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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.01.2023 05:35, Christopher Clark wrote:
> 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?
> 
> 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.

I'm afraid I may not understand your reply: Are you saying there's something
wrong with the change? Or are you saying there's something wrong that merely
becomes apparent due to the change? Or yet something else?

> 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

I can certainly extend the patch to a series, but that'll make sense only
if ...

> - 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.

... it would then also be looked at within a reasonable amount of time. I'm
already sitting on way too many unreviewed patches.

Jan