On Mon, Nov 21, 2022 at 4:41 AM Jan Beulich <jbeul...@suse.com> wrote:
> On 11.10.2022 11:28, Jan Beulich wrote:
> > find_ring_mfn() already holds a page reference when trying to obtain a
> > writable type reference. We shouldn't make assumptions on the general
> > reference count limit being effectively "infinity". Obtain merely a type
> > ref, re-using the general ref by only dropping the previously acquired
> > one in the case of an error.
> >
> > Signed-off-by: Jan Beulich <jbeul...@suse.com>
>
> Ping?
>
Hi Jan,
Sorry it has taken me so long to review this patch and thank-you for
posting it. The points raised are helpful.
Wrt to the patch - I can't ack because:
the general ref that is already held is from the page owner, and it may
actually be foreign; so the second ref acquire is currently ensuring that
it is a match for the owner of the ring. That needs addressing.
Am supportive of points raised:
- review + limit ref counts taken
- better to not need two general page refs
- a type ref rather than general may be sufficient to hold for the ring
lifetime?
- paging_mark_dirty at writes
- p2m log dirty would be better to be allowed than EAGAIN
- allowing mapping of foreign pages may have uses though likely also
challenging
I should let you know that my time available is extremely limited at the
moment, sorry.
Christopher
>
> > ---
> > I further question the log-dirty check there: The present P2M type of a
> > page doesn't really matter for writing to the page (plus it's stale by
> > the time it is looked at). Instead I think every write to such a page
> > needs to be accompanied by a call to paging_mark_dirty().
> >
> > --- a/xen/common/argo.c
> > +++ b/xen/common/argo.c
> > @@ -1429,10 +1429,11 @@ find_ring_mfn(struct domain *d, gfn_t gf
> > ret = -EAGAIN;
> > #endif
> > else if ( (p2mt != p2m_ram_rw) ||
> > - !get_page_and_type(page, d, PGT_writable_page) )
> > + !get_page_type(page, PGT_writable_page) )
> > ret = -EINVAL;
> >
> > - put_page(page);
> > + if ( unlikely(ret) )
> > + put_page(page);
> >
> > return ret;
> > }
> >
>
>
