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SubjectRe: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list)
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.

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