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

[Christopher Clark <christopher.w.clark@xxxxxxxxx>]

On Mon, Nov 21, 2022 at 4:41 AM Jan Beulich <jbeulich@xxxxxxxx> 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@xxxxxxxx>

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;
>  }
>