Re: [PATCH v1 00/15] Keep track of GUPed pages in fs and block

[Jerome Glisse]

On Wed, Apr 17, 2019 at 01:09:22AM +0300, Boaz Harrosh wrote:
> On 16/04/19 22:57, Jerome Glisse wrote:
> <>
> > 
> > A very long thread on this:
> > 
> > https://lkml.org/lkml/2018/12/3/1128
> > 
> > especialy all the reply to this first one
> > 
> > There is also:
> > 
> > https://lkml.org/lkml/2019/3/26/1395
> > https://lwn.net/Articles/753027/
> > 
> 
> OK I have re-read this patchset and a little bit of the threads above (not all)
> 
> As I understand the long term plan is to keep two separate ref-counts one
> for GUP-ref and one for the regular page-state/ownership ref.
> Currently looking at page-ref we do not know if we have a GUP currently held.
> With the new plan we can (Still not sure what's the full plan with this new info)
> 
> But if you make it such as the first GUP-ref also takes a page_ref and the
> last GUp-dec also does put_page. Then the all of these becomes a matter of
> matching every call to get_user_pages or iov_iter_get_pages() with a new
> put_user_pages or iov_iter_put_pages().

So sorry forgot to answer that part. So idea is to do:
    GUP() {
        ...
-       page_ref_inc(page);
+       page_ref_add(page, GUP_BIAS);
        ...
    }

with GUP_BIAS = 1024 or something big but not too big to avoid risk of
overflow by GUP. Then put_user_page() just ref_sub instead of ref_dec
the same amount.

We can have false GUP positive if a page is map so many time or reference
so many time that its refcount reach the GUP_BIAS value but considering
such page as GUPed should not be too harmful (not more harmful than what
we do with GUPed page).

So we want to call put_user_page() for GUPed page and only GUPed page so
that we keep the reference count properly balance.

Cheers,
Jérôme