Re: [Xen-devel] [PATCH 2/2] domain: use PGC_extra domheap page for shared_info

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.03.2020 17:27, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: 06 March 2020 13:46
>> To: Paul Durrant <xadimgnik@xxxxxxxxx>
>> Cc: sstabellini@xxxxxxxxxx; julien@xxxxxxx; Volodymyr_Babchuk@xxxxxxxx; 
>> wl@xxxxxxx;
>> konrad.wilk@xxxxxxxxxx; andrew.cooper3@xxxxxxxxxx; ian.jackson@xxxxxxxxxxxxx;
>> george.dunlap@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; 'David Woodhouse' 
>> <dwmw2@xxxxxxxxxxxxx>
>> Subject: Re: [PATCH 2/2] domain: use PGC_extra domheap page for shared_info
>>
>> On 06.03.2020 14:41, Paul Durrant wrote:
>>>> -----Original Message-----
>>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Sent: 06 March 2020 13:36
>>>> To: Paul Durrant <xadimgnik@xxxxxxxxx>
>>>> Cc: sstabellini@xxxxxxxxxx; julien@xxxxxxx; Volodymyr_Babchuk@xxxxxxxx; 
>>>> wl@xxxxxxx;
>>>> konrad.wilk@xxxxxxxxxx; andrew.cooper3@xxxxxxxxxx; 
>>>> ian.jackson@xxxxxxxxxxxxx;
>>>> george.dunlap@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; 'David 
>>>> Woodhouse' <dwmw2@xxxxxxxxxxxxx>
>>>> Subject: Re: [PATCH 2/2] domain: use PGC_extra domheap page for shared_info
>>>>
>>>> On 06.03.2020 14:26, Paul Durrant wrote:
>>>>>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of 
>>>>>> Jan Beulich
>>>>>> Sent: 06 March 2020 13:24
>>>>>>
>>>>>> On 06.03.2020 14:13, Paul Durrant wrote:
>>>>>>> My aim is to make the separation abundantly obvious by getting rid
>>>>>>> of shared xenheap pages (for non-system domains at least) but I
>>>>>>> can't do that before converting shared_info and grant shared/status
>>>>>>> frames to domheap.
>>>>>>
>>>>>> Following David's various replies - instead of going this route of
>>>>>> replacing the sharing of xenheap pages by different logic, the
>>>>>> same ought to be achievable by making the backing allocations come
>>>>>> from the correct pool?
>>>>>>
>>>>>
>>>>> I still prefer the simplification of not having to clean up the
>>>>> shared xenheap page list in domain_kill() so IMO it is still worth
>>>>> it for that alone.
>>>>
>>>> I don't see anything very special with the cleaning up in
>>>> domain_kill() / domain_relinquish_resources(). What I'd view as
>>>> more desirable in this regard is the general fact of needing
>>>> two lists. But you realize there's a downside to this as well?
>>>> dump_pageframe_info() will reliably show _all_ Xen heap pages
>>>> associated with a domain, but it will only ever show up to 10
>>>> pages on ->page_list for a domain that's not already being
>>>> cleaned up.
>>>
>>> That's not much of a downside though I don't think. The xenheap
>>> (or PGC_extra domheap pages) are 'special' and so info about
>>> them ought to be available via an alternate dump function anyway
>>> (and if not already, it can be added).
>>
>> Whatever you'd add, the logic would need to either traverse the
>> entire ->page_list (can take very long) or have/use out of band
>> information where such pages may have a record (fragile).
>>
> 
> But the shared xenheap pages in question are only shared info, or
> grant table, so their information can be dumped separately.
> I guess it makes more sense to add another patch into the series
> to do explicit dump of shared_info and then exclude 'special'
> pages from dump_pageframe_info().

Bu that's why I said "fragile" - new uses of such pages wouldn't
automatically be picked up, whereas them all landing on xenpage_list
made their dumping a reliable thing.

Jan

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