Re: [Xen-devel] Converting heap page_infos to contiguous virtual

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 13/07/16 21:57, Boris Ostrovsky wrote:
> On 07/13/2016 04:34 PM, Andrew Cooper wrote:
>> On 13/07/2016 21:17, Boris Ostrovsky wrote:
>>> On 07/13/2016 04:02 PM, Andrew Cooper wrote:
>>>> On 13/07/16 20:44, Boris Ostrovsky wrote:
>>>>> I would like to clear a bunch of Xen heap pages at once (i.e. not
>>>>> page-by-page).
>>>>>
>>>>> Greatly simplifying things, let's say I grab (in common/page_alloc.c)
>>>>>     pg = page_list_remove_head(&heap(node, zone, order)
>>>>>
>>>>> and then
>>>>>
>>>>>     mfn_t mfn =
>>>>> _mfn(page_to_mfn(pg));                                        
>>>>>     char *va = mfn_to_virt(mfn_x(mfn));
>>>>>     memset(va, 0, 4096 * (1 << order));
>>>>>
>>>>>
>>>>> Would it be valid to this?
>>>> In principle, yes.  The frame_table is in order.
>>>>
>>>> However, mfn_to_virt() will blow up for RAM above the 5TB boundary.  You
>>>> need to map_domain_page() to get a mapping.
>>> Right, but that would mean going page-by-page, which I want to avoid.
>>>
>>> Now, DIRECTMAP_SIZE is ~128TB (if my math is correct) --- doesn't it
>>> imply that it maps this big a range contiguously (modulo PDX hole)?
>> Your maths is correct, and yet you will end up with problems if you
>> trust it.
>>
>> That is the magic mode for the idle and monitor pagetables.  In the
>> context of a 64bit PV guest, the cutoff is at 5TB, at which point you
>> venture into the virtual address space reserved for guest kernel use. 
>> (It is rather depressing that the 64bit PV guest ABI is the factor
>> limiting Xen's maximum RAM usage.)
> 
> I don't know whether it would make any difference but the pages that I am
> talking about are not in use by any guest, they are free. (This question
> is for scrubbing rewrite that I am working on. Which apparently you
> figured out judged by what you are saying below)

Is this start-of-day scrubbing (when there are no guests), or scrubbing
on guest destruction?

If the former, it seems like it might not be too difficult to arrange
that we're in a context that has all the RAM mapped.

 -George


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