes away. Along with folio->pincount.
