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

[Jan Kara]

On Mon 23-01-23 16:31:32, Matthew Wilcox wrote:
> 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.

Well, but anon COW code is not the only (planned) consumer of the pincount.
Filesystems also need to know whether a (shared pagecache) page is pinned
and can thus be modified behind their backs. And for that VMA tracking
isn't really an option.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR