Re: Ping: [PATCH] Argo: don't obtain excess page references

[Christopher Clark <christopher.w.clark@gmail.com>]

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On Mon, Nov 21, 2022 at 4:41 AM Jan Beulich <jbeulich@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 <jbeulich@suse.com>
>
> Ping?
>

Message received -- I will look at this. Thanks.

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;
> >  }
> >
>
>

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