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

[David Howells]

Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> 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.

Wouldn't that potentially make someone's entire malloc() heap entirely NOCOW
if they did a single DIO to/from it.

Also you only mention DIO read - but what about "start DIO write; fork(); touch
buffer" in the parent - now the write buffer belongs to the child and they can
affect the parent's write.

David